p-Block Elements: Group 18
The Group 18 elements are called noble gases or inert gases because they rarely combine with other elements. They are found only as uncombined elements in nature and some compounds. Nobel gases are colourless at room temperature. Only neon and helium are not yet prepared from their compounds.
The noble gases were discovered and isolated in the 1890s by William Ramsey, Lord Rayleigh, and Morris Travers. Noble gases had actually been first discovered, but not recognized, by Henry Cavendish in 1766. He passed a sequence of electric sparks through a mixture of air and oxygen and collected the gases that were produced at the end of the reaction.Each time he did the experiment, he noticed that around 1% of the gas mixture did not react in the reactor.Ramsay and his colleagues did further experiments along with this analogy and finally isolated a new element, which they called argon. The name argon is derived from the Greek ‘Argos’ meaning lazy or inactive.Once Ramsay had discovered argon, he realised that there was no place in the periodic table for it to fit. He predicted that argon belonged to a whole new group of elements.In 1885 Ramsay began by identifying helium, and in 1888 he isolated neon, krypton and xenon after studying liquid air.Radon was later discovered in 1900.
In the rest of the periodic table, the number of the group is the same as the number of outer shell electrons in the elements of that group.However, this is not true for the noble gases. Helium only has 2 electrons in its outer shell, while the others all have 8. The group’s number is also referred to as 0 because of this.The values of an atomic number of noble gases strongly suggest that they should be placed after halogens and before alkali metals.
Xenon forms a large no. of compounds with oxygen and fluorine in different oxidation states. These are xenon fluorides, xenon oxides and xenon oxyfluoride. Some compounds are discussed below.
|Formula||Name||Oxidn state||Structure||VSEPR explanation|
|XeF2||Xenon difluoride||+2||Linear||Five electron pairs form trigonal bipyramidal with three lone pairs at equatorial positions|
|XeF4||Xenon tetra fluoride||+4||Square Planner||Six electron pairs form octahedron with two positions occupied by lone pairs|
|XeF6||Xenon hexafluoride||+6||Distorted octahedron||Pentagonal bipyramidal or capped octahedron with one lone pair|
|XeO3||Xenon trioxide||+6||Pyramidal tetrahedral (one corner unoccupied)||Three π bonds so that the remaining four electron pairs form a tetrahedron with one corner occupied by a lone pair.|
|XeO2F2||Xenon dioxy difluoride||+6||Trigonal bipyramidal (with one position unoccupied)||Two π bonds so remaining five electron pair form trigonal bipyramid with one equatorial position occupied by a lone pair|
|XeOF4||Xenon oxy tetrafluoride||+6||Square pyramidal (octahedral with one position unoccupied||One π bond so remaining six electron pairs form an octahedron with one position occupied by a lone pair.|
|XeO4||Xenon tetraoxide||+8||Tetrahedral||Four π bonds so remaining four electron pair form a tetrahedron|
|XeO3F2||Xenon trioxy difluoride||+8||Trigonal bipyramidal||Three π bonds so remaining five electron pairs form trigonal bipyramid|
|Ba2[XeO6]-4||Barium perxenate||+8||octahedral||Two π bonds so remaining six electron pair form an octahedron.|