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10th Class - Physical Science-TS

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10th Class - Physical Science-TS
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I. Reflections on concepts

Question 1.
Explain the difference between the valence electrons and the covalency of an element.

Answer:

Valence Electrons Covalency
1. The electrons present in the outermost orbital of an atom are called valence electrons. 1. The total number of covalent bonds that an atom of an element forms is called covalency of the element
2. Valence electrons depend upon the number of electrons present in that atom. 2. Covalency depends upon the valence electrons.
Question 2.
A chemical compound has the following Lewis notation:

(a) How many valence electrons does element Y have?
(b) What ¡s the valency of element Y’ x . X
(c) What is the valency of element X?
(d) How many covalent bonds are there in the molecule? H
(e) To which groups the elements X and Y belong?

Answer:
(a) Six electrons.
(b) Two. because it has combined with two elements namely X and H.
(c) One
(d) Two covalent bonds one in Y - X and another one is Y - H.
(e) X is Hydrogen and Y is oxygen. I suggest the molecule is H2O (water).

Question 3.
How bond energies and bond lengths of molecule help us in predicting their chemical properties? Explain with examples.

Answer:
1. Bond length: Bond length or bond distance ¡s the equilibrium distance between the nuclei of two atoms which form a covalent bond.

2. Bond energy: Bond energy or bond dissociation energy ¡s the energy needed to break a covalent bond between two atoms of a diatomic covalent compound in its gaseous state.

3. If the nature of the bond between the same two atoms changes, the bond length also changes. For example, the bond lengths between two carbon atoms are C-C>C=C>C ≡ C.

4. Thus the various bond lengths between the two carbon atoms are ¡n ethane 1.54 Å, ethylene 1.34 Å. acetylene 1.20 Å.

5. The bond lengths between two oxygen atoms are in H2O2(O-O) Is 1.48 A° and in O2(O = O) is 1.21 Å.

6. Observe the table.

Bond Bond length(Å) Bond (dissociation) energy (KJ mol-1’)
H -H 0.74 436
F-F 1.44 159
Cl-Cl 1.95 243
Br-Br 2.28 193
I-I 2.68 151
H-F 0.918 570
H-Cl 1.27 432
H-Br 1.42 366
H -I 1.61 298
H-O(of H2O) 0.96 460
H-N(of NH3) 1.01 390
H-C(of CH4) 1.10 410

7. When bond length decreases, then bond dissociation energy Increases.
8. When bond length increases, then bond dissociation energy decreases.
9. Bond length of H -H in H2molecule is 0.74 Å and its bond dissociation energy is 436 KJ/mol, whereas bond length of F – F In F2molecule is 1.44 Å and its bond dissociation energy Is 159 K/mol.
10. Melting and boiling points of substances also can be determined by these bond energies and bond lengths.

Question 4.
Draw simple diagrams to show how electrons are arranged in the following covalent molecules:

(a) Calcium oxide (CaO)
(b) Water (H2O)
(c) Chlorine (Cl2)

Answer:
(a) Calcium oxide (CaO)

(b) Water (H2O):

One oxygen and two hydrogen atoms form a water molecule, H2O

(c) Chlorine (Cl2):

Question 5.
Represent each of the following atoms using Lewis notation.

(a) Beryllium
(b) Calcium
(c) Lithium
(d) Bromine gas(Br2)
(e) Calcium Chloride (CaCl2)
(f) Carbon dioxide (CO2)

Answer:
(a) Beryllium:
Beryllium atomic number = 4
Be -Valency electrons = 2

(b) Calcium:
Calcium atomïc number = 20
Ca -Valency electrons = 2

(C) Lithium:
Lithium atomic number = 3
Li -Valency electron = 1

(d) Bromine gas (Br2):

(e) Calcium Chloride (CaCl2):

(f) Carbon dioxide (CO2):

Question 6.
Why do only valence electrons involve in bond formation? Why not electron of Inner shells? Explain.

Answer:

  1. When two atoms come sufficiently close together the valence electrons of each atom experience the attractive force of the nucleus in the other atom.
  2. The nucleus and the electrons ¡n the inner shell remain unaffected when atoms come close together.
  3. only the electrons in outermost shell of an atom get affected.
  4. Thus electrons in valence shell are responsible for the formation of bond between atoms.
Question 7.
List the factors that determine the type of bond that will be formed between two atoms.

Answer:
There are several factors that determine the type of bond which will be formed between two atoms. They are

  • The force of attraction or repulsion between the electrons and protons.
  • Number of valence electrons present in the valence shell of the atom.
  • Electronegative difference between the atoms.
    If the E.N difference between the two atoms is > 1.9, ionic bond is formed.
    If the E.N difference between the two atoms is <1.9, covalent bond is formed.
  • Atomic size
  • Ionisation potential
  • Electron affinity.
Question 8.
Represent the molecule H2O using Lewis notation.

Answer:

Question 9.
What is octet rule? How do you appreciate the role of the ‘Octet rule’ in explaining the chemical properties of elements?

Answer:
Octet rule: “The atoms of element tend to undergo chemical changes that help to leave their atoms with eight outer-shell electrons”.

→ Role of ‘Octet Rule :

  • ‘Octet rule’ helps to explain the chemical activities of atoms of many elements.
  • It explains why some elements are more reactive towards chemical reaction and some are not.
  • It can explain the high reactivity of Alkali, Alkaline earth metals.
  • It can also explain the high reactivity of halogens.
Question 10.
What is hybridization? Explain the formation of the following molecules using hybridization.

(a) BeCl2
(b) BF3

Answer:
(i) Hybridisation is a phenomenon of intermixing of atomic orbitals of almost equal energy which are present in the outer shells of the atom and their reshuffling or redistribution into the same number of orbitals but with equal properties like energy and shape.

