A metal atom becomes a positive cation because it loses electron(s). A non-metal atom becomes a negative anion as it gains electron(s). This occurs, for example, when sodium and chlorine join to form common table salt, NaCl. First, sodium atoms (Na) oxidize and lose an electron to form positively charged sodium ions (Na). Chlorine atoms gain the electrons from the sodium atoms to form negatively charged chloride ions (Cl). Both ions are now oppositely charged and they are held by strong electrostatic forces of attraction.