Noble Gases (Group 18) Elements
Group 18 Elements
Group 18 consists of six elements- helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). All these are gases and chemically unreactive. They form very few compounds. Because of this, they are termed noble gases.
(i) Occurrence: All the noble gases except radon occur in the atmosphere. Their atmospheric abundance in dry air is ~ 1% by volume of which argon is the major constituent. Helium and sometimes neon is found in minerals of radioactive origin e.g., pitchblende, monazite, cleveite. The main commercial source of helium is natural gas. Xenon and radon are the rarest elements of the group.
The most abundant element in the air is Ar. Order of abundance in the air is Ar > Ne > Kr > He > Xe.
(ii) Electronic Configuration: All noble gases have general electronic configuration ns2np6 except helium which has 1s2. Many of the properties of noble gases including their inactive nature are ascribed to their closed-shell structures.
(iii) Ionisation Enthalpy: They have very high ionization enthalpy because of completely filled orbitals. Ionization enthalpy decreases down the group because of an increase in size.
(iv) Atomic radii: Increases down the group because the number of shells increases down the group.
(v) Electron Gain Enthalpy: Since noble gases have stable electronic configurations, they have no tendency to accept the electron and therefore, have larger positive values of electron gain enthalpy.
(vi) Melting and boiling point: Low melting and boiling point because only weak dispersion forces are present.
(vii) Chemical Properties: The reactivity of noble gases has been investigated occasionally ever since their discovery, but all attempts to force them to react to form the compounds were unsuccessful for quite a few years. In March 1962, Neil Bartlett, then at the University of British Columbia, observed the reaction of a noble gas. First, he prepared a red compound which is formulated as O2+ PtF6 –. He then realized that the first ionization enthalpy of molecular oxygen (1175 kJ mol–1) was almost identical to that of xenon (1170 kJ mol–1). He made efforts to prepare the same type of compound with Xe+PtF6– by mixing PtF6 and Xenon. After this discovery, a number of xenon compounds mainly with the most electronegative elements like fluorine and oxygen, have been synthesized.
(viii) Xenon-fluorine compounds: Xenon forms three binary fluorides, XeF2, XeF4 and XeF6 by the direct reaction of elements under suitable conditions.
Xe (g) + F2 (g) 673K, 1 bar XeF2(s)
(xenon in excess) →
Xe (g) + 2F2 (g) 873K, 7 bar XeF4(s)
(1:5 ratio) →
Xe (g) + 3F2 (g) 573K, 60-70bar XeF6(s)
(1:20 ratio) →
XeF6 can also be prepared by the interaction of XeF4 and O2F2 at 143K.
XeF4 +O2F2 →XeF6 +O2
XeF2, XeF4 and XeF6 are colourless crystalline solids.
They are powerful fluorinating agents.
They are readily hydrolyzed even by traces of water. For example, XeF2 is hydrolyzed to give Xe, HF, and O2.
2XeF2 (s) + 2H2O (l) → 2Xe (g) + 4 HF(aq) + O2(g)
Noble Gases (Group 18) Elements
(ix) Structures: XeF2 and XeF4 have linear and square planar structures respectively. XeF6 has seven electron pairs (6 bonding pairs and one lone pair) and thus, have a distorted octahedral structure.
(x) Xenon – Oxygen Compound:
(a) XeO3: It is obtained by the hydrolysis of XeF4and XeF6 with water.
6XeF4 + 12 H2O → 4Xe + 2XeO3 + 24 HF + 3 O2
XeF6 + 3 H2O → XeO3 + 6 HF
(b) XeOF2andXeOF4: Partial hydrolysis of XeF6 gives oxyfluorides, XeOF4 and XeO2F2.
XeF6 + H2O → XeOF4 + 2 HF
XeF6 + 2 H2O → XeO2F2 + 4HF
XeO3 is a colourless explosive solid and has a pyramidal molecular structure. XeOF4 is a colourless volatile liquid and has a square pyramidal molecular structure.
Noble Gases (Group 18) Elements
(xi) Uses of Noble Gases:
(a) Helium is used in filling balloons for meteorological observations. It is also used in gas-cooled nuclear reactors. Liquid helium is used as a cryogenic agent for carrying out various experiments at low temperatures. It is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood.
(b) Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes. Neon bulbs are used in botanical gardens and in greenhouses.
(c) Argon is used to provide an inert atmosphere in high-temperature metallurgical processes and for filling electric bulbs. It is also used in the laboratory for handling substances that are air-sensitive.
(d) Xenon and Krypton are used in light bulbs designed for special purposes.
Noble Gases (Group 18) Elements
