Phosphorus contains 15 protons. Protons, along with neutrons, are particles that make up the nucleus of an atom. Atoms also contain electrons, which neutralize the positive charge of protons.
Most protons in the universe originate from bosons, which are large particles left over from the Big Bang. The number of protons in each atom is fundamental to its composition. For example, helium atoms have two protons, whereas oxygen atoms have 8. Although protons were thought to be fundamental particles, meaning that they cannot be broken down, this theory was refuted in 1974 by the discovery of quarks, which are fundamental particles contained in protons and neutrons.