Core electrons are the electrons in an atom that are not valence electrons and therefore do not participate in bonding. The number of valence electrons of an element can be determined by the periodic table group of the element. With the exception of the transition metals in groups 3-12 and the Lanthanide and Actinide series, the number of valence electrons ranges from 0-8 electrons. All the non-valence electrons for an atom of that element are considered core electrons. Core electrons are tightly bound to the nucleus. Therefore, unlike valence electrons the core electrons play a secondary role in chemical bonding and reactions by screening the positive charge of the atomic nucleus from the valence shell of electrons. In transition metals, the distinction between core and valence electrons is less distinct with electrons in the highest d-shell acting more like valence electrons than core electrons.