The atomic number of an element is equal to the number of protons in the nucleus of an atom of that element. The atomic number of phosphorus is 15, so the phosphorus atom has 15 protons in its nucleus. This also means that a neutral phosphorus atom has 15 electrons orbiting the nucleus.

The number of protons and electrons in an atom determine its chemical properties. For example, the number of electrons in the outermost shell of an atom determines how easily it can form bonds with other atoms. Phosphorus has 5 electrons in its outermost shell, which means that it can form bonds with up to 5 other atoms.

The atomic number of an element can also be used to predict the element's position on the periodic table. The periodic table is a chart that arranges the elements in order of their atomic number. Phosphorus is located in group 15 (also known as the nitrogen family) of the periodic table. The elements in group 15 all have 5 electrons in their outermost shell and share similar chemical properties.