Question

Which atom in the ground state has the same electron configuration as a calcium ion, `Ca^(2+)` in the ground state?

Answer 1

The preceding Noble Gas.

Explanation:

Calcium, `Z=20`, has 20 electrons as the neutral atom. `Ca^(2+)` necessarily has 18 electrons. The preceding Noble `Z=20-2=18` is argon, `Ar`.