does not need to be absorbed directly
by the reacting substances; it is possi-
ble, in photosensitization for exam-
ple, that light can be absorbed by an
inert substance, which subsequently
transfers the absorbed energy (as
thermal energy) to the reactants.
ground state The lowest stable en-
ergy state of a system, such as a mol-
ecule, atom, or nucleus. See energy
level.
group 1.See periodic table. 2.A
mathematical structure consisting of
a set of elements A, B, C, etc., for
which there exists a law of composi-
tion, referred to as ‘multiplication’.
Any two elements can be combined
to give a ‘product’ AB.
(1) Every product of two elements is
an element of the set.
(2) The operation is associative, i.e.
A(BC) = (AB)C.
(3) The set has an element I, called
the identity element, such that IA =
AI = A for all A in the set.
(4) Each element of the set has an in-
verse A–1belonging to the set such
that AA–1= A–1A = I.
Although the law of combination is
called ‘multiplication’ this does not
necessarily have its usual meaning.
For example, the set of integers
forms a group if the law of composi-
tion is addition.
Two elements A, B of a group com-
muteif AB = BA. If all the elements of
a group commute with each other
the group is said to be Abelian. If this
is not the case the group is said to be
non-Abelian.
The interest of group theory in
physics and chemistry is in analysing
symmetry. Discrete groups have a
Ünite number of elements, such as
the symmetries involved in rotations
and reÛections of molecules, which
give rise to point groups. Continuous
groups have an inÜnite number of el-
ements where the elements are con-
tinuous. An example of a continuous
group is the set of rotations about a
Üxed axis. The rotation group thus
formed underlies the quantum
theory of angular momentum, which
has many applications to atoms and
nuclei.group 0 elements See noble
gases.group 1 elements A group of el-
ements in the *periodic table:
lithium (Li), sodium (Na), potassium
(K), rubidium (Rb), caesium (Cs), and
Francium (Fr). They are known as the
*alkali metals. Formerly, they were
classiÜed in group I, which consisted
of two subgroups: group IA (the main
group) and group IB. Group IB con-
sisted of the *coinage metals, copper,
silver, and gold, which comprise
group 11 and are usually considered
with the *transition elements.group 2 elements A group of el-
ements in the *periodic table: beryl-
lium (Be), magnesium (Hg), calcium
(Ca), strontium (Sr), barium (Ba), and
radium (Ra). They are known as the
*alkaline-earth metals. Formerly,
they were classiÜed in group II,
which consisted of two subgroups:
group IIA (the main group, see alka-
line-earth metals) and group IIB.
Group IIB consisted of the three met-
als zinc (Zn), cadmium (Cd), and mer-
cury (Hg), which have two s-electrons
outsideÜlled d-subshells. Moreover,
none of their compounds have
unÜlled d-levels, and the metals are
regarded as nontransition elements.
They now form group 12 and are
sometimes called the zinc group. Zinc
and cadmium are relatively elec-
tropositive metals, forming com-
pounds containing divalent ions Zn2+
or Cd2+. Mercury is more unreactive
and also unusual in forming mer-
cury(I) compounds, which contain
the ion Hg 2 2+.253 group 2 elements
g