<P> The farther right in each transition metal series, the lower the energy of an electron in ad subshell and the less such an electron has the properties of a valence electron . Thus, although a nickel atom has, in principle, ten valence electrons (4s 3d), its oxidation state never exceeds four . For zinc, the 3d subshell is complete and behaves similarly to core electrons . </P> <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>

Why is it useful to know the number of valence electrons in an atom
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