Core electron

Core electrons are the electrons in an atom that are not valence electrons and do not participate in chemical bonding.^[1] 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.^[2]

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.