There is a further series of this type
in the next period of the table: be-
tween yttrium ([Kr]4d 5 s^2 ) and cad-
mium ([Kr]4d^105 s^2 ). This is the second
transition series. In the next period
of the table the situation is rather
more complicated. Lanthanum has
the conÜguration [Xe]5d^16 s^2. The
level of lowest energy then becomes
the 4f level and the next element,
cerium, has the conÜguration
[Xe]4f^15 d^16 s^2. There are 7 of these f-
orbitals, each of which can contain
2 electrons, andÜlling of the f-
levels continues up to lutetium
([Xe]4f^145 d^16 s^2 ). Then theÜlling of the
5 d levels continues from hafnium to
mercury. The series of 14 elements
from cerium to lutetium is a ‘series
within a series’ called an inner transi-
tion series. This one is the lan-
thanoid series. In the next period
there is a similar inner transition se-
ries, the actinoid series, from tho-
rium to lawrencium. ThenÜlling of
the d-level continues from element
104 onwards.
In fact, the classiÜcation of chemi-
cal elements is valuable only in so far
as it illustrates chemical behaviour,
and it is conventional to use the term
‘transition elements’ in a more re-
stricted sense. The elements in the
inner transition series from cerium
(58) to lutetium (71) are called the
lanthanoids; those in the series from
thorium (90) to lawrencium (103) are
the actinoids. These two series to-
gether make up the f-block in the pe-
riodic table. It is also common to
include scandium, yttrium, and lan-
thanum with the lanthanoids (be-
cause of chemical similarity) and to
include actinium with the actinoids.
Of the remaining transition el-
ements, it is usual to speak of three
main transition series: from titanium
to copper; from zirconium to silver;
and from hafnium to gold. All these
elements have similar chemical prop-
erties that result from the presence
of unÜlled d-orbitals in the element
or (in the case of copper, silver, and
gold) in the ions. The elements from
104 to 109 and the undiscovered el-
ements 110 and 111 make up a
fourth transition series. The el-
ements zinc, cadmium, and mercury
haveÜlled d-orbitals both in the el-
ements and in compounds, and are
usually regarded as nontransition el-
ements forming group 12 of the peri-
odic table.
The elements of the three main
transition series are all typical metals
(in the nonchemical sense), i.e. most
are strong hard materials that are
good conductors of heat and elec-
tricity and have high melting and
boiling points. Chemically, their be-
haviour depends on the existence of
unÜlled d-orbitals. They exhibit vari-
able valency, have coloured com-
pounds, and form *coordination
compounds. Many of their com-
pounds are paramagnetic as a result
of the presence of unpaired elec-
trons. Many of them are good cata-
lysts. They are less reactive than the
s- and p-block metals.transition point (transition tem-
perature) 1.The temperature at
which one crystalline form of a sub-
stance changes to another form.
2.The temperature at which a sub-
stance changes phase. 3.The tem-
perature at which a substance
becomes superconducting. 4.The
temperature at which some other
change, such as a change of magnetic
properties, takes place.
transition state (activated com-
plex)The association of atoms of
highest energy formed during a
chemical reaction. The transition
state can be regarded as a short-lived
intermediate that breaks down to
give the products. For example, in a
SN2 substitution reaction, one atom533 transition state
t