Core electrons are the electrons in an atom that are not valence electrons and do not participate directly 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):
All other non-valence electrons for an atom of that element are considered core electrons.