(a) Formation of BeCl2(Beryllium chloride) molecule:
(ii) The atomic number of Beryllium is 4
(iii) The electronic configuration of Beryllium atom in its ground state is 1s22s2.
(iv) The electronic configuration of Beryllium atom in its excited state is 1s22s12p1.

(i) In the excited Beryllium atom its ‘2s’ and ‘2px‘ orbitals intermix to give two equivalent ‘ sp ‘ hybrid orbitals.
(ii) The electronic configuration of Be is 1S22s12px1. It has one half-filled ‘p’ orbital.
(iii) The electronic configuration of 17 Cl is 1s2 2 s22p63s23px23py23pz1

(iv) The half-filled 3pzorbitals of two chlorine atoms overlap with ‘sp’ hybrid orbitals of beryllium atom in their axes to form two σsp-pbonds.
(v) BeCl2 molecule so formed has linear shape. The bond angle in BeCl2 is 180°.

(b) Formation of Boron Trifluoride BF3:
(i) The central atom in BF3 is boron.
(ii) The electronic configuration of boron atom in its excited state is 1s22s12p2
B(Z=5)is

(iii) In the excited boron atom 2s’ orbital and two ‘2p’ orbitals intermix to give three equivalent sp2 hybrid orbitals.
(iv) In the formation of BF3 molecule, three sp2 hybrid orbitals of boron overlap with half-filled 2pz orbitals of three fluorine atoms. in their axes to give three bonds.
(v) BF3 molecule so formed has trigonal planar structure.

(iv) The bond angle ¡n BF3is 120°.

Application of Concepts

Question 1.
Explain the formation of sodium chloride and calcium oxide on the basis of the concept of electron transfer from one atom to another atom.

Answer:
1. Formation of sodium chloride (NaCl): Sodium chloride is formed from the elements sodium and chlorine. It can be explained as follows.
(a) Formation of Cation: When sodium atom loses one electron to get octet electron configuration it forms a cation (Na+) and gets electron configuration that of Neon (Ne)
Na → Na++e
E.C: 2, 8, 1 2,8 +1e

(b) Formation of anion: Chlorine has shortage of one electron to get octet in its valence shell. So It gains the election that was lost by Na to form anion (Cl) and gets electron configuration of Argon (Ar)
Cl + e→ Cl
E.C: 2,8,7 2,8,8

(c) Formation of the compound NaCl from ions: Transfer of electrons take place between ‘Na’ and ‘Cl’ atoms while they form Na+and Clions. These oppositely charged ions get attracted towards each other due to
electrostatic forces and form the compound sodium chloride (NaCl).
Na+(g)+ Cl(g)→ NaCl(s)

2. FormatIon of calcium oxide (CaO) Calcium Oxide Is formed from the elements Calcium and Oxygen. It can be explained as follows :
(a) Formation of Cation: When Calcium atom loses two electrons to get octet electronic configuration it forms a cation (Ca+2) and gets electron configuration of Argon (Ar)

(b) Formation of anion: Oxygen has shortage of two electrons to get octet in its valence shell. So it gains the electrons that were lost by Ca to form anion (O-2) and gets electron configuration of Neon (Ne)
O + 2e-→ O-2
E.C: 2, 6 → 2,8

(c) Formation of the compound CaO from ions : Transfer of electrons between ‘Ca’ and ‘O’ atoms takes place while they form Ca+2and O-2ions. These oppositely charged ions get attracted towards each other due to
electrostatic forces and form the compound calcium oxide (CaO).
Ca+2(g)+ O-2(g)→CaO(s)

Question 2.
A, B and C are three elements with atomic numbers 6, 11. and 17 respectively.

(i) Which of these cannot form Ionic bond? Why?
(ii) Which of these cannot form covalent bond? Why?
(iii) Which of these can form Ionic as well as covalent bonds?

Answer:
(i) ‘A’ cannot form ionic bond. Its valence electrons are 4. It is difficult to lose or gain 4e–to get octet configuration. So it forms covalent bond [Z of A is 6 so it is carbon (c)].

(ii) ‘B’ cannot form covalent bond. Its valence electrons are 1 only. So it is easy to donate the electron for other atom and become an ion. So it can form ionic bond [Z of B is 11, so it is sodium (Na)].

(iii) Element C can form ionic as well as covalent bonds. The element with atomic number 17 is Cl. It is able to participate with Na in ionic bond forming NaCl and with hydrogen in HCl molecule as covalent bond.

Question 4.
How Lewis dot structure helps in understanding bond formation between atoms?

Answer:

  1. The valence electrons in an atom are represented by putting dots (.)on the symbol of the element, one dot for each valence electron.
  2. By knowing the valence electrons of two different atoms by Lewis dot structure, we can understand which type of bond is going to establish between them and forms corresponding molecule.
Question 5.
Explain the formation of the following molecules using valence bond theory.

(a) N2molecule,
(b) O2molecule

Answer:
(a) Formation of N2molecule:
7N has electronic configuration 1s22s22px12py12pz1Suppose that ‘px’orbital of one ‘N’ atom overlaps the ‘px’ orbital of the other ‘N’ atom giving σpx-pxbond along the inter-nuclear axis. The pyand pzorbitals of one ‘N’ atom overlap the pyand pzorbital of other ‘N’ atom laterally, respectively perpendicular to internuclear axis giving 2π py-pyand pz– pzbonds. Therefore N2molecule has a triple bond between two nitrogen atoms. (N ≡ N)

b) Formation of O2molecule:

  • 8O has electronic configuration 1s22s22px22py12pz1
  • If the ‘py’ orbital of one ‘O’ atom overlaps the ‘py’ orbital of other ‘O’ atom along the internuclear axis, a sigma py– pybond ((σpy- py) is formed.
  • pzorbital of one ‘O’ atom overlaps the pzorbital of other ‘O’ atom laterally, perpendicular to the inter-nuclear axis giving a Πpz-pzbond.
  • O2molecule has a double bond between two oxygen atoms. (O=O)
Question 6.
Predict the reasons for low melting points for covalent compounds when compared with ionic compounds.

