IV, which consisted of two sub-
groups: IVB (the main group) and
group IVA. Group IVA consisted of ti-
tanium (Ti), zirconium (Zr), and
hafnium (Hf), which now form group
4 and are generally considered with
the *transition elements.
The main valency of the elements
is 4, and the members of the group
show a variation from nonmetallic to
metallic behaviour in moving down
the group. Thus, carbon is a non-
metal and forms an acidic oxide
(CO 2 ) and a neutral oxide. Carbon
compounds are mostly covalent. One
allotrope (diamond) is an insulator,
although graphite is a fairly good
conductor. Silicon and germanium
are metalloids, having semiconduc-
tor properties. Tin is a metal, but
does have a nonmetallic allotrope
(grey tin). Lead is deÜnitely a metal.
Another feature of the group is the
tendency to form divalent com-
pounds as the size of the atom in-
creases. Thus carbon has only the
highly reactive carbenes. Silicon
forms analogous silylenes. Germa-
nium has an unstable hydroxide
(Ge(OH) 2 ), a sulphide (GeS), and
halides. The sulphide and halides dis-
proportionate to germanium and the
germanium(IV) compound. Tin has a
number of tin(II) compounds, which
are moderately reducing, being oxi-
dized to the tin(IV) compound. Lead
has a stable lead(II) state. See inert-
pair effect.
In general, the reactivity of the el-
ements increases down the group
from carbon to lead. All react with
oxygen on heating. TheÜrst four
form the dioxide; lead forms the
monoxide (i.e. lead(II) oxide, PbO).
Similarly, all will react with chlorine
to form the tetrachloride (in the case
of theÜrst four) or the dichloride (for
lead). Carbon is the only one capable
of reacting directly with hydrogen.
The hydrides all exist from the stable
methane (CH 4 ) to the unstable
plumbane (PbH 4 ).group 15 elements A group of el-
ements in the *periodic table: nitro-
gen (N), phosphorus (P), arsenic (As),
antimony (Sb), and bismuth (Bi),
which all have outer electronic
conÜgurations ns^2 np^3 with no partly
Ülled inner levels. Formerly, they
were classiÜed in group V, which
consisted of two subgroups: group VB
(the main group) and group VA.
Group VA consisted of vanadium (V),
niobium (Nb), and tantalum (Ta),
which are generally considered with
the *transition elements:
The lighter elements (N and P) are
nonmetals; the heavier elements are
metalloids. The lighter elements are
electronegative in character and have
fairly large ionization energies. Nitro-
gen has a valency of 3 and tends to
form covalent compounds. The other
elements have available d-sublevels
and can promote an s-electron into
one of these to form compounds
with the V oxidation state. Thus,
they have two oxides P 2 O 3 , P 2 O 5 ,
Sb 2 O 3 , Sb 2 O 5 , etc. In the case of bis-
muth, the pentoxide Bi 2 O 5 is difÜcult
to prepare and unstable – an exam-
ple of the increasing stability of the
III oxidation state in going from
phosphorus to bismuth. The oxides
also show how there is increasing
metallic (electropositive) character
down the group. Nitrogen and phos-
phorus have oxides that are either
neutral (N 2 O, NO) or acidic. Bismuth
trioxide (Bi 2 O 3 ) is basic. Bismuth is
the only member of the group that
forms a well-characterized positive
ion Bi3+.group 16 elements A group of el-
ements in the *periodic table: oxy-
gen (O), sulphur (S), selenium (Se),
tellurium (Te), and polonium (Po),
which all have outer electronic
conÜgurations ns^2 np^4 with no partly255 group 16 elements
g