Lecture 7
Metallic and non-metallic
elements
- Metallic elements form solids that are good
conductors of electricity, and have structures
. Non-metallic elements form molecules or
covalent solids, which are generally poor
conductors of electricity
. All elements of the s, d and f blocks are metallic
(except
hydrogen), non-metallic ones being confined to
hydrogen and to the upper right-hand part of the
p block
Different types of chemical behavior are associated with
the two kinds of element.
.
• Typical metallic elements are good reducing agents (for
example, reacting with water to produce dihydrogen)
and form hydrated cations in aqueous solution (Na+,
Mg2+, etc.).
They have solid halides and oxides. The oxides are
basic and either react with water to produce hydroxide
ions (OH−) or, if insoluble under neutral
conditions, dissolve in acidic solutions. Their hydrides are
solids with some ionic (H−)
character.
Typical non-metallic elements form ionic
compounds with electropositive metals.
They form anions in water, either monatomic
(e.g. Cl−) or oxoanions (e.g. NO3−, ).
Their oxides are either molecular or polymeric
covalent in structure, and are acidic, reacting
with water (as do halides) to produce oxoacids
(H2CO3,H2So4, etc
Periodic table showing metallic and (heavily shaded) non-metallic elements
.
A metalloid is a chemical element with properties that are in-between
or a mixture of those of metals and nonmetals, and which is
considered to be difficult to classify y as either a metal or a
nonmetal.
The six elements commonly recognized as metalloids are boron,
silicon, germanium, arsenic, antimony and tellurium. They are
metallic-looking brittle solids, with intermediate to relatively good
electrical conductivities, and each having the electronic band
structure of either a semiconductor or a semimetal. Chemically,
they mostly behave as (weak) nonmetals, have intermediate
ionization energy and electronegativity values, and form
amphoteric or weakly acidic oxides. The electrical properties of
silicon and germanium, in particular, enabled the establishment of
the semiconductor industry in the 1950s and the development of
solid state electronics.