groups 3–12 See transition el-
ements.group 13 elements A group of el-
ements in the *periodic table: boron
(B), aluminium (Al), gallium (Ga), in-
dium (In), and thallium (Tl), which all
have outer electronic conÜgurations
ns^2 np^1 with no partlyÜlled inner lev-
els. They are theÜrst members of the
p-block. The group differs from the
alkali metals and alkaline-earth met-
als in displaying a considerable varia-
tion in properties as the group is
descended. Formerly, they were
classiÜed in group III, which con-
sisted of two subgroups: group IIIB
(the main group) and group IIIA.
Group IIIA consisted of scandium
(Sc), yttrium (Yt), and lanthanum (La),
which are generally considered with
the *lanthanoids, and actinium (Ac),
which is classiÜed with the *acti-
noids. Scandium and yttrium now be-
long to group 3 (along with lutetium
and lawrencium).
Boron has a small atomic radius
and a relatively high ionization en-
ergy. In consequence its chemistry is
largely covalent and it is generally
classed as a metalloid. It forms a
large number of volatile hydrides,
some of which have the uncommon
bonding characteristic of *electron-
deÜcient compounds. It also forms a
weakly acidic oxide. In some ways,
boron resembles silicon (see diago-
nal relationship).
As the group is descended, atomic
radii increase and ionization energies
are all lower than for boron. There is
an increase in polar interactions and
the formation of distinct M3+ions.
This increase in metallic character is
clearly illustrated by the increasing
basic character of the hydroxides:
boron hydroxide is acidic, aluminium
and gallium hydroxides are ampho-
teric, indium hydroxide is basic, and
thallium forms only the oxide. As theelements of group 13 have a vacant
p-orbital they display many electron-
acceptor properties. For example,
many boron compounds form
adducts with donors such as ammo-
nia and organic amines (acting as
Lewis acids). A large number of com-
plexes of the type [BF 4 ]–, [AlCl 4 ]–,
[InCl 4 ]–, [TlI 4 ]–are known and the
heavier members can expand their
coordination numbers to six as in
[AlF 6 ]3–and [TlCl 6 ]3–. This acceptor
property is also seen in bridged
dimers of the type Al 2 Cl 6. Another
feature of group 13 is the increasing
stability of the monovalent state
down the group. The electron
conÜguration ns^2 np^1 suggests that
only one electron could be lost or
shared in forming compounds. In
fact, for the lighter members of the
group the energy required to pro-
mote an electron from the s-subshell
to a vacant p-subshell is small. It is
more than compensated for by the
resulting energy gain in forming
three bonds rather than one. This en-
ergy gain is less important for the
heavier members of the group. Thus,
aluminium forms compounds of the
type AlCl in the gas phase at high
temperatures. Gallium similarly
forms such compounds and
gallium(I) oxide (Ga 2 O) can be iso-
lated. Indium has a number of
known indium(I) compounds (e.g.
InCl, In 2 O, In 3 I[InIIICl 6 ]). Thallium has
stable monovalent compounds. In
aqueous solution, thallium(I) com-
pounds are more stable than the cor-
responding thallium(III) compounds.
See inert-pair effect.group 14 elements A group of el-
ements in the *periodic table: carbon
(C), silicon (Si), germanium (Ge), tin
(Sn), and lead (Pb), which all have
outer electronic conÜgurations ns^2 np^2
with no partlyÜlled inner levels. For-
merly, they were classiÜed in groupgroups 3–12 254g