Classes of metals products with soft bases, which are bases with covalent bonds. The IUPAC Gold Book provides a more general definition of a class (a) metal ion, as "A metal ion that combines preferentially with ligands containing ligating atoms that are the lightest of their Periodic Group", and a class (b) metal as one that combines preferentially with ligands containing ligating atoms other than the lightest of their Periodic Group. Classes of metals Class A metals are metals that form hard acids. Hard acids are acids with relatively ionic bonds. These metals, such as iron, aluminium, titanium, sodium, calcium and the lanthanides,
Period 2 element O, but also O and O. Oxygen is the third-most common element by mass in the universe (although carbon forms more of the atoms it is a lighter atom). It is highly electronegative and non-metallic, usually diatomic, gas down to very low temperatures. Only fluorine is more reactive among non-metallic elements. It is two electrons short of a full octet and readily takes electrons from other elements. It reacts violently with alkali metals and white phosphorus at room temperature and less violently with alkali earth metals heavier than magnesium. At higher temperatures it burns most other metals and many non-metals