Answer:

  1. The melting point is low due to the weak Vander Waal’s forces of attraction between the covalent molecules.
  2. The force of attraction between the molecules of a covalent compound is very weak.
  3. Only a small amount of heat energy is required to break these weak molecular forces, due to which covalent compounds have low melting points and low boiling points. :
  4. But some of the covalent solids like diamond and graphite have, however very high melting points and boiling points.

Higher Order Thinking questions

Question 1.
Two chemical reactions are described below.

(i) Nitrogen and hydrogen react to form ammonia.
(ii) Carbon and hydrogen bond together to form a molecule of methane (CH4)

For each reaction, give
a) The valence of each of the atoms involved in the reaction.
b) The Lewis structure of the product that ¡s formed.

Answer:
i) Nitrogen reacts with hydrogen to form Ammonia. The reaction is
N2+ 3H2→ 2NH3
a) The valency of each atom involved in the reaction.
Valence of Nitrogen = 3
Valence of Hydrogen = 1

(b) The Lewis structure of the product that is formed

(i) Carbon reacts with hydrogen to form a molecule of methane. The reaction is
C + 2H2→ CH4
(a) The valency of each atom involved in the reaction
Valence of Carbon = 4
Valence of Hydrogen = 1

(b) The Lewis structure of the product

Suggested Projects

Question 1.
Collect the information about properties and uses of covalent compounds and prepare a report?

Answer:
A. Properties of covalent compounds:

  1. Covalent compounds are usually liquids or gases, only some of them are solids.
  2. They are usually liquids or gases due to the weak force of attraction between their molecules.
  3. They have usually low melting and low boiling points.
  4. They are usually insoluble in water but they are soluble in organic solvents.
  5. They do not conduct electricity.
  6. They show molecular reactions.

Uses of covalent compounds:

  1. Covalent compounds form 99% of our body.
  2. Water is a covalent compound. We know many uses of water.
  3. Sugars, food substances, tea, and coffee are all covalent compounds.
  4. Air we breathe in contains covalent molecules of oxygen and nitrogen.
  5. Almost everything on earth other than most simple inorganic salts are covalent compounds.

Page 150

Question 1.
How do they(elements)usually exist?

Answer:
Elements usually exist as group of atoms.

Question 2.
Do they exist as a single atom or as a group of atoms?

Answer:
Except inert elements, all others exist as group of atoms. Inert elements exist as single atoms.

Question 3.
Are there elements which exist as atoms?

Answer:
Inert elements exist freely as atoms.

Question 4.
Why do some elements exist as molecules and some as atoms?

Answer:
Inert elements exist as atoms as they won’t form any bond other atoms form bonds and exist as molecules.

Question 5.
Why do some elements and compounds react vigorously while others are inert?

Answer:
Elements which do not have octet conflgurtion in their valence shelf react vigorously with other elements to form stable entities and which have octet configuration in their valence shell are chemically inert in nature.

Question 6.
Why is the chemical formula for water H20 and for sodium chloride NaCl, why not HO2 and NaCl2’?

Answer:
In water, oxygen atom bonds with two hydrogen atoms where as sodi + ion forms single bond with chloride in NaCl – ion.

Question 7.
Why do some atoms combine while others do not?

Answer:
Elements which do not have octet configuration in their valence shell combine with other elements and which have octet in their valence shell are chemically inert in nature.

Question 8.
Are elements and Compounds simply made up of separate atoms Individually arranged?

Answer:
No. They are arranged

Question 9.
Is there any attraction between atoms?

Answer:
Yes, there is attraction chemical bond.

Question 10.
What is that holdlng them together?

Answer:
Force of attraction called chemical bond.

Page 152

Question 11.
Why there is absorption of energy in certain chemical reactions and release of energy In other reactions?

Answer:
The absorption of energy in chemical reactions occurs when the reactants ‘‘ less chemical energy than the product where as release of energy in chemical reactions occurs when the reactants have higher chemical energy than the products.

Question 12.
Where the absorbed energy goes?

Answer:
The energy absorbed by the molecules makes electrons to reach excited s e and Increases kinetic energy of the molecule.

Question 13.
Is there any relation to energy and bond formation between atoms?

Answer:
The interacting energy is the potential energy between the atoms. It is negative if the atoms are bound and positive if they can move away from each other. The interaction energy is the integral of the force over the separation distance so these two quantities are directly related. The interaction energy is turning at the equilibrium position. This value of the energy Is called the bond energy and is the energy needed to separate completely to infinity (the work that needs to be done to overcome the attractive force.)

Question 14.
What could be the reason for the change in reactivity of elements?

Answer:
Number of valence electrons In the atoms of the element.

Question 15.
What could be the reason for the less reactivity of noble gases?

Answer:
All the noble gases have eight electrons in the outermost shell, except Helium (He). Thus they have no valency electrons and are less reactive or not at all reactive.

Page 155

Question 16.
What have you observed from tSe above conclusions about the main groups?

Answer:
Main group elements lose or gain electrons to get noble gas electronic configuration.

Question 17.
Why do atoms of elements try to combine and form molecules?

Answer:
To get stable electronic configuration In their valence shell.

