p – Block Elements Part - 1a | Class 12th

Group 15 Elements

The elements of group 15 – Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi). This group is also known as Nitrogen family.

Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a typical metal. All the elements of this group are polyatomic. Dinitrogen is a diatomic gas while all others are solids.

The valence shell electronic configuration of these elements is ns²np³. The s orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable.

Ø Atomic and Ionic Radii: Covalent and ionic (in a particular state) radius increase in size down the group. There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled d and / or f orbitals in heavier members.

Ø Ionisation Enthalpy:  It goes on decreasing down the group due to increase in atomic size. Group 15 elements have higher ionisation energy than group 14 elements due to smaller size of group 15 elements. Group 15 elements have higher ionization energy than group 16 elements because they have stable electronic configuration i.e., half-filled p-orbitals. The order of successive ionisation enthalpies, as expected is Δ_i H₁ < Δ_i H₂ < Δ_i H₃.

Ø Electronegativity: The electronegativity value, in general, decreases down the group with increasing atomic size. However, amongst the heavier elements, the difference is not that much pronounced.

ü Chemical Properties:

Ø Oxidation States and trends in a chemical reactivity: The common oxidation states of these elements are – 3, + 3 and + 5. The tendency to exhibit – 3 oxidation state decreases down the group, bismuth hardly forms any compound in –3 oxidation state. The stability of + 5 oxidation state decreases down the group. The only well characterized Bi (V) compound is BiF₅.

The stability of + 5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group.

Nitrogen exhibits +1, + 2, + 4 oxidation states also when it reacts with oxygen. Phosphorus also shows + 1 and + 4 oxidation states in some oxoacids.

In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution. For example: 3 HNO₂ → HNO₃ + H₂O + 2NO.

Nitrogen is restricted to a maximum covalency of 4 since only four (one s and three p) orbitals are available for bonding. The heavier elements have vacant d orbitals in the outermost shell which can be used for bonding (covalency) and hence, expand their covalence as in PF₆^–.

Ø Anomalous Properties of Nitrogen: Nitrogen differs from the rest of the members of this group due to its smaller size, high electro negativity, high ionisation enthalpy and non-availability of d orbitals. Some of the anomalous properties shown by nitrogen are:

1.    Nitrogen has the ability to form pπ-pπ multiple bonds with itself and with other elements like C and O. Other elements of this group do not form pπ-pπ bonds.

2.    Nitrogen exists as a diatomic molecule with a triple bond (one s and two p) between the two atoms. So its bond enthalpy is very high. While other elements of this group are poly atomic with single bonds.

3.        The single N–N bond is weak. So the catenation tendency is weaker in nitrogen.

4.      Due to the absence of d orbitals in its valence shell, the maximum covalency of nitrogen is four.

N cannot form dπ-pπ bond. While Phosphorus and arsenic can form dπ-dπ bond with transition metals and with C and O.