Periodicity of valence or Oxidation State The electrons present in the outermost shell of an atom are called valence electrons and the number of these electrons determine the valence or the valency of the atom. The orbitals present in the valence shell are called valence orbitals. The valence of an atom equal to either the number of valence electrons are equal to 8 … [Read more...] about Periodic Trends In Chemical Properties