Page 156

Question 18.
Is it accidental that IA to VIlA main group elements durIng chemical reactions get eight electrons In the outermost shells of their ions, similar to noble gas atoms?

Answer:
No. It cannot be simply accidental, Eight electrons in the outermost shell definitely gives stability to the ion or atom. Based on the above observations a statement known as The Octet Rule” Is framed.

Page 157

Question 19.
Explain the formation of ionic compounds NaCl, MgCl2, Na2O and AlCl3 through Lewis electron-dot symbols (formulae).

Answer:
(1) Lewis electron-dot symbol for NacI :

Formation of sodium chloride (NaCl) :
Sodium chloride Is formed from the elements sodium and chlorine. It can be explained as follows.
Na(s)+ 1/2 Cl2(g)→ Nacl(s)

Cation formation: When Sodium (Na) atom loses one electron to get octet electron configuration it forms a cation (Na+) and gets electron configuration that of Neon (Ne) atom.

Anion formation: Chlorine has shortage of one electron to get octet in its valence shell. So It gains the electron from Na atom to form anion and gets electron configuration as that of argon (Ar).

Formation of compound NaCl from its ions: Transfer of electrons between Na and cl atoms, results in the formation of Na+, and Cl- ions. These oppositely charged ions get attracted towards each other due to electrostatic forces and form the compound sodium chloride (NaCl).
Na+(g)+ Cl-(g)→ Na+(g)Cl-(a)or NaCl

2. Lewis electron-dot symbol for MgCl2:

Formation of magnesium chloride (MgCl2) :
Magnesium chloride is formed from the elements magnesium and chlorine. The bond formation MgCl2 in brief using chemical equation is as follows :
Mg(a)+Cl2(g)→ MgCl2(a)
Cation Formation :

Anion formation :

The compound MgCl2 formation from its ions :
Mg2+ gets Ne configuration and Each Cl- gets Ar configuration.
Mg2+(g)+ 2Cl-(g)→ MgCl2(a)
One ‘Mg’ atom transfers two electrons one each to two ‘Cl’ atoms and so formed Mg2+and 2Cl- attract to form MgCl2.

3. Lewis electron-dot symbol for (Na2O) :

Formation of di-sodium monoxide (Na2O) :
Di-sodium monoxide formation can be explained as follows :
Cation formation (Na+ formation) :

Anion formation (O2-, the oxide formation) Electronic configuration

The compound Na20 formation from Its ions is as shown.
2Na+(g) + O2-(g) → Na2O(g)
Two ‘Na; atoms transfer one electrons each to one oxygen atom to form 1Na+ and 02-.
Each Na+ gets ‘Ne’ configuration and 02- gets ‘Ne’ configuration.
These ions (2Na+ and 02-) attract to form Na2O.

4. Lewis electron-dot symbol for (AlCl3) :

Question 20.
How do cations and anions of an ionic compound exIst in its solid state?

Answer:
Cations and anions are surrounded themselves In three-dimensional lattices to give properly shaped crystals.

Question 21.
Do you think that pairs of Na+Cl-as units would be present in the solid crystal?

Answer:
No, electrostatic forces are non-directional. Therefore, it is not possible for one Na+to be attracted by one Cl-and vice-versa. Depending upon the size and charge of a particular ion, number of oppositely charged Ions get attracted by it, but, in a definite number. In sodium chloride crystal each Na is surrounded by 6 Cl-and each Cl-by six Na ions. Ionic compounds in the crystalline state consist of orderly arranged cations and anions held together by electrostatic forces of attractions in three dimensions

Page 161

Question 22.
Can you explain the reasons for all these?

Answer:
An ionic bond is formed between atoms of elements with electronegativity difference equal to or greater than 1.9.

Page 162

Question 23.
Can you say what type of bond exists between atoms of nitrogen molecules?

Answer:
Triple Bond

Page 164

Question 24.
What do you understand from bond lengths and bond energies?

Answer:
Bonds formed between two atoms in different molecules have different bond lengths and bond energies

Page 165

Question 25.
Are the values not different for the bonds between different types of atoms?

Answer:
Different for different molecules.

Page 167

Question 26.
What is the bond angle in a molecule?

Answer:
It is the angle subtended by two imaginary lines that pass from the nuclei of two atoms which form the covalent bonds with the central atom through the nucleus of the central atom at the central atom.

Page 169

Question 27.
How is HCl molecule formed?

Answer:
The ‘is’ orbital of ‘H’ atom containing unpaired electron overlaps with ‘3p’ orbital of chlorine atom containing unpaired electron of opposite spin.

TS 10th Class Physical Science Chemical Bonding Activities

Activity 1

Question 1.
Write the Lewis structure of the given elements ¡n the table. Also, consult the periodic table and fill in the group number of the element.

Answer:

Question 2.
Look at the periodic table. Do you see any relation between the number of valence electrons and group numbers’?

Answer:
For groups 2-6 the number of valence electrons is its group number. Group 1 has one outer electron, group 2 has two, where groups 13-17 number of valence electrons is (Group number-10). 3, 2, 1 respectively. (ie, 8-5=3;8-6=2;8-7= 1)

Question 3.
What did you notice in Lewis dot structure of noble gases and electronic configurations of the atoms of these elements shown in table – 1.

Answer:
It was found that the elements get octet or ns2 np6 configuration except helium, duplet.

Important Question

TS 10th Class Physical Science Important Questions Chapter 8 Chemical Bonding

Question 1.
How is covalent bond formed?

Answer:
A covalent bond is formed by the sharthg of electrons between two atoms.
Eg: Bonding In H2 molecule, O2 molecule, N2 molecule etc.

Question 2.
What is ‘Octet rule’?

