Core electron

Core electrons are the electrons in an atom that are not valence electrons and do not participate in chemical bonding. The nucleus and the core electrons of an atom form the atomic core. Core electrons are tightly bound to the nucleus. Therefore, unlike valence electrons, core electrons play a secondary role in chemical bonding and reactions by screening the positive charge of the atomic nucleus from the valence electrons.

The number of valence electrons of an element can be determined by the periodic table group of the element (see valence electron):

• For main-group elements, the number of valence electrons ranges from 1 to 8 (ns and np orbitals).
• For transition metals, the number of valence electrons ranges from 3 to 12 (ns and (n−1)d orbitals).
• For lanthanides and actinides, the number of valence electrons ranges from 3 to 16 (ns, (n−2)f and (n−1)d orbitals).

All other non-valence electrons for an atom of that element are considered core electrons.