Ions
A neutral atom has the same number of protons and electrons. Often an atom has a deficit or excess of electrons, that is less than or more than the number of protons. An atom that has lost an electron has a plus one charge (+1). An atom that has lost two electrons has a plus 2 charge (+2). These are called cations. Positive charges up to +7 occur in common compounds. If an atom has gained electrons it has a negative charge. One excess electron gives a minus one charge, and so on. Negative ions are called anions.
A sodium atom will commonly lose one electron and become a sodium ion (Na+). This occurs in an ionic solid such as sodium chloride (NaCl) which consists of sodium ions (Na+) and chloride ions (Cl-). The sodium ions have given up their outer electrons to the chloride ions. The crystal as a whole has the same number of protons and electrons and is neutral. If the solid is dissolved in water the solution consists of sodium ions and chloride ions surrounded by the polar water molecules that are oriented about the ions to form a hydration sphere. The water molecule has a negative and positive end. The negative end of the water molecule is directed toward a positive ion and the positive end is turned toward a negative ion.
Ions can consist of two or more atoms. This is the case with the ammonium ion (NH4+). Another example is the divalent sulfate ion (SO42-).