Atomic Structure Question 3

Which of the following is the electron configuration of an excited atom that is likely to emit a quantum of energy?

1s^22s^22p^63s^23p^1

1s^22s^22p^63s^23p^5

1s^22s^22p^63s^1

1s^22s^22p^63s^13p^1

Approach

According to the aufbau principle, electrons fill subshell in order of lowest available energy. Electrons normally fill subshells in this order:

1s \rarr 2s \rarr 2p \rarr> 3s \rarr 3p

In the last choice, an electron is present in $3p$ despite $3s$ having a slot for an electron. This implies that the electron is in an excited state. Typically, this doesn't last a long time, and the electron will fall back down to the ground state, releasing energy in the process.