CBSE Class 12 Chemistry Notes: The p Block Elements

The elements of groups 15 are nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb) and bismuth (Bi). The first two elements N and P are non – metal. The next two, arsenic and antimony are metalloids whereas the last element, bismuth is a metal. The general electronic configuration of this group is ns²np³.

General Introduction to the nitrogen family members

Nitrogen (N)

Nitrogen gets its name from a mineral known as “niter” (potassium nitrate), from which it can be prepared. It was discovered by a Scottish physician Daniel Rutherford in 1770.

Nitrogen is a fairly common element in the earth’s crust. It is most important constituent of atmosphere, making up to 78.08 %. The greatest use of nitrogen is in ammonia which is used for fertilizer production and to produce nitric acid. Nitrogen gas is used where inert atmosphere is needed, such as in an ordinary light bulb. Nitrogen is also used in protecting historic documents. Nitrogen is also used in storage of food stuff and in explosives.

Nitrogen is absolutely essential to all living organisms. It is important part of all protein molecules which, among other functions, are the building materials in all kinds of cells.

Nitrogen oxides (from combustion) are one of the cause of toxic rain.

Phosphorous (P)

Phosphorous got its name because it glows in the dark. “Phosphorous” is from the greek word phos (light) and phoros (bearer). It was first isolated in 1699 by German physician Henning Brand. He discovered it from urine.

It is a non – metals. As a gas it is colourless, but it occurs in 3 allotropic form in solid state. There are white (or yellow) phosphorous, red phosphorous and black phosphorous. The most common and reactive of which is white (or yellow) phosphorous which looks like a waxy. Transparent solid with a garlic like odour. Red phosphorous is much less reactive. It is red powder and does not dissolve in most liquids.

Black phosphorous is produced by heating white phosphorous in the presence of a mercury catalyst and a seed crystal of black phosphorous. It is least reactive, does not ignite easily.

Commercially, phosphorous compounds are used in the manufacture of phosphorus acid (H₃PO₄) which is found in soft drinks, used in fertilizers. Also phosphorous is used in fireworks, phosphorescent compounds. Phosphorous is used in forming china clay. Also used in glass making.

Arsenic (As)

It was first recognized as an element by alchemists. Although it is not certain where and when arsenic was actually discovered, credit for the discovery usually goes to alchemist Albert the great in 1250 AD.

Arsenic is occasionally found as a free element, but it is usually found in a number of minerals. Elemental arsenic occurs in two solid modifications: Yellow and grey or metallic. It is a steel gray, very brittle, crystalline metalloid. Arsenic does not melt when heated but changes directly in to a vapour (sublimation). Arsenic is stable in dry air, but the surface oxidizes slowly in moist air to give a bronze tarnish and finally a black covering to the element. When heated in air, arsenic combines with oxygen to form arseneous oxide (As₂O₃). A blue flame is produced and the compounds give off distinctive garlic – like odour.

It is necessary ultra trace element for humans, red algae, chickens, rats, goats and pigs. A deficiency of arsenic results in inhibited growth. Most infamous use of arsenic is a poison. Arsenic also used in semi conductor devices along with phosphorus.

Antimony (Sb)

The name antimony is derived from the Latin words “anti + monos” which means “not alone”. The symbol for antimony comes from its original Latin name stibium. It was studied by Nicholos Lemery in 17th century.

The standard state of antimony is solid at 298 K. It is very brittle and flakes easily. Antimony has a bluish – white metallic sheen. It is a poor conductor of heat and electricity. It is known to be severely toxic. It causes damage to the liver and is used in some cases to induce vomiting and sweating.

It is added to iron and iron alloys to increase its hardness, stiffness and reduce corrosion. Antimony sulphides are used in the vulcanization of rubber as well as in pigments.

Bismuth (Bi)

It’s name came from the German word Wissmuth which means “white mass”. Its symbol is Bi from Bismuth. It was distinguished from tin and lead by Claude Geoffroy in 1753.

Bismuth is the heaviest non – radioactive naturally occurring element. It is a hard, brittle metal with an unusually low melting point (271°C). Bismuth is a white – silvery crystalline, brittle metal with a pinkish tinge. It is rare metal usually arrociated with copper, lead, tin, silver and gold ores.

It unusual property to expand upon solidification makes it useful in type – metal alloys and for coating. Bismuth is used in wide variety of pigments ranging form cosmetics to the coloration of glass.

Illustration 1.
Why the first ionization energy of group 15 elements much higher than those of corresponding group 14 elements?

Solution: This is due to increase in nuclear charge and extra stable configuration of the elements of this group. The extra stability of configuration is attributed to the exactly half filled p – orbitals in their valence shells.

Illustration 2.
Why the tendency of catenation is less in nitrogen as compared to carbon?
Solution: The lesser tendency of nitrogen to catenate as compared with carbon can be understood in terms of their bond energies, as listed below.