General characteristic of P- block element.

P-Block elements.

The elements in which the last electrons enters P subshell of the valence shell of their atoms are known as P-block elements. These elements are located at the extreme right of the periodic table. The general valence shell electronic configuration of P-block elements is ns^₂np^1-6.

The general trend of P-Block elements.

The general trend of P-block elements except noble gases reacts with hydrogen, oxygen, and halogen to form various hydrides, oxides, and halide respectively.The element present in noble gases have a characteristic of almost zero electron affinity and also a character of very high ionisation enthalpies.Under normal condition, the element present in the noble gases atom has little tendency to gain or lose an electron.On moving down in the periodic group of these elements show more or less trend is observed in the properties of these compounds.

Electronic configuration.

The general electronic configuration of P-block elements follows ns²np^1-6 which is given below.

General properties.

Boron is a non-metal and always forms covalent bonds. Normally it forms three covalent bonds at 120 degrees. using the sp2 hybrid orbital.There is no tendency to form univalent compounds.All BX3, compounds are electron deficient and may accept an electron pair from another atom, thus forming coordinate bonds. BF3 is commercially important as a catalyst. Boron also forms a large number of compounds in which the boron atoms form an open basket type of structure, and some which are a closed polyhedron.Other atoms such as carbon, may be included in the polyhedron. The bonding in these compounds is off considering theoretical interest and involves multi-center bonds.

The four elements Al,Ga, In, Tl all four forms trivalent compounds. The heavier members show the 'inert pair effect' and universal compounds become increasingly important in the order Ga In Ti. These four elements are more metallic and more ionic than boron. They are moderately reactive metal.Their compounds are on the borderline between the ionic and covalent. Their compounds are on the borderline when anhydrous, but they form ions in solution.

Occurrence and abundance.

Boron is fairly rare elements, but it is well known because it occurs as concentrated deposits of borax Na₂[B₄O₅(OH)₄].8H₂O and kernite Na₂[B₄O₅(OH)₄].2H₂O.

Aluminium is the most abundant metal and the third most abduent elements after oxygen and silicon by weight in the earth's crust.It is well known commercial important. Aluminium metal is produced on a vast scale. Primary production was 19.4 million tonnes in 1992 and is the addition of 5 million tonnes is recycled.

Gallium is twice as abundant as boron, but indium and thallium and are much less common.As three elements, Ga, In, and Th occur as sulphides, Galium, In, and Thallium are not very well known. This is partly because they do not occur as concentrates ore, and partly because there are no major uses for them.Small amounts of Galium are found in ores of the elements adjacent to it in the periodic table.(Aluminium, Zn,and Ge). Traces of In and thallium are found on ZnS and PbS ores.Production of In was 145 tonnes, Ga 28 tonnes and Te 14.5 tonnes in 1993.

Melting Points, Boiling points and Structures.

The melting point of Group 13 elements do not show a regular trend and did the metals of group 1 and 2. The group 13 values are not strictly comparable because B and Ga have unusual crystal structures.

Boran has the unusual crystal structure which results in the melting point being very high.There are at least four different allotropic forms.Boron has insufficient electrons to fill the valence shell even after forming the bonds.The variety and complexity of the allotropic forms illustrate the number of ways in which boron attempts to solve this problem.Other elements solve this problem by metallic bonding, but the small size and high ionisation energy make this impossible to boron. All four allotropic forms contain icosahedral unit with boron atoms at all 12 corners.In this unit, twelve B atoms form a regular shape and each boron atom is bonded to five equivalent neighbours at distance 1.77 degrees Angstrom.The difference between the allotropic forms arises in the way the icosahedral and bonded together.The simplest form is a rhombohedral boron.In this half, the atoms are bonded to one atom in another icosahedral at a distance of 1.71 degrees Angstrom, and one-half the atoms are bonded to atoms in two different icosahedra at a distance of 2.03gegree Angstrom.

The elements Al. In, and Tl, all have close-packed metal structures. Gallium has an unusual structure. Each metal atom has one closed neighbour at a distance of 2.43 degree Angstrom and six more distant neighbours at a distance between 2.70 degrees Angstrom.and 2.79 degrees Angstrom.This remarkable structure tends towards discrete diatomic molecules rather than a metallic structure.This accounts for the incredibly by a low melting point of gallium of 30degree C. Ga is also unusual because the liquid expands when it forms the solid,i,e the solid is less dense than the liquid.

Gallium, Indium and Thallium.'

Gallium is found in bauxite ore, and the ratio of Gallium and Aluminium is about 1/5000. During the Bayer processes for the purification alumina, the concentration of gallium in the alkaline solution gradually increases to about 1/250.Gallium is extracted by electrolysis of this solution. Indium and thallium, occur in minute quantities in ZnS and PbS ores. These sulphites ores are roasted with air in a smelter, to convert them to ZnO and PbO> The Ga and In are recovered from flue dust, and they are extracted by electrolysis of aqueous solutions of their salts.

These metal are soft , silvery coloured, and reactive. They are dissolved in acids. There is no larger scale used for Ga, In and Th, but a small amount of Ga are used to dope crystals to make transistors.This is obtained by zone refining. Galium arsenic is isoelectronic with Ge and is used in the light emitting diode.and a laser diode. Indium is used to dope crystal to make p-n-p transistors and its thermistors.It is also used in low melting soldier and other low melting alloys.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.