Trends in Physical Properties of Group 15 Elements

October 22, 2020

Group 15 of the modified long form periodic table consists of five elements: Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi).
These elements are collectively called the elements of nitrogen family.
Nitrogen and Phosphorus are non-metals, arsenic and antimony are semi-metals or metalloids: bismuth is metallic although not so strong.
Some important physical properties of group 15 elements are given below:

i) Physical state and occurrence

Nitrogen is a gas, while rest of the four elements of this group is solids, under ordinary conditions.
Nitrogen forms about 80% by volume of the earth’s atmosphere. In the combined state, nitrogen exists mostly as nitrates.
Phosphorus is the only element of this group which does not occur free in nature. Phosphorus occurs in nature as phosphates. As, Sb and Bi occur as sulphides or oxides.
Nitrogen exists as diatomic molecule while Phosphorus, arsenic, and antimony as tetra–atomic (P₄, As₄, Sb₄) molecules.

ii) Atomic radii

Atomic radii of Group 15 elements are smaller than those of Group 14 elements.
This is due to the increased effective nuclear charge for Group 15 elements.
The atomic radius gradually increase as one goes from N to Bi. This is due to the addition of new electronic shell at each element down the group.

iii) Ionic radii

Nitrogen, phosphorus form M^3-anions. Arsenic, antimony and bismuth form M³⁺ and M⁵⁺ cations.
The ionic radii of anions are bigger than the corresponding atomic radii. This is due to the increased repulsions between the electrons belonging to the same shell.
The ionic radii of cations of As, Sb and Bi are smaller than atomic radii.

iii) Melting and boiling points

The melting points first increase from N to As and then decrease.
The boiling points however, show a gradual increase down the group (except for As, which sublimes).
Lowering of the melting points for antimony (Sb) and bismuth (bi) is because, due to their inert-pair effect these elements are able to form only three covalent bonds instead of five.

iv) Ionization energy

a) The ionization energies of Group 15 elements are higher than the corresponding values of the Group 14 elements.

This is due to the increased nuclear charge for group 15 elements relative to those of group 14.
The p-orbitals in the valence shell of these elements are half-filled.
This gives extra stability to these electronic configurations. Therefore, the ionization energy of these elements increases.

b) The ionization energy decreases in going from N to Bi down the group. The decrease is however, is not very smooth.

The decrease in the ionization energy down the group is due to an increase in the atomic size in going form N to Bi.

v) Electronegativity

Group 15 elements are more electronegative than Group 14 elements.
Electronegativity of this group of elements shows a gradual decrease in going from N to Bi (down the group).

Group 15 elements have smaller atomic radii and higher nuclear charge as compared to the Group 14 elements. These factors lead to an increase in the electronegativity of these elements as compared to that of the group 14 elements.

The electronegativity decreases in going from N to Bi due to an increase in the atomic size down the group.

vi) Metallic character

The elements of group 15 are less metallic than the elements of Group 14. This is due to smaller atomic radii and higher nuclear charge for group 15 elements.
The metallic character of Group 15 elements increases in going from N to Bi.
First two elements of this group i.e., N and P are non-metals, next two As and Sb are metalloids while Bi is a typical metal.

N, P As, Sb Bi

Non-metals metalloids metal

→ → → Metallic character increases → → →

This increase in the metallic character down the group is due to the increased atomic size of these elements in that direction.

vii) Oxidation states

Group 15 elements have five electrons in their outermost shell i.e., ns²np³.
These elements can therefore complete their outer shell in the following two ways:

a) By electron transfer

To complete their outer shell, these elements should either lose 5 valence electrons or gain 3 more electrons.
Due to their smaller size and higher nuclear charge, nitrogen (N) and phosphorus (P) cannot lose their five valence electrons.
However, these elements can accept three electrons to form nitride (N^3-) and phosphide (P^3-) anions.
Nitrogen, because of its smaller size has higher tendency to form N^3- Phosphorus forms P^3- anions less readily.
Nitrogen and phosphorus form nitrides and phosphides respectively only with highly electropositive elements such as magnesium and calcium.
Other members of Group 15 i.e., As, Sb and Bi do not form M^3- type of anions because of their lower electronegativity.
The heavier elements of this group tend to lose their electrons to form M³⁺ and M⁵⁺cations.
Since, the inert-pair effect becomes more prominent near at the bottom of the group hence Sb and Bi form only M³⁺ cations, i.e., Sb³⁺ and Bi³⁺.
The M⁵⁺ ions do not exist, and the +5 oxidation state is realized only through covalent bonding.

b) By electron sharing

Elements of Group 15 contain 5 valence electrons (ns²np³). Out of these five, three electrons are unpaired electrons (p_x¹, p_y¹ and p_z¹).
Thus, these elements can form three covalent bonds by mutual sharing.
As is in the case of NH₃, PH₃, AsH₃, SbH₃ and BiH₃: the two s-electrons remain on the atoms of these elements as lone pair.
If one of the two s-electrons is promoted to a vacant d-orbital, then the total number of unpaired electrons becomes 5.
As a result, an atom of that element can make five covalent bonds.
Since, all other elements (except N) have d-orbital in their valence shell these elements can form 5 covalent bonds. For example: phosphorus forms PF₅, PCl₅, etc.
The strength of covalent bonding decreases as we go down the group.

viii) Catenation

The catenation tendency of Group 15 elements is less than that of carbon.
This is because of the lower M-M bond energies of the Group 15 elements as compared to that of carbon.

Bond : C – C N – N P – P As – As

Bond energy (kJ/mol) : 353 164 202 147

Nitrogen and phosphorus show catenation tendencies to form the following compounds.
Phosphorus shows a distinct tendency for catenation by forming both cyclic and open chain compounds. Nitrogen shows little tendency for catenation due to lower N-N bond energy.

Nitrogen (N) : H₂N – NH₂(hydrazine), H – N = N≡ N (hydrazoic acid)

Phosphorus (P) : H₂P –PH₂