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: Orbital diagrams
Si: $[Ne]3s^2 3p^2$. With two electrons in three $3p$ orbitals, Hund's rule puts one in each of two orbitals, so there are 2 unpaired spins.
Cr: $Z=24$. Half-filled $d^5$ plus half-filled $s^1$ is the lowest energy arrangement, so it is $[Ar]3d^5 4s^1$.

Step 2: Count spins
In Cr all six electrons outside argon ($5$ in $3d$, $1$ in $4s$) have unpaired spins. So the count is 6.
The spin-only moment would be $\sqrt{6\times 8} \approx 6.9\ \text{BM}$.

Step 3: Answer
The pair is 2 for Si and 6 for Cr, which matches option (B).

Final Answer:
Si has two unpaired and Cr has six unpaired electrons. \[ \boxed{\text{(B)}\ 2\ \text{and}\ 6} \]