Chemistry Group 15 Elements

Topics Covered :

● Occurrence
● Electronic Configuration
● Atomic and Ionic Radii
● Ionisation Enthalpy
● Electronegativity
● Physical Properties
● Chemical Properties
● Anomalous Properties of Nitrogen

Introduction :

`=>` We know that the p-block elements are placed in groups `13` to `18` of the periodic table.

`=>` Their valence shell electronic configuration is `color{red}(ns^2, np^(1-6))` (except `color{red}(He)` which has `color{red}(1s^2)` configuration). The properties of `p`-block elements like that of others are greatly influenced by atomic sizes, ionisation enthalpy, electron gain enthalpy and electronegativity.

`=>` The absence of `d`-orbitals in second period and presence of `d`- and/or `f`-orbitals in heavier elements (starting from third period onwards) have significant effects on the properties of elements.

`=>` In addition, the presence of all the three types of elements; metals, metalloids and non-metals bring diversification in chemistry of these elements.

Group 15 Elements :

`=>` Group 15 includes nitrogen, phosphorus, arsenic, antimony and bismuth.

`=>` As we go down the group, there is a shift from non-metallic to metallic through metalloidic character.

`=>` Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a typical metal.

Occurrence :

`=>` Molecular nitrogen comprises `78%` by volume of the atmosphere. In the earth’s crust, it occurs as sodium nitrate, `color{red}(NaNO_3)` (called Chile saltpetre) and potassium nitrate (Indian saltpetre).

`=>` It is found in the form of proteins in plants and animals.

`=>` Phosphorus occurs in minerals of the apatite family, `color{red}(Ca_9(PO_4)_6. CaX_2 (X = F, Cl text(or) OH))` (e.g., fluorapatite `color{red}(Ca_9 (PO_4)_6. CaF_2)`) which are the main components of phosphate rocks. Phosphorus is an essential constituent of animal and plant matter.

`=>` It is present in bones as well as in living cells.

`=>` Phosphoproteins are present in milk and eggs.

`=>` Arsenic, antimony and bismuth are found mainly as sulphide minerals.

`=>` The important atomic and physical properties of this group elements along with their electronic configurations are given in Table 7.1.

Trends of some of the atomic, physical and chemical properties of the group are discussed below.

Electronic Configuration :

`=>` The valence shell electronic configuration of these elements is `color{red}(ns^2 np^3)`.

`=>` The `s`-orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable.

Atomic and Ionic Radii :

`=>` Covalent and ionic (in a particular state) radii increase in size down the group.

`=>` There is a considerable increase in covalent radius from `N` to `P`.

`=>` But 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.

Ionisation Enthalpy :

`=>` Ionisation enthalpy decreases down the group due to gradual increase in atomic size.

`=>` Because of the extra stable half-filled `p`-orbitals electronic configuration and smaller size, the ionisation enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods.

`=>` The order of successive ionisation enthalpies, as expected is `color{red}(ΔiH_1 < ΔiH_2 < ΔiH_3)` (Table 7.1).

Electronegativity :

`=>` The electronegativity value, in general, decreases down the group with increasing atomic size.

`=>` But for the heavier elements, the difference is not that much pronounced.

Physical Properties :

`=>` All the elements of this group are polyatomic.

`=>` Dinitrogen is a diatomic gas while all others are solids.

`=>` Metallic character increases down the group.

`=>` Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a metal.

`=>` This is due to decrease in ionisation enthalpy and increase in atomic size.

`=>` The boiling points, in general, increase from top to bottom in the group but the melting point increases upto arsenic and then decreases upto bismuth. Except nitrogen, all the elements show allotropy.

Chemical Properties :

Trends in some of the chemical properties is given as follows :

Oxidation states and trends in 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 due to increase in size and metallic character.

`=>` Last member of the group, bismuth hardly forms any compound in `-3` oxidation state.

`=>` The stability of `+5` oxidation state decreases down the group. The only well characterised `Bi (V)` compound is `BiF_5`.

`=>` 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,

`color{red}(3HNO_2 → HNO_3+H_2O +2NO)`

`=>` Similarly, in case of phosphorus nearly all intermediate oxidation states disproportionate into `+5` and `-3` both in alkali and acid.

`=>` However `+3` oxidation state in case of arsenic, antimony and bismuth become increasingly stable with respect to disproportionation.

