AN ELECTRONIC DISTINCTION BETWEEN METALS AND NONMETALS R. T. SANDERSON State University of Iowa, Iowa City, Iowa
THEproperties of matter must originate with the properties of its atoms. The properties of atoms must in turn originate with their composition and strncture. One certainly ought, from a knowledge of the atomic structure of an element, to be able to state whether it is metallic or nonmetallic. Yet apparently there is no recognized fundamental basis for making this important although admittedly somewhat arbitrary distinction. It is not ionization potentials. Although metals as a class have lower values than nonmetals as a class, the ionization potentials are alike for mercury and iodine, and the metals zinc, cadmium, palladium antimony, osmium, iridium, platinum, and gold as well as mercury have higher values than boron or silicon. It is not electronegativity. In general, metals are less electronegative than nonmetals, but the electronegativities of boron and silicon are exceeded by those of tin, germanium, antimony, gold, mercury, thallium, lead, and bismuth, and for hydrogen and germanium they are essentially equal. It is not atomic radius, for although nonmetal atoms tend to be smaller, atoms of iodine and tellurium are larger than those of vanadium, chromium, manganese, iron, and cobalt. I t is not the number of outer shell electrons. Usually metal atoms have fewer than nonmetal atoms, but although boron, carbon, and silicon have fewer than five, the metals antimony, bismuth, and polonium have five or six. Nor is it the type of orbital available for bonding. Although many elements having low-energy d-orbital vacancies are metals, many other elements having only 8- and porbital vacancies are also. metals. Evidently -some combination of such fundamental properties as these must determine whether an element is metallic, metalloid, or nonmetallic. The average experienced chemist has an intuitive understanding of
VOLUME 34, NO. 5, MAY, 1957
these qualities that removes any conscious need of some arbitrary means of distinction. Not so the student, for whom the following simple empirical rule is stated: W i t h the single exception of hydrogen, all elements are metals if the number of electrons i n the outermost shell of their atoms i s equal to or less than the period number of the element (which i s the same as the principal quantum number of that shell). Hydrogen and all other elements are nonmetals, but i f the number of electrons in the outennost shell i s one (or two) greater than their principal quantum number, they may show some metallic charactetistics. As shown by the principal electron distributions: Principal quantum no.: Period 2: Period 3: Period 4: Period 5: Period 6:
1
Be B A1 Si Ge As Sb Te Po
At
2 2 2 2 2 2 2 2 2 2
2
2 3 8 8 8
8 8 8 8 8
3
4
6
6
3 4 18 18 18 18 18 18
4 5 18 18 32 32
5 6 18 18
6 7
this rule establishes the familiar metal-nonmetal diagonal of the periodic table just beyond beryllium, aluminum, germanium, antimony, and polonium. I t recognizes certain metallic properties in boron (and carbon), in silicon (and phosphorus), in arsenic (and selenium), in tellurium (and iodine), and in astatine. Unfortunately, the rule does not explain why this electronic distinction exists. A simple, fundamental explanation would be a worthwhile contribution to chemistry.
