Question

four out of the five electron configurations below represent ground states for the given species. which one of following represents an excited state (i.e. not a ground state)?
as: ( 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{3} )
sc: ( 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}4p^{1} )
al: ( 1s^{2}2s^{2}2p^{6}3s^{2}3p^{1} )
se²⁻: ( 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6} )
cu: ( 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^{10} )

Explanation:

• For \(As\) (atomic number \(33\)), the electron configuration \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{3}\) is correct. The sum of electrons \(2 + 2+6 + 2+6 + 2+10 + 3=33\), and it follows the Aufbau principle (\(4s\) fills before \(3d\), and then \(4p\)).
• For \(Sc\) (atomic number \(21\)), the ground - state electron configuration should be \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{1}4s^{2}\). In the given configuration \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}4p^{1}\), an electron is excited from the \(3d\) orbital (lower - energy) to the \(4p\) orbital (higher - energy).
• For \(Al\) (atomic number \(13\)), the electron configuration \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{1}\) is correct. The sum of electrons \(2+2 + 6+2+1=13\), and it follows the Aufbau principle.
• For \(Se^{2 -}\) (atomic number of \(Se\) is \(34\), and with two extra electrons, total electrons \(36\)), the configuration \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}\) is correct. The sum of electrons \(2+2 + 6+2+6+2+10 + 6=36\), and it follows the Aufbau principle.
• For \(Cu\) (atomic number \(29\)), the electron configuration \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^{10}\) is correct. Due to the stability of the fully - filled \(3d\) sub - shell, one electron from the \(4s\) sub - shell moves to the \(3d\) sub - shell.

Answer:

Sc: \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}4p^{1}\)