To achieve a noble-gas configuration, a phosphorous atom will form a P3- anion rather than forming a P5+ cation because less energy is required to add three electrons than to remove five electrons.
The electron configuration of phosphorus, P, is [Ne]3s²3p³.
Each P atom has five valence electrons: two in the 3s orbital and three in the 3p orbital.
Elements can achieve a noble gas electron configuration three ways.
1. An element can give up electrons to achieve the electron configuration of the next lowest noble gas.
2. An element can attract additional electrons to achieve the electron configuration of the next highest noble gas.
3. Two (or more elements) can share electrons so that each attains the electron configuration of the nearest noble gas.
See MA.edu's Achieving a Noble Gas Electron Configuration to learn more.