A valence electron is an electron that is the most likely to be involved in a chemical reaction. They are typically the electrons with the highest value of the principle quantum number, Magnesium's ground state electron configuration is 1s22s2p63s2, the valence electrons would be the 3s electrons.Bromine's ground state electron configuration is 1s22s2p63s2p6d104s2p5, the valence electrons would be the 4s and 4p electrons. a single electron or one of two or more electrons in the outer shell of an atom that is responsible for the chemical properties of the atom. When elements share electrons, a strong covalent bond is formed. When an element gives away an outer electron, it results in oppositely charged ions that are held together by a weaker ionic bond