Physical properties
(1) Physical state : Nitrogen– (gas), phosphorus – (solid) (vaporises easily), As, Sb, Bi–solids.
Note : Nitrogen is the most abundant gas in the atmosphere. It constitutes about 78% by volume of the atmosphere. Phosphorus is the most reactive element in this group and its yellow form is always kept under water.
(2) Atomic radii : Atomic radii increases with atomic number down the group i.e., from N to Bi due to addition of extra principal shell in each succeding elements.
(3) Ionisation energy : The ionisation values of the elements of this group decreases down the group due to gradual increases in atomic size.
(4) Electronegativity : Generally the elements of nitrogen family have high value of electronegativity. This value shows a decreasing trend in moving down the group from nitrogen to bismuth.
(5) Non-metallic and metallic character : Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids (semi-metal) and bismuth a typical metal.
(6) Molecular state : Nitrogen readily forms triple bond (two pπ –pπ bonds) and exists as discrete diatomic gaseous molecule (N ≡ N) at room temperature. Phosphorus, arsenic and antimony exist in the form of discrete tetra atomic molecules such as P4, As4 Sb4 bonds. In which the atoms are linked to each other by single
(7) Melting and boiling points : The melting points and boiling points of group 15 elements do not show a regular trend.
Note : M.pt. first increases from N to As and then decreases from As to Bi. Boiling point first increases from N to Sb. Boiling point of Bi is less than Sb.
(8) Allotropy : All the members of group 15 except Bi exhibit the phenomenon of allotropy.
(i) Nitrogen exists in two solid and one gaseous allotropic forms.
(ii) Phosphorus exists in several allotropic forms such as white, red, scarlet, violet and black form.
(a) White or yellow phosphorus : White phosphorus is prepared from rock phosphate Ca3(PO4 )2 SiO2 and coke which are electrically heated in a furnace.
2Ca3 + (PO4)2 + 6SiO2 
6CaSiO + P4O10.
P4O10 + 10C P4 + 10CO
When exposed to light, it acquires a yellow colour.
(b) Red phosphorus : It is obtained by heating yellow phosphorus, between 240 –250oC in the presence of an inert gas. Yellow phosphorus can be separated from red phosphorus by reaction with NaOH (aq) or KOH (aq) when the former reacts and the latter remains unreacted.
(iii) Arsenic exists in three allotropic forms namely grey, yellow and black. Antimony also exists in three forms, viz., metallic, yellow and explosive.
(9) Oxidation state : The members of the group 15 exhibit a number of positive and negative oxidation states.
(i) Positive oxidation states : The electronic configuration (ns2 np3) for the valence shell of these elements shows that these elements can have +3 and +5 oxidation states. In moving down this group, the stability of +3 oxidation state increases. It may be pointed out here that nitrogen does not exhibit an oxidation state of +5, because it fails to expand its octet due to nonavailability of vacant d-orbitals.
(ii) Negative oxidation states : For example oxidation state of nitrogen is –3. The tendency of the elements to show –3 oxidation state decreases on moving down the group from N to Bi.
(10) Catenation (self linkage) : Elements of group 15 also show some tendency to exhibit catenation. This tendency goes on decreasing in moving down the group due to gradual decrease in their bond (M–M) energies.
Note :
- Out of the various allotropic forms of phosphorus, black phosphorus is a good conductor of electricity (similarity with graphite).
- Proteins, the building blocks of our body contain 16% of nitrogen in them.
- Radioactive phosphorus (P32 ) is used in the treatment of leukemia (blood cancer).
- The disease caused by the constant touch with white phosphorus is called Phossy Jaw.
