Question

which electron configuration is not a configuration of a stable main - group element? a 1s² b 1s²2s² c 1s²2s²2p⁶ d 1s²2s²2p⁶3s²3p⁶

Explanation:

Stable main - group elements usually have a full outer - shell electron configuration. Helium has a stable configuration of $1s^{2}$. Neon has $1s^{2}2s^{2}2p^{6}$, and argon has $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$. The configuration $1s^{2}2s^{2}$ is that of beryllium, which is not a stable noble - gas configuration. Its outer shell is not full.

Answer:

B. $1s^{2}2s^{2}$