Scandium, element 21 on the periodic table, has 2 valence electrons. Valence electrons are the electrons that occupy the highest energy orbital of an atom. They are the electrons in an atom that participate in chemical reactions.
Scandium's electron configuration is [Ar] 3d1 4s2. The 4s orbital is the one that houses its valence electrons. Since valence electrons are the primary electrons involved in chemical reactions, they are a key factor in determining an element's chemical properties. Elements in the same column, or group, of the periodic table have the same number of valence electrons, so they frequently have similar chemical and physical properties; scandium is a group 3 element, and it shares many physical properties with other group 3 elements, such as lutetium and yttrium. For example, these elements have very similar melting points.