Beryllium is chemically inert to air and water and is not readily attacked by acids because of the presence of a BeO film on the surface of the metal. BeSO₄ is readily soluble in water. This is because Be²⁺ ions overcome the lattice enthalpy factor. Beryllium does not have a coordination number of more than 4. Beryllium oxide is not purely acidic but amphoteric in nature.