A borate ion is a compound of one atom of boron and three atoms of oxygen that carries a total charge of minus three. The borate ion is an anion, which is an ion with a negative charge. An ion with a positive charge is called a cation.
The minus three charge of the borate ion means the ion contains three more electrons, which carry a negative charge, than it does protons, which carry a positive charge. The borate ion can combine with cations to form a neutral molecule. For example, when it combines with the calcium cation, the two bond to form the compound calcium borate.