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

##### 1 Answer
Apr 19, 2016

The preceding Noble Gas.

#### Explanation:

Calcium, $Z = 20$, has 20 electrons as the neutral atom. $C {a}^{2 +}$ necessarily has 18 electrons. The preceding Noble $Z = 20 - 2 = 18$ is argon, $A r$.