266Categorization and Periodic Table Territory
Metalloids are generally regarded as a third category of chemical elements, alongside
metals and nonmetals. They have been described as forming a (fuzzy) buffer zone
between metals and nonmetals. The make-up and size of this zone depends on the
classification criteria being used. Metalloids are sometimes grouped instead with metals,
regarded as nonmetals or treated as a sub-category of same.
Metalloid
Border(^) H He
(^) Li Be B C N O F Ne
(^) Na Mg (^) Al Si P S Cl Ar
(^) K Ca (^) Zn Ga Ge As Se Br Kr
(^) Rb Sr (^) Cd In Sn Sb Te I Xe
(^) Cs Ba (^) Hg Tl Pb Bi Po At Rn
(^) Fr Ra (^) Cn Uut Fl Uup Lv Uus Uuo
Periodic table extract showing elements that have
sometimes] been classified as metalloids:
Elements that appear commonly to rarely in the list
of metalloid lists.
Elements that appear still less frequently.
Outlying elements showing that the metalloid net
is sometimes cast very widely. Although they do
not appear in the list of metalloids lists, isolated
references to their designation as metalloids can
be found in the literature.
Metalloids cluster on either side of the dividing line between metals and nonmetals.
This can be found, in varying configurations, on some periodic tables (see mini-example,
right). Elements to the lower left of the line generally display increasing metallic behavior;
elements to the upper right display increasing nonmetallic behavior.
When presented as a regular stair-step, elements with the highest critical temperature for
their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line.
The diagonal positioning of the metalloids represents somewhat of an exception to the
phenomenon that elements with similar properties tend to occur in vertical columns. Going
across a periodic table row, the nuclear charge increases with atomic number just as there
is as a corresponding increase in electrons. The additional 'pull' on outer electrons with
increasing nuclear charge generally outweighs the screening efficacy of having more
electrons.