Answer:
Octet rule: The presence of 8 electrons in the outermost shell of an atom or a molecule is called ‘octet rule.

Question 3.
What is’Bondlength’?

Answer:
Bond length: ft is the inner-nucleus distance between the two atoms in a molecule. It is measured In Angstrom, 1 Å = 10-8 cm.

Question 4.
What is a chemical bond?

Answer:
An attractive force between two atoms In a molecule Is called a chemical bond.

Question 5.
How is a cation formed?

Answer:
A cation Is formed when an atom loses electrons.

Question 6.
How is an anion formed?

Answer:
An anion Is formed when an atom gains electrons.

Question 7.
What are the forces present in an Ionic bond?

Answer:
Electrostatic forces of attraction are present in ionic bonds.

Question 8.
Which type of compounds are more soluble In polar solvents?

Answer:
Ionic compounds are more soluble In polar solvents.

Question 9.
Which compounds exhibit high melting and boiling points?

Answer:
Ionic compounds exhibit high melting and boiling points.

Question 10.
What is electronic configuration’?

Answer:
A systematic arrangement of electrons In the atomic orbits Is called electronic configuration.

Question 11.
Why are molecules more stable than atoms?

Answer:
Molecules have lower energy than that of the combined atoms. Molecules are more stable than atoms since chemical species with lower energy are more stable.

Question 12.
Which forces are weaker forces and where are they operative?

Answer:
Vander Waal’s forces are very weak forces.
They are operative between non-polar molecules.

Question 13.
Which compounds exhibit low melting and boiling points?

Answer:
Covalent compounds exhibit low melting and boiling points when compared to ionic compounds.

Question 14.
What is ionic linkage?

Answer:
When two oppositely charged Ions are engaged in a bond, it is known as ionic linkage.

Question 15.
On which factors do anions depend?

Answer:
Atomic size
Ionization potential
Electron affinity
Electronegativity

Question 16.
What is VSEPRT’?

Answer:
VSEPRT means ‘valence - shell - electron - pair repulsion - theory’.

Question 17.
How do you know the valence of a metal?

Answer:
The number of electrons lost from a metal atom is the valence of its elements which is equal to its group number. Eq: Na and Mg have valences 1 and 2 respectively.

Question 18.
How do you know the valence of a non-metal?

Answer:
The number of electrons gained by a non-metal element for its atom to attain octet configuration is its valency, which is equal to 8 - its group number. Eg: The valency of chlorine is (8 - 7) = 1.

Question 19.
Why do atoms combine and form molecules?

Answer:
The energy of molecule is less than the total energy of constituent atoms. Therefore atoms combine and go to a stable state of lower energy.

Question 20.
What is ‘orbital concept of bond formation’?

Answer:
Atoms with hair-filled or vacant orbitals try to get paired electrons in those orbitals by bond formation i.e., by losing, gaining or sharing of electrons.

Question 21.
Name the bonds present In the molecules i) BaCl2 ii) C2H4.

Answer:
(i) In BaCl2 - Ionic.
(ii) In C2H4 - Covalent (double bond H2C = CH2)

Question 22.
What s the use of VSEPR Theory?

Answer:
It predicts shapes of molecule without consideration of hybridization.

Question 23.
Why a molecule of Hydrogen is more stable than the uncombined atoms?

Answer:
When a molecule of hydrogen is formed from the atoms, energy is released (104 Kcal/ mol) Thus the molecule possessing lower energy is more stable that the atoms.

Question 24.
Why ionic compounds are good electrolytes?

Answer:
Electrolytes produce ions in solution, which carry current.
Ionic compounds is the fused state and aqueous solutions contain ions moving freely. Hence they conduct electricity.

Question 25.
When is ionic bond termed between atoms?

Answer:
Ionic bond is readily formed between atoms of elements with a low ionisation energy and atoms of elements with high electronegativity.

Question 26.
What is crystal lattice’?

Answer:
In a crystal of an ionic compound each Ion Is surrounded by oppositely charged ions Th ions arrange themselves at an optimum distance with regular periodicity in a well-defined three-dimensional network called crystal lattice.

Question 27.
What is lattice energy?

Answer:
the energy released when gaseous positive and negative ions are brought together from infinity to form one mole ionic crystal Is called lattice energy.

Question 28.
Represent the formation of nitrogen molecules by Lewis notation. (AS1)

Answer:

Question 29.
Represent Fluorine a molecule by using lewies notation.

Answer:

Question 30.
How many sigma and pl bonds are present in acetylene molecule?

Answer:
HC = CH, one sigma and two pi bonds are present.

Question 31.
How is a σ (sigma) bond formed?

Answer:
By the axial or tendon overlap of pure orbitals or hybrid orbitals of two atoms.

Question 32.
How is a π (pi) bond formed?

Answer:
By the lateral or parallel overlap of only Pure atomic orbitals after the σ bond formation.

Question 33.
Which type of atoms easily enter into ionic bonding?

Answer:
An atom with low ionization potential and another with high electron affinity.

Question 34.
What is a polar bond?

Answer:
A covalent bond in a heteroatomic molecule. Eg : HCl.

Question 35.
What is meant by inter-nuclear axis?

Answer:
The hypothetical line joining the center of nuclei of two atoms in a molecule.

Question 36.
What are multiple bonds?

Answer:
Double and triple bonds are multiple bonds.

Question 37.
How many σ and π bonds are in O2 molecule?

Answer:
One σ and two π bonds.

Question 38.
What are Lewis structures?

Answer:
The symbol of the atom gives the core (or kernel) of the atom is, which valence electrons are shown as dots (.), cIrcles (O) or crosses (x). Thus in Nax, Na represents the core of sodium atom and the cross (x) represents the valence electron of sodium.

