Electronic configuration

Calcium (Ca) has an atomic number ($Z$) of 20. This means a neutral calcium atom has 20 electrons.

• The $1s$ orbital holds 2 electrons: $1s^2$ (Remaining: $20 - 2 = 18$)
• The $2s$ orbital holds 2 electrons: $2s^2$ (Remaining: $18 - 2 = 16$)
• The $2p$ orbital holds 6 electrons: $2p^6$ (Remaining: $16 - 6 = 10$)
• The $3s$ orbital holds 2 electrons: $3s^2$ (Remaining: $10 - 2 = 8$)
• The $3p$ orbital holds 6 electrons: $3p^6$ (Remaining: $8 - 6 = 2$)
• The $4s$ orbital holds 2 electrons (before $3d$): $4s^2$ (Remaining: $2 - 2 = 0$)

Combining the filled orbitals:

The noble gas preceding Calcium is Argon (Ar), which has an electronic configuration of $1s^2 2s^2 2p^6 3s^2 3p^6$. We can replace this core configuration with $[\text{Ar}]$.

The electronic configuration of Calcium is:
$\boxed{\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^6 \text{4s}^2}$ or $\boxed{[\text{Ar}] \text{4s}^2}$