1s² 2s² 2p⁶ 3s² 3p³

Here's how to understand it:

* 1s²: The first energy level (n=1) has one subshell (s) with 2 electrons.

* 2s²: The second energy level (n=2) has one subshell (s) with 2 electrons.

* 2p⁶: The second energy level (n=2) has one subshell (p) with 6 electrons.

* 3s²: The third energy level (n=3) has one subshell (s) with 2 electrons.

* 3p³: The third energy level (n=3) has one subshell (p) with 3 electrons.

Explanation:

* Phosphorus has an atomic number of 15, meaning it has 15 protons and 15 electrons.

* We fill the orbitals in order of increasing energy levels, following the Aufbau principle.

* The s subshell can hold up to 2 electrons, and the p subshell can hold up to 6 electrons.

* The 3p subshell only has 3 electrons in phosphorus, leaving 3 empty spaces. This is why phosphorus is highly reactive and tends to gain 3 electrons to achieve a stable octet.