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The Noble GasesThe Noble Gases (inert gases, Group 0, Group 18 or the helium group) are
notoriously unreactive elements (‘noble’ means unreactive in chemistry) and in
their elemental state they exist as monoatomic gases – gases whose ‘molecules’
are single atoms of the element, since the atoms are reluctant to react with
anything, including one-another. This inertness is due to the fact that they have
stable outer electron shells, with stable octets of electrons (full s and p subshells)
except helium, which has a stable full inner shell. The electronic configurations
are:
Helium (He): 1s2
Neon (Ne): 1s2 2s2
2p6 Argon (Ar): [Ne]
3s2 3p6
Krypton (Kr): [Ar] 3d10 4s2
4p6 Xenon (Xe): [Kr] 4d10
5s2 5p6 Radon (Rn): [Xe]
5d10 6s2 6p6
Nevertheless, this group does have some interesting chemistry and also exhibit
interesting physical properties. Reactivity increases down the group. Often helium
is included as the first member of the group.
Helium (He)
Helium is chemically a highly unreactive element. It only forms transient species
when electric discharges are passed through a mixture of helium gas and another
gaseous element. for example, passing an electric discharge through a mixture of
helium and hydrogen forms the transient molecule HHe, which has a very short
half-life. HHeF is metastable.
Neon (Ne)
Neon is chemically the most unreactive element. It forms no true compounds, and
no neutral molecules. Ionic molecules are known, e.g. (NeAr)+, (NeH)+, (HeNe)+
and Ne+.
Argon (Ar)
The unstable argon fluorohydride, HArF, is known. Ar also forms clathrates (see
krypton) with water and highly unstable ArH+ and ArF are known. ArF exists as an
excited dimer (excimer) with a half-life of nanoseconds. ArF is thought to be a
Rydberg cluster (Rydberg matter) – a state of matter formed with atoms in a very
high energy state, but not quite ionised. The electron in such atoms is usually
found quite far from the nucleus and can be delocalised into a weak molecular
orbital, which decreases the electron energy slightly. Such clusters generally
1
,contain less than 100 atoms.
Krypton (Kr)
Krypton is also very unreactive. Some clathrate compounds occur – compounds in
which helium atoms are sandwiched or caged in by other molecules without
forming any definite chemical bonds with the encaging molecules. For example,
3C6H4(OH)2·0.74Kr, in which some krypton atoms become trapped inside crystals
of C6H4(OH)2 or para-quinol (p- hydroxybenzene, ‘para’ meaning that the two –OH
groups are opposite one-another):
2
, OH
p-hydroxybenzene
OH
KrF2 is unstable and has the weakest element-fluroine bond energy (50 kJ/mol)
known. it decomposes into Ke and F2 at room temperature (at the rate of about 10%
per hour):
KrF2 → Kr(g) + F2(g)
+ + – –
The ions KrF and Kr F are known to form ionic salts with MFor M F ions, where
M is
2 3 6+ 2 11
antimony (Sb), niobium (Nb) or tantalum (Ta), e.g. KrO2F Sb F –.
2 11
Radon
(Rn)
RnF2 has been reported. The RnF+ ion forms similar salts to the KrF+ ion. The cost
and radioactivity of radon have inhibited a study of its chemistry, which is expected
to be diverse.
The chemistry of xenon is discussed below.
Physical Properties
The elements exist as monoatomic gases at room temperature and pressure, but
can be solidified at extremely low temperatures, due to weak (induced) van der
Waals forces between the atoms, e.g. neon freezes at -249 oC, helium at -270oC.
The larger the atomic mass, the lower the freezing temperature (as the van der
waal’s forces are stronger between larger atoms with more electrons to
contribute to the inter-atomic attractive forces). The solids have the face-centred
cubic (fcc) close-packed arrangement.
Solid xenon is a white crystalline material (fcc) and at high pressures (140 GPa) it
switches to a metallic sky-blue solid with a hexagonal close-packed (hcp)
structure. In this hcp form, the bonding between the atoms is metallic – xenon
bonds with itself under these extreme conditions to form a metal!
Discharge tubes. The gases can all be made to fluoresce. In discharge tubes, such
as neon lights, the inert gas is at low pressure and several thousand volts are
applied across two metallic electrodes. The electric voltage ionises the gas,
completing the circuit, and causing the gas to fluoresce (light be emitted by
electron transitions in the ions). the characteristic colour of the light produced
depends on the element as follows:
Helium (He) pink
3
, Neon (Ne) red-
orange Argon (Ar)
violet Krypton (Kr)
white
Xenon (Xe) blue/lavendar
4
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