Question

Covalency of chlorine atom after the excitation of two electrons will be [NCERT Pg. 102]

• A. 2
• B. 5
• C. 3
• D. 7

Answer 1

Answer: (B)

5

Answer 2

The covalency of an atom is determined by the number of unpaired electrons available for bonding. Chlorine (Cl) is a third-period element, so it has vacant 3d orbitals available for excitation.

1. Ground State Electronic Configuration of Chlorine (Cl):
   The atomic number of Cl is 17.
   Electronic configuration: $1s^2 2s^2 2p^6 3s^2 3p^5$
   Valence shell configuration: $3s^2 3p^5$

Orbital diagram for the valence shell in the ground state:
$3s$: $\uparrow\downarrow$
$3p$: $\uparrow\downarrow \uparrow\downarrow \uparrow$
$3d$: $\_ \_ \_ \_ \_$ (empty)

In the ground state, Cl has 1 unpaired electron (in the 3p orbital). Thus, its covalency in the ground state is 1 (e.g., in HCl, Cl$_2$).

2. First Excited State (Excitation of one electron):
   An electron from a paired 3p orbital is promoted to an empty 3d orbital.
   $3s$: $\uparrow\downarrow$
   $3p$: $\uparrow\downarrow \uparrow \uparrow$
   $3d$: $\uparrow \_ \_ \_ \_$

Now, there are 3 unpaired electrons (one from 3s, two from 3p, and one from 3d).

3. Second Excited State (Excitation of two electrons):
   The question asks for the covalency after the excitation of two electrons. This means a second electron is excited from the first excited state configuration.
   From the first excited state ($3s^2 3p^4 3d^1$), the next available paired electron is in the remaining paired 3p orbital. This electron is promoted to another empty 3d orbital.

Configuration after excitation of two electrons:
$3s$: $\uparrow\downarrow$
$3p$: $\uparrow \uparrow \uparrow$ (all three 3p orbitals now have unpaired electrons)
$3d$: $\uparrow \uparrow \_ \_ \_$ (two 3d orbitals now have unpaired electrons)

Counting the unpaired electrons:
From 3s: 0 (still paired)
From 3p: 3 unpaired electrons
From 3d: 2 unpaired electrons
Total unpaired electrons = 3 + 2 = 5.

Therefore, after the excitation of two electrons, the chlorine atom has 5 unpaired electrons, and its covalency will be 5 (e.g., in ClF$_5$).