Phosphorous has five electrons in its valence shell. Two are in the 3s orbital. The remaining three are distributed evenly among the three orthogonal 3p orbitals.
Phosphorous has a total of fifteen electrons. Two are located in the 1s orbital and two are located in the 2s orbital. The second energy level has three orthogonal 2p orbitals, holding a total of six electrons. This adds up to ten electrons which provide a neon core. The remaining five electrons are the valence electrons.
According to Hund's rule, the 3p orbitals are degenerate, or have the same energy. Therefore, each orbital holds one electron. Because of these three single electrons, phosphorous usually forms three bonds.