proton

A proton is a positively charged particle that sits in an atom's nucleus. The number of protons is the atomic number, and that number decides which element the atom is: one proton is hydrogen, six is carbon, eleven is sodium. Because each proton carries one unit of positive charge, the protons set the positive charge of the nucleus, and in a neutral atom an equal number of electrons cancels it. Protons are never gained or lost in ordinary chemical reactions — only electrons are — which is why the proton count is a stable identity while the electron count can change when an atom becomes an ion. Each proton weighs about one atomic mass unit, so the proton count is part of what the mass number adds up.