Question 39.
In case of Ionic substances, a more appropriate term Is formula weight, rather than ‘molecular weight’. Why?

Answer:
Molecules are not present in ionic substances. Only ions are present. The formula of an ionic substance represents the simpler ratio of ions in one mole of crystal Hence a more appropriate term Is ‘formula weight’.

Question 40.
NaCl dissolves in water but not In benzene. Explain.

Answer:
NaCl dissolves in water because of hydration. Water being a polar molecule has positive and negative ends which hydrate Na+ and Cl- ions. Benzene being nonpolar cannot dissolve the ions of NaCl.

Question 41.
What are the bond angles in H2O and NH3 molecule?

Answer:
Bond angle In H2O molecule is 104° 3′.
Bond angle In NH3 molecule is 107°.

Question 42.
Why noble gases (inert gases) are stable?

Answer:
Noble gases are stable because their outermost orbit contains 8 or 2 electrons.
Noble gases exist as individual atoms.

Question 43.
What is an ion?

Answer:
An ion is an electrically charged atom (or group of atoms).

Question 44.
Sulphur dioxide is a diamagnetic molecule. Explain.

Answer:
Sulphur dioxide (SO2) is a diamagnetic molecule because it has all electrons paired (no free electrons).

Question 45.
What are the structures of sodium chloride and calcium chloride crystals?

Answer:
Sodium chloride has face-centered cubic structure. Calcium chloride has body-centered cubic structure.

Question 46.
Write a short note on bond angles.

Answer:
It is the average angle between two adjacent atoms bonded to the central atom in a molecule. Molecules with larger bond angles are more stable than those with smaller angles.

Question 47.
What Is ionisatIon? Give one example.

Answer:
The process of removal of electron (s) from an atom or molecule is termed
Ionisation. g : Na → Na+ + e-
Dissociation of an ionic solid into constituent ions upon Its dissolution in a suitable solvent is also called ionisation. Eg : NaCl?(aq) → Na+(aq) +Cl-(aq)

Question 48.
What is a Coordination number? (ASI)

Answer:
The number of ions of opposite charge that surround a given ion of given charge is known as coordination number of that given Ion.

Question 49.
Show the formation of HCl molecule with Lewis dot structures using the information given below.

Answer:

Question 50.
Bond angle of Water reduced to 104°27’ from 109.28 said, Rajesh. Is he corred? Justify your answer.
(or) Explain, why bonding angle (HOH) In water Is 104°31’ instead of 109° 281?

Answer:
Yes. He Is correct.
justification:
In water molecule oxygen atom has two lone pairs of electrons (: 0 ?? and two bond pairs (O- H) of electrons.
Due to this two lone pairs of electrons shows repulsion on bond pairs.
So, the bond angle reduced to 104°.27 from 109°,28


2 Marks Questions

Question 1.
Explain Ionic bond with suitable example.

Answer:
Sodium (Na) looses one electron and forms Sodium ion (Na+).
Na → Na++1e-
Chlorine (Cl) gains one electron and forms chloride ion (Ci).
Cl+le- → Cl-
Positive sodium on (Na) and negative chloride (Ci) ion come together due to electrostatic forces, participate in ionic bond and form NaCl.
Na+ + Cl- → NaCl

Question 2.
Distinguish between a sigma bond and a pi bond.

Answer:

Sigma bondPi bond
1. It is formed by the end-on-end overlap of orbitals. 1. It is formed by the lateral overlap of orbitals.
2. It has Independent existence.2. It has no independent existence.
3. It Is a strong bond. Because axial overlap is more. 3. It is a weak bond. because lateral overlap is less.
4. There can be only one sigma bond. 4. There can be one or two it bonds between two atoms.
Question 3.
Write about ‘Hydrogen bond’.

Answer:
1. Hydrogen bond Is formed between molecules in which hydrogen atom s attached to an atom of an element with large electronegativity and very small size (F. O. N). Because of hydrogen bond the molecules associate themselves and hence possess higher B.P’s and M.P’s. ,
2. The hydrogen bond formed between two molecules is called inter-molecular hydrogen bond
3. The hydrogen bond formed between different groups of the same Molecule is called intra-molecular hydrogen bond. "

Question 4.
Bring out the difference between Ionic and covalent bonds.

Answer:

Ionic bondCovalent bond
1. It is formed by transference of electrons from one atom to the other. 1. It is formed by the sharing of electron pairs by two atoms.
2. Electrostatic.2. Not electrostatic, but rigid.
3. Ionic substances are formed by ionic bonds. 3. Molecules are formed by covalent bond.
4. on - directional.4. Directional.
Question 5.
Write about VSEPR theory.

Answer:
It predicts the possible shapes of the molecules based on the number of electron pairs In the valence shell of the central atom.

Electron pairs Shape of molecules
TwoLinear
Three Equilateral triangle
Four Tetrahedral
Five Trigonal bipyramidal
SixOctahedral
Question 6.
Write a short note on octet rule with example.

Answer:
Atoms try to get 8 electrons in their outermost shell (inert gas structure) by combining with other atoms. In order to get 8 outer electrons, atoms may lose to, gain from or share electrons with other atoms.


Question 7.
Why a large amount of energy is needed to remove an electron from a neutral gaseous neon atom than the energy needed to remove an electron from gaseous sodium atom?

Answer:
Na(g) + 118.4 K.cal → Na’ + e
Ne(g) + 497.0 K.cal → Ne + e
Orbitals which are fully filled are very stable, so large amount of energy is needed to remove an electron from them.

Question 8.
Why there is absorption of energy in certain chemical reactions and release of energy In other reactions?

