Question 1.
What information does the electronic configuration of an atom provide? (AS1)


  • The distribution of electrons in shells, sub-shells and orbital in an atom is known as electronic configuration.
  • It provides the information of position of an electron in the space of atom.
  • The distribution of electrons in various atomic orbitals provides an understanding of the electronic behaviour of the atom and in turn its reactivity.
  • The short hand notation is as shown below.
Question 2.
a) How many maximum number of electrons that can be accommodated in a principal energy shell?

The maximum number of electrons that can be accommodated in a principal energy shell is 2n2;. Here n is principal quantum number.

b) How many maximum number of electrons that can be accommodated in a sub-shell?

The maximum number of electrons that can be accommodated in a sub-shell is 2(2l +1) (where l is orbital quantum number).

c) How many maximum number of electrons can that be accommodated in an orbital?

The maximum number of electrons that can be accommodated in an orbital is 2.

d) How many sub-shells are present in a principal energy shell?

The number of sub-shells in a principal energy shell is n (n is principal quantum number).

e) How many spin orientations are possible for an electron in an orbital?

The spin orientations possible for an electron in an orbital are 2.

Question 3.
In an atom the number of electrons in M-shell is equal to the number of electrons in the K and L-shell. Answer the following questions. (AS1)
a) Which is the outermost shell?

The outermost shell is N shell.

b) How many electrons are there in its outermost shell?

Two electrons are there in outermost shell.

c) What is the atomic number of element?

The atomic number of element is 22.

d) Write the electronic configuration of the element.

The element is Ti (Titanium). Its electronic configuration is 1s2;2s2;2p63s2;3p64s2;3d2;.

Reason :

  • Electrons enter M shell after completion of K and L.
  • So the number of electrons in M shell is 10.
  • But after completion of 3p orbital electron enters 4s before entering to 3d.
  • So outermost orbit or shell is N shell.
  • So the atomic number of element is 22.
  • Its electron configuration is 1s2; 2s2; 2p63s2; 3p64s2; 3d2;.
Question 4.
Rainbow is an example for continuous spectrum - explain. (AS1)
Which is naturally occurring continuous spectrum ? Explain.


  • Rainbow is a spectrum of different colours (VIBGYOR) with different wavelengths.
  • These colours are continuously distributed.
  • There is no fixed boundary for each colour.
  • Hence, rainbow is a continuous spectrum.
Question 5.
How many elliptical orbits are added by Sommerfeld in third Bohr’s orbit ? What was the purpose of adding these elliptical orbits? (AS1)

Sommerfeld added two elliptical orbits to Bohr’s third orbit.

Purpose of adding elliptical orbits :

  • Bohr’s model failed to account for splitting of line spectra and line spectrum.
  • In an attempt to account for the structure of line spectrum, Sommerfeld modified Bohr’s atomic model by adding elliptical orbits.
Question 6.
What is absorption spectrum?

Absorption spectrum: The spectrum formed by the absorption of energy when electron jumps from lower energy level to higher energy level is called absorption spectrum. It contains dark lines on bright background.

Question 7.
What is an orbital? How it is different from Bohr’s orbit? (AS1)
Comparison between orbit and orbital.

The region of space around the nucleus where the probability of finding electron is maximum is called orbital. Whereas orbit is the path of the electron around the nucleus.

These two are differentiated like this.

Orbit Orbital
1. Path of electron around nucleus. 1) Probability of finding electron around nucleus.
2. Orbits are represented by letters K, L, M, N, 0, …….etc. 2. Orbitals are represented by letters s, p, d, f, g, …….etc.
3) Its information is given by principal 3) Its information is given by orbital quantum number.
4) It is two dimensional. 4) It is three dimensional.
5) It does not satisfy Heisenberg’s uncertainty principle. 5) It satisfies the Heisenberg’s principle of uncertainty.
Question 8.
Explain the significance of three quantum numbers in predicting the positions of an electron in an orbit. (AS1)
How are quantum numbers helpful to understand the atomic structure?

Significance of three quantum numbers in predicting the positions of an electron in an orbit.

1) Principal quantum number (n) :
The principal quantum number explains about the size and energy of shells (or) orbitals. It is denoted by n.

As ‘n’ increases, the orbitals become larger and the electrons in those orbitals are farther from the nucleus.

It takes values 1, 2, 3, 4, ……………. for that the shells are represented by letters K, L, M, N, ……….

The number of electrons in a shell is limited to 2n2;.

