Question

What is the number of unpaired electrons present in ground state of Silicon and Chromium, respectively ?

• A. 2 and 4
• B. 2 and 6
• C. 0 and 6
• D. 4 and 5

Hint:

Write the ground state configurations; Cr is an exception with a half-filled 3d and 4s.

Answer 1

The Correct Option is B

Step 1: Write the configuration of Silicon
Silicon has atomic number \(14\), so its configuration is \(1s^2 2s^2 2p^6 3s^2 3p^2\).
The two \(3p\) electrons occupy different orbitals with parallel spins (Hund's rule).
So silicon has \(2\) unpaired electrons.

Step 2: Write the configuration of Chromium
Chromium has atomic number \(24\). The expected \([Ar]3d^4 4s^2\) is not the real one.
A half-filled \(3d\) subshell is extra stable, so one \(4s\) electron moves into \(3d\).
The ground state is \([Ar]3d^5 4s^1\).

Step 3: Count the unpaired electrons in Chromium
The five \(3d\) orbitals each hold one electron and the \(4s\) orbital holds one electron.
That gives \(5+1 = 6\) unpaired electrons.

Step 4: Compare with the options
Silicon has 2 and chromium has 6, which is option (B). Option (A) uses \(4\) for chromium, which follows the wrong configuration \(3d^4 4s^2\). Option (C) gives silicon 0, which would need both \(3p\) electrons paired. Option (D) gives silicon 4, which ignores that \(3s\) is full.

Final Answer:
Silicon has 2 unpaired electrons and chromium has 6. \[ \boxed{\text{(B)}\ 2\ \text{and}\ 6} \]