There is a further series of this type
in the next period of the table: between yttrium ([Kr]4d5s
2 ) and cadmium ([Kr]4d
10 5s
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
1 6s
2 . The
level of lowest energy then becomes
the 4f level and the next element,
cerium, has the conÜguration
[Xe]4f
1
5d
1
6s
2 . There are 7 of these forbitals, each of which can contain
2 electrons, and Ülling of the flevels continues up to lutetium
([Xe]4f
14 5d
1
6s
2 ). Then the Ülling of the
5d levels continues from hafnium to
mercury. The series of 14 elements
from cerium to lutetium is a ‘series
within a series’ called an inner transition series. This one is the *lanthanoid series. In the next period
there is a similar inner transition series, the *actinoid series, from thorium to lawrencium. Then Ülling of
the d-level continues from element
104 onwards.
In fact, the classiÜcation of chemical 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 restricted 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 together make up the f-block in the periodic table. It is also common to
include scandium, yttrium, and lanthanum with the lanthanoids (because of chemical similarity) and to
include actinium with the actinoids.
Of the remaining transition elements, 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 properties 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 elements 110 and 111 make up a
fourth transition series. The elements zinc, cadmium, and mercury
have Ülled d-orbitals both in the elements and in compounds, and are
usually regarded as nontransition elements forming group 12 of the periodic 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 electricity and have high melting and
boiling points. Chemically, their behaviour depends on the existence of
unÜlled d-orbitals. They exhibit variable valency, have coloured compounds, and form *coordination
compounds. Many of their compounds are paramagnetic as a result
of the presence of unpaired electrons. Many of them are good catalysts. They are less reactive than the
s- and p-block metals.
transition point (transition temperature) 1. The temperature at
which one crystalline form of a substance changes to another form.
2. The temperature at which a substance changes phase. 3. The temperature 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 complex) 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
S N 2 substitution reaction, one atom
533
transition state
t
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in the next period of the table: between yttrium ([Kr]4d5s
2 ) and cadmium ([Kr]4d
10 5s
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
1 6s
2 . The
level of lowest energy then becomes
the 4f level and the next element,
cerium, has the conÜguration
[Xe]4f
1
5d
1
6s
2 . There are 7 of these forbitals, each of which can contain
2 electrons, and Ülling of the flevels continues up to lutetium
([Xe]4f
14 5d
1
6s
2 ). Then the Ülling of the
5d levels continues from hafnium to
mercury. The series of 14 elements
from cerium to lutetium is a ‘series
within a series’ called an inner transition series. This one is the *lanthanoid series. In the next period
there is a similar inner transition series, the *actinoid series, from thorium to lawrencium. Then Ülling of
the d-level continues from element
104 onwards.
In fact, the classiÜcation of chemical 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 restricted 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 together make up the f-block in the periodic table. It is also common to
include scandium, yttrium, and lanthanum with the lanthanoids (because of chemical similarity) and to
include actinium with the actinoids.
Of the remaining transition elements, 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 properties 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 elements 110 and 111 make up a
fourth transition series. The elements zinc, cadmium, and mercury
have Ülled d-orbitals both in the elements and in compounds, and are
usually regarded as nontransition elements forming group 12 of the periodic 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 electricity and have high melting and
boiling points. Chemically, their behaviour depends on the existence of
unÜlled d-orbitals. They exhibit variable valency, have coloured compounds, and form *coordination
compounds. Many of their compounds are paramagnetic as a result
of the presence of unpaired electrons. Many of them are good catalysts. They are less reactive than the
s- and p-block metals.
transition point (transition temperature) 1. The temperature at
which one crystalline form of a substance changes to another form.
2. The temperature at which a substance changes phase. 3. The temperature 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 complex) 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
S N 2 substitution reaction, one atom
533
transition state
t
www.AzShimi.ir www.AzShimi.com