`=>` 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 `PF_6^-`.

Anomalous properties of nitrogen :

`=>` Nitrogen differs from the rest of the members of this group due to its smaller size, high electronegativity, high ionisation enthalpy and non-availability of `d`-orbitals.

`=>` Nitrogen has unique ability to form `pπ -pπ` multiple bonds with itself and with other elements having small size and high electronegativity (e.g., `C`, `O`).

`=>` Heavier elements of this group do not form `pπ -pπ` bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping.

`=>` Thus, nitrogen exists as a diatomic molecule with a triple bond (one `s` and two `p`) between the two atoms. Consequently, its bond enthalpy (941.4 kJ mol`text()^(–1)`) is very high.

`=>` On the contrary, phosphorus, arsenic and antimony form single bonds as `P–P`, `As–As` and `Sb–Sb` while bismuth forms metallic bonds in elemental state.

`=>` However, the single `N–N` bond is weaker than the single `P–P` bond because of high interelectronic repulsion of the non-bonding electrons, owing to the small bond length. As a result the catenation tendency is weaker in nitrogen.

`=>` Another factor which affects the chemistry of nitrogen is the absence of `d` orbitals in its valence shell.

`=>` Besides restricting its covalency to four, nitrogen cannot form `dπ –pπ` bond as the heavier elements can e.g., `color{red}(R_3P = O)` or `color{red}(R_3P = CH_2)` (`R` = alkyl group).

`=>` Phosphorus and arsenic can form `dπ –dπ` bond also with transition metals when their compounds like `color{red}(P(C_2H_5)_3)` and `color{red}(As(C_6H_5)_3)` act as ligands.

(i) `color{green}("Reactivity towards hydrogen ")` All the elements of Group 15 form hydrides of the type `color{red}(EH_3)` where `color{red}(E = N, P, As, Sb)` or `color{red}(Bi)`.

● Some of the properties of these hydrides are shown in Table 7.2.

● The hydrides show regular gradation in their properties.

● The stability of hydrides decreases from `color{red}(NH_3)` to `color{red}(BiH_3)` which can be observed from their bond dissociation enthalpy.

● As a result, the reducing character of the hydrides increases.

● Ammonia is only a mild reducing agent while `color{red}(BiH_3)` is the strongest reducing agent amongst all the hydrides.

● Basicity also decreases in the order `color{red}(NH_3 > PH_3 > AsH_3 > SbH_3 le BiH_3)`.

(ii) `color{green}("Reactivity towards Oxygen ")` All these elements form two types of oxides : `color{red}(E_2O_3)` and `color{red}(E_2O_5)`.

● The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state.

● Their acidic character decreases down the group.

● The oxides of the type `color{red}(E_2O_3)` of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth is predominantly basic.

(iii) `color{green}("Reactivity towards Halogens" )` : These elements react to form two series of halides : `color{red}(EX_3)` and `color{red}(EX_5)`.

● Nitrogen does not form pentahalide due to non-availability of the `d` orbitals in its valence shell.

● Pentahalides are more covalent than trihalides.

● All the trihalides of these elements except those of nitrogen are stable.

● In case of nitrogen, only `color{red}(NF_3)` is known to be stable.

● Trihalides except `BiF_3` are predominantly covalent in nature.

(iv) `color{green}("Reactivity towards Metals ")` : All these elements react with metals to form their binary compounds exhibiting `–3` oxidation state, such as, `color{red}(Ca_3N_2)` (calcium nitride) `color{red}(Ca_3P_2)` (calcium phosphide), `color{red}(Na_3As_2)` (sodium arsenide), `color{red}(Zn_3Sb_2)` (zinc antimonide) and `color{red}(Mg_3Bi_2)` (magnesium bismuthide).

Q 3080291117

Though nitrogen exhibits +5 oxidation state, it does not form pentahalide. Give reason.


Nitrogen with `n = 2`, has s and p orbitals only. It does not have d orbitals to expand its covalence beyond four. That is why it does not form pentahalide.
Q 3000291118

`PH_3` has lower boiling point than `NH_3`. Why?


Unlike `NH_3, PH_3` molecules are not associated through hydrogen bonding in liquid state. That is why the boiling point of `PH_3` is lower than `NH_3`.