Which of the following is a reasonable ground-state electron configuration? a) 1s21p62d2b) 1s22s42p6c) 1s22s22p5d) 1s22s22d6

Explanation:

Ground-state is the configuration of the electrons when every electron of the atom occupies the lowest possible energy level.

The rank of energy of the orbitals may be remembered using Aufbau rules, and it is:

1s² < 2s² < 2p⁶ < 3s² < 3p⁶ < 4s² < 3d¹⁰ < 4p⁶ < 5s² < 4d¹⁰ < 5p⁶ < 6s² < 4f¹⁴ < 5d¹⁰ < 6p⁶ < 7s² < 5f¹⁴ < 6d¹⁰ < 7p⁶

Then, to find whether an electron configuration corresponds to a ground-state one, you must check that the previous order is preserved.

Option A). 1s² 1p⁶ 2d²

The orbital 1p does not exist. So this one is discarded.

Option B) 1s² 2s⁴ 2p⁶

The maximum number of electrons in an s orbital is 2, so 2s⁴ is impossible, and this option is discarded.

Option C) 1s² 2s² 2p⁵

This is correct because the orbitals are filled in the correct increasing energy order, without jumping any one.

Option D) 1s² 2s² 2d⁶

Orbital 2d does not exist, so this option is also discarded.