Here's why:

* Phosphorus's electron configuration: Phosphorus has an atomic number of 15, meaning it has 15 protons and 15 electrons. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p³.

* Valence shell: The valence shell is the outermost electron shell. In phosphorus's case, the valence shell is the third shell (n=3).

* Valence electrons: The electrons in the valence shell are called valence electrons. Phosphorus has 2 electrons in the 3s subshell and 3 electrons in the 3p subshell, totaling 5 valence electrons.