What is the bonding in sodium chloride like? The particles in sodium chloride are closely packed and regularly arranged. Sodium chloride contains sodium ions. It also contains chloride ions. Sodium ions are smaller than chloride ions. Sodium ions are positively charged. Chloride ions are negatively charged. The electrostatic force of attraction between oppositely charged ions is called an ionic bond. Ions are formed when atoms, or groups of atoms, lose or gain electrons. Each sodium atom transfers its outer electron to a chlorine atom. A sodium ion, Na +, and a chloride ion, Cl , form. Sodium is in Group 1 and has one electron in its outer shell. Chlorine is in Group 7 and has seven electrons in its outer shell. The outer electron is transferred from the sodium atom to the chlorine atom. The sodium atom and chlorine atom now both have full outer shells. They are now a sodium ion, Na + and a chloride ion, Cl . A lot of energy is needed to overcome ionic bonds, so ionic compounds have high melting points and high boiling points. The strength of the ionic bond increases when the change on the ions increases. So magnesium oxide has a higher melting point than sodium chloride. The strength of the ionic bond increases when the size of the ions decreases. The strength of the ionic bond also increases when the size of the ions decreases. Fluoride ions are smaller than chloride ions, so calcium fluoride has a higher melting point than calcium chloride.