<P> Because the number of valence electrons which actually participate in chemical reactions is difficult to predict, the concept of the valence electron is less useful for a transition metal than for a main group element; the d electron count is an alternative tool for understanding the chemistry of a transition metal . </P> <P> The number of electrons in an atom's outermost valence shell governs its bonding behavior . Therefore, elements whose atoms can have the same number of valence electrons are grouped together in the periodic table of the elements . As a general rule, a main group element (except hydrogen or helium) tends to react to form a closed shell, corresponding to the electron configuration s p . This tendency is called the octet rule, because each bonded atom has eight valence electrons including shared electrons . </P> <P> The most reactive kind of metallic element is an alkali metal of group 1 (e.g., sodium or potassium); this is because such an atom has only a single valence electron; during the formation of an ionic bond which provides the necessary ionization energy, this one valence electron is easily lost to form a positive ion (cation) with a closed shell (e.g., Na or K). An alkaline earth metal of Group 2 (e.g., magnesium) is somewhat less reactive, because each atom must lose two valence electrons to form a positive ion with a closed shell (e.g., Mg). </P> <P> Within each group (each periodic table column) of metals, reactivity increases with each lower row of the table (from a light element to a heavier element), because a heavier element has more electron shells than a lighter element; a heavier element's valence electrons exist at higher principal quantum numbers (they are farther away from the nucleus of the atom, and are thus at higher potential energies, which means they are less tightly bound). </P>

Where are valence electrons located in transition metals