A neutral atom containing 12 protons is of the element magnesium. If the atom had a positive or negative charge, it would still be of the element magnesium; however, it would then be classified as an ion.
Protons determine the elemental identity of an atom. If one removes or adds protons to an atom, it changes identity. For example, if one removed a proton from an atom of magnesium, it would then be a sodium atom. The number of protons in an element's atom is displayed in the upper left corner of the cells in most periodic tables; it is called the atomic number. Protons are positively charged. The other major parts of the atom are negatively charged electrons and uncharged neutrons. Atoms with fewer or more electrons than the atomic number are positively and negatively charged, respectively. Atoms with varying numbers of neutrons are called "isotopes."