Ülled inner levels. They are also
called the chalcogens. Formerly, they
were classiÜed in group VI, which
consisted of two subgroups: group
VIB (the main group) and group VIA.
Group VIA consisted of chromium
(Cr), molybdenum (Mo), and tungsten
(W), which now form group 6 are
generally classiÜed with the *transition elements.
The conÜgurations are just two
electrons short of the conÜguration
of a noble gas and the elements are
characteristically electronegative and
almost entirely nonmetallic. Ionization energies are high, (O 1314 to Po
813 kJ mol
–1
) and monatomic cations
are not known. Polyatomic cations
do exist, e.g. O 2
+
, S 8
2+ , Se 8
2+
, Te 4
2+ .
Electronegativity decreases down the
group but the nearest approach to
metallic character is the occurrence
of ‘metallic’ allotropes of selenium,
tellurium, and polonium along with
some metalloid properties, in particular, marked photoconductivity. The
elements of group 16 combine with a
wide range of other elements and the
bonding is largely covalent. The elements all form hydrides of the type
XH 2 . Apart from water, these materials are all toxic foul-smelling
gases; they show decreasing thermal
stability with increasing relative
atomic mass of X. The hydrides dissolve in water to give very weak
acids (acidity increases down the
group). Oxygen forms the additional
hydride H 2 O 2 (hydrogen peroxide),
but sulphur forms a range of sulphanes, such as H 2 S 2 , H 2 S 4 , H 2 S 6 .
Oxygen forms the Ûuorides O 2 F 2
and OF 2 , both powerful Ûuorinating
agents; sulphur forms analogous
Ûuorides along with some higher
Ûuorides, S 2 F 2 , SF 2 , SF 4 , SF 6 , S 2 F 10 . Selenium and tellurium form only the
higher Ûuorides MF 4 and MF 6 ; this is
in contrast to the formation of lower
valence states by heavier elements
observed in groups 13, 14, and 15.
The chlorides are limited to M 2 Cl 2
and MCl 4 ; the bromides are similar
except that sulphur only forms S 2 Br 2 .
All metallic elements form oxides
and sulphides and many form selenides.
group 17 elements A group of elements in the *periodic table:
Ûuorine (F), chlorine (Cl), bromine
(Br), iodine (I), and astatine (At). They
are known as the *halogens. Formerly, they were classiÜed in group
VII, which consisted of two subgroups: group VIIB (the main group)
and group VIIA. Group VIIA consisted
of the elements manganese (Mn),
technetium (Te), and rhenium (Re),
which now form group 7 and are
usually considered with the transition elements.
group 18 elements A group of elements in the *periodic table: helium (He), neon (Ne), argon (Ar),
krypton (Kr), xenon (Xe), and radon
(Rn). Formerly classiÜed as group 0
elements, they are usually referred to
as the *noble gases.
group representation A group of
mathematical objects that is homomorphic to (i.e. has the same mathematical structure as) the original
group. In particular, group representations made up of square matrices
are of interest. The dimension of the
representation is the number of rows
or group representation columns of
the matrix. The *irreducible representations of a group, i.e. the representations that cannot be expressed
in terms of lower-dimensional representations, are of great importance
in quantum mechanics since the energy levels of a quantum mechanical
system are labelled by the irreducible
representations of the symmetry
group of the system. This enables
*selection rules for the system to be
derived.
group 17 elements
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g
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called the chalcogens. Formerly, they
were classiÜed in group VI, which
consisted of two subgroups: group
VIB (the main group) and group VIA.
Group VIA consisted of chromium
(Cr), molybdenum (Mo), and tungsten
(W), which now form group 6 are
generally classiÜed with the *transition elements.
The conÜgurations are just two
electrons short of the conÜguration
of a noble gas and the elements are
characteristically electronegative and
almost entirely nonmetallic. Ionization energies are high, (O 1314 to Po
813 kJ mol
–1
) and monatomic cations
are not known. Polyatomic cations
do exist, e.g. O 2
+
, S 8
2+ , Se 8
2+
, Te 4
2+ .
Electronegativity decreases down the
group but the nearest approach to
metallic character is the occurrence
of ‘metallic’ allotropes of selenium,
tellurium, and polonium along with
some metalloid properties, in particular, marked photoconductivity. The
elements of group 16 combine with a
wide range of other elements and the
bonding is largely covalent. The elements all form hydrides of the type
XH 2 . Apart from water, these materials are all toxic foul-smelling
gases; they show decreasing thermal
stability with increasing relative
atomic mass of X. The hydrides dissolve in water to give very weak
acids (acidity increases down the
group). Oxygen forms the additional
hydride H 2 O 2 (hydrogen peroxide),
but sulphur forms a range of sulphanes, such as H 2 S 2 , H 2 S 4 , H 2 S 6 .
Oxygen forms the Ûuorides O 2 F 2
and OF 2 , both powerful Ûuorinating
agents; sulphur forms analogous
Ûuorides along with some higher
Ûuorides, S 2 F 2 , SF 2 , SF 4 , SF 6 , S 2 F 10 . Selenium and tellurium form only the
higher Ûuorides MF 4 and MF 6 ; this is
in contrast to the formation of lower
valence states by heavier elements
observed in groups 13, 14, and 15.
The chlorides are limited to M 2 Cl 2
and MCl 4 ; the bromides are similar
except that sulphur only forms S 2 Br 2 .
All metallic elements form oxides
and sulphides and many form selenides.
group 17 elements A group of elements in the *periodic table:
Ûuorine (F), chlorine (Cl), bromine
(Br), iodine (I), and astatine (At). They
are known as the *halogens. Formerly, they were classiÜed in group
VII, which consisted of two subgroups: group VIIB (the main group)
and group VIIA. Group VIIA consisted
of the elements manganese (Mn),
technetium (Te), and rhenium (Re),
which now form group 7 and are
usually considered with the transition elements.
group 18 elements A group of elements in the *periodic table: helium (He), neon (Ne), argon (Ar),
krypton (Kr), xenon (Xe), and radon
(Rn). Formerly classiÜed as group 0
elements, they are usually referred to
as the *noble gases.
group representation A group of
mathematical objects that is homomorphic to (i.e. has the same mathematical structure as) the original
group. In particular, group representations made up of square matrices
are of interest. The dimension of the
representation is the number of rows
or group representation columns of
the matrix. The *irreducible representations of a group, i.e. the representations that cannot be expressed
in terms of lower-dimensional representations, are of great importance
in quantum mechanics since the energy levels of a quantum mechanical
system are labelled by the irreducible
representations of the symmetry
group of the system. This enables
*selection rules for the system to be
derived.
group 17 elements
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g
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