2) The Angular - momentum quantum number (l) :
The angular momentum quantum number defines the shape of the orbital occupied by the electron and the orbital angular momentum of the electron, is in motion.

l takes values from 0 to n - 1 for these values the orbitals are designated by letters s, p, d, f, ………….. etc.

l also governs the degree with which the electron is attached to nucleus. The larger the value of l, the smaller is the bond with which it is maintained with the nucleus.

3) Magnetic orbital quantum number (ml) :
The orientation of orbital with external magnetic field determines magnetic orbital quantum number.

mlhas integer values between - l and l including zero.

The number of values for m, are 2l + l, which give the number of orbitals per sub-shell. The maximum number of electrons in orbitals in the sub-shell is 2 (2l + l).

Question 9.
What is nlxmethod? How is it useful? (AS1)
What is nlxmethod? How is it useful in electronic configuration?

The shorthand notation consists of the principal energy level (n value) the letter representing sub - level (l value), and the number of electrons (x) in the sub-shell is written as superscript nlx.

It is useful in writing electron configuration of elements. For example, in Hydrogen (H), the set of quantum numbers is n = 1, l = 0, ml = 0, ms = ½ or - ½. The electronic configuration is

Question 10.
Following orbital diagram shows the electronic configuration of nitrogen atom. Which rule does not support this? (AS1)

Write the correct electronic configuration of the given nitrogen atom with the help of Hund’s rule.

  • This electron configuration does not support Hund’s rule.
  • According to Hund’s rule, the orbitals of equal energy are occupied with one elec-tron each before pairing of electrons starts.
  • Here, pairing of electrons in 2pxorbital was taken place without filling of an elec-tron in 2pzorbital.
  • Hence the correct electron configuration is as follows.
Question 11.
Which rule is violated in the electronic configuration 1s02s2; 2p4?


  • Aufbau principle is violated in this electronic configuration because according to Aufbau principle, electron enters orbital of lowest energy.
  • Among 1s, 2s and 2p, Is has least energy.
  • So Is orbital must be filled before the electron should enter 2s.
Question 12.
Write the four quantum numbers for the differentiating electron of sodium (Na) atom. (AS1)

The electronic configuration of sodium (Na) is 1s² 2s² 2p63s1;. So the differentiating electron enters 3s. Therefore the four quantum numbers are

Question 13.
What is emission spectrum?
When radiation is emitted what is the name given to such spectrum? Explain such spectrum.


  • The spectrum produced by the emitted radiation is known as emission spectrum.
  • This spectrum corresponds to liberation of energy when an excited electron returns back to ground state.

Emission spectrum is of two types :

1) Continuous spectrum :
When white light passes through a prism it dissociates into seven colours. This spectrum is called continuous spectrum.

2) Discontinuous spectrum :
Discontinuous spectrum is of two types.

a) Line spectrum :
The spectrum with sharp and distinct lines. It is given by gaseous atoms.

b) Band spectrum :
The spectrum very closely spaced lines is known as band spectrum. It is given by molecule.

Question 14.
i) An electron in an atom has the following set of four quantum numbers to which orbital it belong to : (AS2)


This electron belongs to 2s orbital.
Spin is in clockwise direction. ⇒ 2s1;

ii) Write the fojur quantum numbers for Is1 electron. (AS1)
The four quantum numbers for Is1 electron are

Question 15.
Which electronic shell is at a higher energy level K or L? (AS2)

L – shell is at higher energy level, because it is far from nucleus than K shell.

Question 16.
Collect the information regarding wavelengths and corresponding frequencies of three primary colours red, blue and green. (AS4)

The wavelengths and corresponding frequencies of three primary colours red, blue and green are given below.

Primary colours Wavelength in nm
(1 nm = 10-9m)
Frequency in Hz
Red 700 4.29 × 1014
Green 530 5.66 × 1014
Blue 470 6.38 × 1014
Question 17.
The wavelength of a radio wave is 1.0 m. Find its frequency. (AS7)



c = 3 × 108 m/s ; ? = 1m ; c = v? ? v = c?=3×108/1 = 3 × 108 Hz.:

Question 18.
Why are there exemptions in writing the electronic configurations of Chromium and Copper?

1. Elements which have half-filled or completely filled orbitals have greater stability.

2. So in chromium and copper the electrons in 4s and 3d redistributes their energies to attain stability by acquiring half-filled and completely filled d-orbitals.

3. Hence the actual electronic configuration of chromium and copper are as follows.

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