Oxidation number, also known as oxidation state, describes the degree to which an atom has gained or lost electrons in a chemical compound. When an element is assigned an oxidation number of 2, it signifies that the atom has lost two electrons. This loss of electrons results in a positive charge on the atom.

For example, consider the compound magnesium oxide (MgO). In this compound, magnesium has an oxidation number of 2, while oxygen has an oxidation number of -2. This means that each magnesium atom has lost two electrons, which have been transferred to oxygen atoms to form oxide ions (O2-).

The oxidation number of an element in a compound is determined by considering the electronegativity of the atoms involved. Electronegativity is a measure of an atom's ability to attract electrons. In general, the more electronegative an atom is, the greater its ability to attract electrons.

In the case of MgO, oxygen is more electronegative than magnesium. This means that oxygen atoms attract electrons more strongly than magnesium atoms. As a result, magnesium atoms lose electrons to oxygen atoms, resulting in the formation of Mg2+ and O2- ions.

It's important to note that the oxidation number of an element does not necessarily correspond to its formal charge. Formal charges are assigned to atoms in a molecule based on the assumption that electrons are shared equally between atoms. In some cases, the oxidation number and formal charge of an atom may be the same, while in other cases they may differ.