Physical properties include physical state, metallic character, melting and boiling points, density, and allotropy. Nitrogen is a diatomic gas, while the remaining elements are solids. As we move down a group, metallic character increases & the ionisation enthalpy of the elements decrease with an increase in their atomic size.
Trends in melting and boiling points:
The melting point increases from nitrogen to arsenic due to the gradual increase in atomic size. The very low melting point of nitrogen is due to its discrete diatomic molecules. On the other hand, the high melting point of arsenic is attributed to its giant layered structure in which the layers are closely packed.
Although the atomic size increases from arsenic to antimony, there is a decrease in their melting points. Although antimony has a layered structure, it has a low melting point than arsenic because of the relatively loose packing of atoms. Furthermore, the melting point of bismuth is less than antimony due to the loose packing of atoms by metallic bonding. On the other hand, the boiling point gradually increases from nitrogen to bismuth.
Density of group fifteen elements:
The density of these elements increases regularly from nitrogen to bismuth.
All the elements in group fifteen, except for bismuth, show allotropy. Nitrogen exists in two allotropic forms, that is, alpha nitrogen and beta nitrogen. Phosphorus exists in many allotropic forms. Of these, the two important allotropic forms are white phosphorus and red phosphorus.
Arsenic exists in three important allotropic forms - yellow, grey and black. Antimony also has three important allotropic forms, namely, yellow, explosive and metallic.
All the elements of group 15 have 5 electrons in their outermost orbit. They need only 3 electrons to complete their octet configuration. The octet can be achieved either by gaining 3 electrons or by sharing 3 electrons by means of covalent bonds. As a result, the common negative oxidation state of these elements is -3. As we move down the group, the tendency to exhibit -3 oxidation state decreases. This is due to the increase in atomic size and metallic character.
Group 15 elements also show positive oxidation states of +3 & +5 by forming covalent bonds. Due to the inert pair affect the stability of +5 oxidation state decreases down the group, while that of +3 oxidation state increases. Nitrogen has only s- and p-orbitals, but no d-orbitals in its valance shell. Therefore, nitrogen can show a maximum covalency of 4.A covalency of four is obtained by sharing its lone pair of electron with another atom or ion.
Phosphorus and the remaining elements can exhibit a covalency of five and a maximum covalency, also called expanded covalency of six. This is possible because of the presence of vacant d-orbitals in the valence shell. All the compounds of group fifteen elements, which exhibit a +5 oxidation state, are covalent.
In case of +3 oxidation state, both ionic and covalent compounds are formed.
In group fifteen elements, the covalent character decreases from nitrogen to bismuth. Nitrogen, because of its smaller size, high electro-negativity and strong tendency to form p pi – p pi multiple bonds, it exhibits various oxidation states from -3 to +5.
|N2O Nitrous oxide
|NO Nitric oxide
|N2O3 Nitrogen trioxide
|N2O4 Nitrogen tetroxide
|N2O5 Nitrogen pentoxide
In case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution.
In phosphorus, nearly all intermediate oxidation states disproportionate into +5 & -3 both in alkali and acid.