OCCURRENCE OF GP 15 ELEMENTS

Molecular nitrogen comprise 78% by volume of the atmosphere. In the earth’s crest, it occurs as Sodium nitrate NaNO₃ (called Chine salt petre) and Potassium nitrate KNO₃ (Indian saltpeter. It is found in the form of proteins in plants and animals. Phosphorus occurs in minerals of the apatite family, which is mixed salt of phosphate and fluorides (halides) of calcium ([Ca₃(PO₄)₂]₃.CaX₂) e.g. fluorapatite, Ca₉(PO₄)₆.CaF₂ photosphorous is an essential constituent of animal and plant matter. It is present in bones as well as in living cells. Phospho-proteins are present in milk and eggs. Arsenic, Antimony and Bismuth are found mainly as sulphide minerals.

Anomalous Behavior of Nitrogen:-

Nitrogen, the first member of group 15 elements, shows anomalous behavior and differs from rest of the members of its family it is due to:
(i) Exceptionally small atomic size.  (ii) High electronegativity

(iii) High ionization energy.       (iv) Non availability of d-orbitals in its valence shell.

Some of the properties of Nitrogen in which it differs from rest of the members of its group are:-

(1) pπ – pπ multiple bonds:- Due to the small size and high electronegativity value, Nitrogen forms pπ – pπ multiple bonds. Hence it exists as dinitrogen (N₂). Because of small surface area N₂ exists as gas. The other elements of the group do not have a tendency to from π-bonds hence exist as tetra-atomic molecules such as P₄, As₄, Sb₄, etc. Due to their large size they have strong intermolecular forces of attraction and exist as solids at room temp.

(2) Catenation:- Catenation is the property of self linking of the elements. The elements of group 15 show the property of catenation, but to a much smaller extent than carbon. It is due to the weak bond strength of these elements than that of C – C bond (347 KJ/mole). Nitrogen has a very little tendency to form catenation due to large inter-electronic repulsions. Phosphorous has maximum tendency to form Catenation. It forms cyclic as well as open chain compounds.

(3) Reactivity:- Due to the presence of a triple bond between two Nitrogen atoms, the bond length is very small: hence the bond dissociation energy is very high. As a result, N₂ is inert and unreactive in its elemental state. Other elements form only the single bonds hence have low bond dissociation energy and are reactive.

(4) pπ – dπ multiple bonds:- Nitrogen due to non-availability of d-orbitals has no tendency to form pπ – dπ bonds. Such type of bonding is very prominent in R₃P = CH₂.

(5) Formation of pentavalent compounds:- Nitrogen contains no d – orbital in its outermost orbit, hence  it cannot expand its octet and does not show pentavalency. But other elements of the group have vacant d – orbitals and can expand their octet hence shows pentavalency. Therefore NF₅, NCl₅, R₃N = O type compounds do not exist while PCl₅, R₃P = O type compounds exists.