Answer:
If bond dissociation energy of reactants is more than bond energy of products then energy ¡s absorbed n the chemical reaction.
If bond dissociation energy of reactants is less than bond energy of products then energy s released’ in the chemical reaction.

Question 9.
Electronic configuration of x is 2, 8, 1 and electronic configuration of Y is 2, 8, 7 ExplaIn what type of bond Is formed between them.

Answer:
The element with electronic configurations 2, 8, 1 is sodium which Is a metal. Similarly, the element with electronic configurations 2, 8, and 7 Is chlorine which is a non-metal. The electronegativity difference between these two elements Is more than 1.9 so they form ionic bond.

Question 10.
Why ionic compounds dissolve in polar solution and covalent compounds dissolve in non-polar solution?

Answer:
Ionic compounds are polar in nature so they are soluble in polar solvents whereas covalent compounds are non-polar in nature so they are soluble in non-polar solvents.

Question 11.
What are favourable conditions for the formation of anion?

Answer:
The favourable conditions for the formation of anion is
High Electron Affinity.
Low charge on the anion.
Small size of the non-metal.
Configuration is very near to inert gas configuration.

Question 12.
Bond angle of ammonia reduced to 107°48 from 109°28" said Ramya. Is she correct? Justify your answer.

Answer:
Yes. She Is correct.
Justification:
In NH3 there are three bond pairs (N-H) and one lone pair of electrons (N) Is present around the central atom of nitrogen.
So, the lone pair electron shows repulsion on bond pairs.
Hence, to minimise the repulsion force bond angle changes from 109°28 to 107°48.
At the same time the shape also changes from tetrahedral to pyramidal.


Question 13.
Write the ‘Octet Rule’. How does Mg (12) get stability while reacting with chlorine as per this rule?

Answer:
Octet rule: The atoms of elements tend to undergo chemical changes that help to leave their atoms with eight outer-shell electrons.
Electronic configuration of ‘Mg’ is - 2, 8, 2
So, it loses two electrons in its outer shell to get octet configuration and becomes Mg+2.
So, it gives 2 electrons to two chlorine atoms to form Magnesium chloride (Mgcl2) and gets stability.

Question 14.
Draw the diagram to show the formation of Oxygen molecules by Valence bond theory.

Answer:

4 Marks Questions

Question 1.
If the electronic configurations of atoms A and B are 3S2, 2S2, 2P6,3S2, 3P1’ and 2, 61S2, 2P4 respectively, then
a) Which atom forms negative ion
b) Which atom forms positive ion?
c) What Is the valency of atom A?
d) What Is the molecular formula of the compound form by atoms A and B?

Answer:
Given electronic configuration of atom A is Is’ 2s2 2p6 3s2 3p1 i.e., Aluminium and B is 1S2, 2S2, 2p6 i.e., Carbon.
a) The atom ‘B’ tends to form negative ion by gaining two electrons order to get nearest inert gas neon configuration is 1s22s22p6.
b) The atom ‘A’ tends to form positive ion by losing three electrons n order to get nearest inert gas. Neon configuration is 1s22s22p6.
c) Valency of atom ‘A’ is ‘3’.
d) According to Criss-Corss method, the molecular formula of the compound formed by atoms both A and B Is A3B3 i.e., AlO3.

Question 2.
a) Draw the electron dot structures of Sodium, Oxygen and Magnesium.
b) Show in the form of a picture, the formation of Na20 and MgO.(AS5)

Answer:
a) Sodium atom has one valency electron in its valency shell, so its electron dot structure Is Nax(or)Na Oxygen atom has two valency electrons in its valency shell, so Its electron dot structure is O (or) O:
Magnesium atom also having two valency electrons in its valency shell, so its electron dot structure is Mg: (or) Mg :


Question 3.
Explain VSEPRT theory.

Answer:
VSEPRT theory:
The full form of VSEPRT is Valence Shell Electron Pair Repulsion Theory.
VSEPRT proposes that molecules get specific shapes due to repulsion between bond pair and lone pair.
Lone pair occupies more space around the central atom than bond pair.
Thus the presence of lone pairs on the central atom causes slight distortion of bond angles from the expected regular shape.
If two bond pairs without lone pair then shape of the molecule is linear with bond angle 180°.
It three bond pairs without lone pair then shape is trigonal planar with bond angle 120°.
If there are four bond pairs without lone pair then shape is tetrahedron with bond angle 109°28.
If there are three bond pairs and one lone pair then due to repulsions between bond pair and lone pair the shape is pyramidal.
If there are two bond pairs and two lone pairs then due to lone pair and bond pair repulsions the shape of the molecule is V.

Question 4.
Explain the formation of BF3 molecule with the help of Valency Bond theory.

Answer:
Formation of BF3 molecule:
3B has electronic configuration 1s22s22px1.
The excited electronic configuration of 5B is 1s2 2s1 2px1 2p1y.
As It forms three identical B-F bonds In BF3.
It Is suggested that excited ‘B atom undergoes hybridisation.
There is an intermixing of 2s, 2px, 2py orbitals and their redistribution into three Identical orbitals called sp2 hybrid orbitals.
For three sp2 orbitals to get separated to have minimum repulsion the angle between any two orbitals is 120° at the central atom and each sp2 hybrid orbitals.


Now three fluorine atoms overlap their 2p2 orbitals containing unpaired electrons (F9, 1s2 2s2 2px2 2px2 2py2,2pz1) the three sp2 orbitals of ‘B that contain unpaired electrons to form three σsp2-p bonds.

Question 5.
What Is hybridisatlon?

