1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Here's how it works:
* 1s²: The first energy level (n=1) has one subshell (s) which can hold 2 electrons.
* 2s² 2p⁶: The second energy level (n=2) has two subshells (s and p). The s subshell holds 2 electrons and the p subshell holds 6 electrons.
* 3s² 3p⁶: The third energy level (n=3) also has two subshells (s and p) with the same capacity as the second level.
* 4s²: The fourth energy level (n=4) starts with the s subshell, which holds 2 electrons.
Calcium has 20 electrons, and this configuration accounts for all of them.
