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 1-8 electrons (ns and np orbitals). For transition metals, the number of valence electrons ranges from 3-12 electrons (ns and (n−1)d orbitals). For lanthanides and actinides, the number of valence electrons ranges from 3-16 electrons (ns, (n−2)f and (n−1)d orbitals). All other non-valence electrons for an atom of that element are considered core electrons. Orbital theory A more complex explanation of the difference between core and valence electrons can be described with atomic orbital theory. In atoms with a single electron the energy of an orbital is determined exclusively by the principle quantum number n. The n = 1 orbital has the lowest possible energy in the atom. For large n, the energy increases so much that the electron can easily escape from the atom. In single electron atoms, all energy levels with the same principle quantum number are degenerate, and have the same energy. In atoms with more than one electron, the energy of an electron depends not only on the properties of the orbital it resides in, but also on its interactions with the other electrons in other orbitals. This requires consideration of the ℓ quantum number. Higher values of ℓ are associated with higher values of energy; for instance, the 2p state is higher than the 2s state. When ℓ = 2, the increase in energy of the orbital becomes large enough to push the energy of orbital above the energy of the s-orbital in the next higher shell; when ℓ = 3 the energy is pushed into the shell two steps higher. The filling of the 3d orbitals does not occur until the 4s orbitals have been filled. The increase in energy for subshells of increasing angular momentum in larger atoms is due to electron–electron interaction effects, and it is specifically related to the