Answer:
In the formation of molecules, the atomic orbitals of the atoms may hybridise.
It is the process of mixing up of atomic orbitals of an atom to form identical hybrid orbitals.
This takes place only during the formation of bond.
There should not be much difference in the energies of the orbitals that hybridise.
The number of hybrid orbitals formed is equal to the number of hybridising atomic orbitals.
Hybrid orbitals form sigma bonds only not t bonds.

Question 6.
What are the important characteristic features of hybridisation?

Answer:
Orbitals on a single atom only would undergo hybridization.
The orbitals involved In hybridisation should not differ largely In their energies.
The number of hybrid orbitals formed is equal to the number of hybridising orbitals.
The hybrid orbitals form stronger directional bonds than the pure s, p, d atomic orbitals.
It is the orbitals that undergo hybridisation and not the electrons.
Concept of hybridisation is useful in explaining the shape of molecules.

Question 7.
Can you suggest an experiment to prove that ionic compounds possess strong bonds when compared to that of covalent bonds? Explain the procedure.

Answer:
Take a small amount of sodium chloride (NaO) on a metal spatula (having an insulated handle).
Heat it directly over the flame of a burner.
We will see that sodium chloride (NaCl) does not melt easily.
Sodium chloride melts (and becomes a liquid) only on strong heating.
This shows that sodium chloride which is an Ionic compound possesses strong bonds.
So it has a high melting point.
Whereas covalent compounds like naphthalene and carbon tetra chloride has low boiling points such as 80° C and 77° C respectively.
The force of attraction between the molecules of a covalent compound is very weak.
Only a small amount of heat energy Is required to break these weak molecular forces, due to which covalent compounds have low melting points and low boiling points.

Question 8.
A list of five substances is given.
a) NaCl
B) CH4
C) CCl4
d) N2
e) Ar

Which of these substances correspond to the options (i) to (x)
i) A substance with high melting point
ii) A substance with low heat of vapourisation
iii) Diatomic
iv) An insulator
v) Soluble In water
vi) A liquid which is immutable in water
vii) Monoatomic
viii) A gas which Is insoluble In water
ix) An Ionic compound
x) Covalent compound

Answer:
(i) NaCl
(ii) CCl4
(iii) N2
(iv) CCl4
(v) NaCl
(vi) CCl4
(vii) Ar
(viii) N2 and CH4
(ix) Nacl
(x) CH4, CCl4, N2.

Question 9.
Explain the formation of Ammonia molecules by using Lewis notation.

Answer:
Ammonia (NH3) molecule: In ammonia molecule, three N - H single covalent bonds are present -Electron configuration of 7N is 2, 5 and 1H is 1. Nitrogen (N) needs 3 electrons to have octet In Its valence shell. Therefore it shows ‘3’ electrons with ‘3’ hydrogen(H) atoms to form ammonia (NH3) molecule.


Question 10.
Explain the formation of Methane molecules by using Lewis notation.

Answer:
Methane (CH4) molecule: In the formation of methane, (CH4) molecule, carbon contributes 4 electrons (one electron to each hydrogen atom) and 4 hydrogen atoms contribute one electron each. Thus in CH4 molecule, there are four CH4 covalent bonds as shown below:


Question 11.
Write the difference between ionic bond and covalent bond?

Answer:

Ionic bond Covalent bond
1. Ionic bond is formed by the transfer of one or more electrons. 1. Covalent bond is formed sharing of electrons.
2. It is formed between metal and non-metal. 2. It is formed between non-metals.
3. It is also called electrovalent bond and Is due to electro valency. 3. It Is called electron pair bond and is due to covalency.
4. Ionic bond consists of electrostatic force of attraction between the oppositely charged eons. 4. Covalent bond consists of shared pair or pairs of electrons which an attracted by both the nuclei.
5. Ionic bond is non rigid and non-directional.5. Covalent bond is rigid and directional.
6. lonic bond is polar in nature.6. Covalent bond may be polar or non-polar.
Question 12.
An element X has 3 electrons In the outermost shell of the atom. (AS1)
a) State whether ‘X’ Is a metal or a non-metal?
b) Write the symbol of Its ion?
c) Write the formula of its oxide?

Answer:
a) An element which has 3 electrons In the outermost shell is Aluminium. It is confirmed by writing electronic configuration 1S2 2S22P63S13p1.
b) The symbol is Al+3. It is a metal
c) Its oxide Is Al2O3
4Al + 3O2 → 2Al2O3

Question 13.
Give electron dot formula for the following.
a) Magnesium chloride
b) Carbon-di-oxide
c) Carbon tetrachloride
d) Hydrogen bromide

Answer:

Do You Known
Davy’s experiment
Humphry Davy (1778-1829), a Professor of Chemistry at the Royal Institute in London, constructed a battery of over 250 metallic plates. In 1807, using electricity from this battery, he was able to extract highly reactive metals like potassium and sodium by electrolysis of fused salts (Page 151)

It was seen that the metal part of the compound migrated towards the negative electrode and the
non-metal part towards the positive electrode. So it was proposed that metals are responsible for
positively charged particles and non-metals are responsible for negatively charged particles. The oppositely charged particles are held together by electrostatic forces in a compound. Do you agree with this explanation? Why? While this explanation could explain bonding in Nacl, KCl etc. It could not explain bonding in carbon compounds or diatomic molecules of elements.

2. The number of electrons lost from a metal atom is the valence of its element which is equal to its group number (Page 157) Eg: Na and Mg have valencies 1 and 2 respectively
3. The number of electrons gained by a non-metal element for its atom is it valency, which is equal to (8 - its group number) (Page 157) Eg: The valency of chlorine is (8-7) - 1
4. An angstrom (Å) is a unit of length equal to 10-10 m, or 0.1 nanometre, or 101 picometres. 1 nanometre = 10-9 metre; 1 picometre = 10-12 metre. (Page 164)

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