The number of valence electrons an atom has depends on its atomic number. The atomic number of an element is the number of protons in the nucleus of an atom. The number of valence electrons an element has is equal to the atomic number minus the number of inner-shell electrons.
Inner-shell electrons are the electrons that are not in the outermost energy level of an atom. They are held more tightly to the nucleus than valence electrons, so they do not participate in chemical reactions.
The number of valence electrons an element has can be used to predict its chemical properties. For example, elements with one valence electron are highly reactive and tend to form ionic compounds. Elements with seven valence electrons are also highly reactive, but they tend to form covalent compounds. Elements with a full outer energy level, which means they have eight valence electrons, are relatively unreactive and tend to form stable compounds.
The valence electrons of an atom can be used to determine its valence number. The valence number of an element is the number of electrons it can gain, lose, or share in a chemical reaction. For example, an element with one valence electron has a valence number of 1, an element with two valence electrons has a valence number of 2, and so on.
The valence electrons of an atom can also be used to determine its oxidation number. The oxidation number of an element is the charge it would have if all of its valence electrons were lost. For example, an element with one valence electron has an oxidation number of 1+, an element with two valence electrons has an oxidation number of 2+, and so on.
