Nitrogen Family- Group 15 Elements

Nitrogen Family:

The elements Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi) together constitute group 15 of the long form of Periodic table and are collectively known as members of nitrogen family.

General Trends in Physical Properties of Nitrogen Family:

(i) Electronic Configuration- The general outer electronic configuration of group 15 elements in ns² np³ (n = 2 to 6) which shows that these elements have 5 electrons in the valence shell, two in s-orbital and three in p-orbital.

Element / Symbol	Atomic Number	Electronic Configuration
Nitrogen, N	7	1s², 2s² 2p³ or [He] 2s² 2p³
Phosphorous, P	15	1s², 2s² 2p⁶, 3s² 3p³ or [Ne] 3s² 3p³
Arsenic, As	33	1s², 2s² 2p⁶, 3s² 3p⁶ 3d¹⁰, 4s² 4p³ or [Ar] 3d¹⁰, 4s² 4p³
Antimony, Sb	51	1s², 2s² 2p⁶, 3s² 3p⁶ 3d¹⁰, 4s² 4p⁶ 4d¹⁰, 5s² 5p³ or [ Kr] 4d¹⁰, 5s² 5p³
Bismuth, Bi	83	1s², 2s² 2p⁶, 3s² 3p⁶ 3d¹⁰, 4s² 4p⁶ 4d¹⁰ 4f¹⁴, 5s² 5p⁶ 5d¹⁰, 6s² 6p³ or [Xe] 4f¹⁴, 5d¹⁰, 6s² 6p³

(ii) Density- On moving down the group from N to Bi, there is a regular increase in density with the increase in atomic number.

(iii) Atomic radii and Ionic radii- The atomic radii of group 15 elements are smaller than corresponding elements of group 14 due to their greater nuclear charge. However, on moving down the group from N to Bi, atomic radii show an increasing trend due to an increase in screening effect which overcomes the effect of increased nuclear charge. Like atomic radii, ionic radii of group 15 elements also shows an increasing trend on moving down the group.

(iv) Melting points and Boiling points- The melting points do not show regular trend on moving down the group from N to Bi. There is an increase in melting points from N to As and then decrease from As to Bi.

(v) Electronegativity- On moving down the group from N to Bi, there is a gradual decrease in electronegativity with the increase in atomic number. However, it remains the same from Sb to Bi.

(vi) Ionisation energy- Because of small atomic size and high nuclear charge than group 14 elements, the ionisation energy of group 15 elements are comparatively higher than the group 14 elements. However, on moving down the group from N to Bi, the ionisation energy goes on decreasing due to increase in atomic size leading to a decrease in the forces of attraction between the nucleus and valence electrons.

(vii) Electropositive Character ( or Metallic Character)- The elements of group 15 are less electropositive than group 14 elements because of their high ionisation energy. Electropositive (or metallic) character increases on moving down the group 15 due to decrease in ionisation energy. Example- N and P are distinctly non-metals, As and Sb are metalloids while Bi is true metal.

(viii) Oxidation States- The elements of group 15 show two types of oxidation states-

Positive Oxidation States- The outer electronic configuration of group 15 elements is ns² np³ which shows that these elements exhibit oxidation states of +3 (using only three p-electrons in bonding) and + 5 (involving all the 5 valence electrons in bonding). On moving down the group, the tendency to show +5 oxidation state decreases and that of +3 oxidation state increases, because with the increase in atomic mass, the s-electrons become inert due to inert pair effect and only p-electrons are involved in bonding. Example- Bi shows only +3 oxidation state while oxidation state of N and P in N₂O₅ and P₂O₅ is +5.
Negative Oxidation States- These elements require three electrons to acquire the stable noble gas configuration. Nitrogen because of its small size and high electronegativity can accept electrons to form Nitrides (N^3-). Example- Mg₃N₂, Ca₃N₂ etc. thereby showing an oxidation state of -3. It also shows the -3 oxidation state in NH₃ (forming covalent bonds). The tendency to show -3 oxidation state decreases on moving down the group due to a decrease in electronegativity. Phosphorous shows -3 oxidation state in PH₃ and Phosphides (P^3-) but to a lesser extent. The other elements of the group form covalent compounds with more electropositive elements thereby showing an oxidation state of -3. Example- sodium arsenide (Na₃As), zinc antimonide (Zn₃Sb₂) and magnesium bismuthide (Mg₃Bi₂).

(ix) Catenation- The elements of group 15 show the property of catenation but to a much less extent than group 14 elements. It further goes on decreasing on moving down the group with the rise in atomic number. Example- N- can form chain extending up to three N-atoms (e.g. Azide in N₃^–) while in other members of the family, these chains are limited only up to two atoms. Example- diphosphine (P₂H₄).