Electron pairing in atoms

Question

A question asks:

This question refers to isolated gaseous atoms. In which atom are all electrons paired? Ba, Br, S or Si?

I thought it was Ba as its electron configuration is [Xe] 6s2.

However, the answer says Si, which seems incorrect to me.

Could someone please confirm the answer for me?

Thanks

Answer 1

The configuration for the free atoms is:

• Ba - $\mathrm{[Xe]\ 6s^2}$
• Br - $\mathrm{[Ar]\ 3d^{10} 4s^2 4p^5}$
• S - $\mathrm{[Ne]\ 3s^2 3p^4}$
• Si - $\mathrm{[Ne]\ 3s^2 3p^2}$

Thus:

• Barium has no unpaired electrons.
• Bromine has one unpaired electron in 4p subshell.
• Sulfur has two unpaired electrons in the 3p subshell.
• Silicon has two unpaired electrons in the 3p subshell.