Phosphorous has five valence electrons. This means there are five electrons in the element's outermost shell. The number of valence electrons is determined by looking at the atomic number of the element on the periodic table.
The atomic number of Phosphorous is 15. This means the element has 15 protons and 15 electrons in its neutral state. The electrons orbit the nucleus in shells. Each shell can contain a set maximum number of electrons. The innermost shell of an atom can contain a maximum of two electrons. The second shell can hold up to eight electrons. The total number of electrons in the first and second shells adds up to 10 electrons. When this number is subtracted from the original 15 electrons, five electrons remain. This gives Phosphorous a valence number of five.
Unless the element in question is Helium or a transitional metal, the valence number may be quickly determined by counting the number of columns from the left, skipping the transitional metals in the center. Thus, elements in the first column have a valence number of one. After skipping columns three through 12, elements in the 15th column have a valence number of five.