General Properties of Group 15 p – Block elements

 

The elements in which the last electron enters in the valence p-sub shell are called the p-block elements. They include elements from groups 13 to 18. Their general electronic configuration is ns2np1-6 where n = 2 (except He which has 1s2 configuration). They, includes metals, non-metals and metalloids.

 

Elements belonging to the s and p-blocks in the periodic table are called the representative elements or main group elements.

Inert pair effect:  The tendency of ns2 electron pair to participate in a bond formation decreases with the increase in atomic size. Within a group, the higher oxidation state becomes less stable with respect to the lower oxidation state as the atomic number increases. This trend is called the ‘inert pair effect’. In other words, the energy required to unpair the electrons is more than the energy released in the formation of two additional bonds.

General Properties of Group 15 p – Block elements

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 the 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 orbitals in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable.

Nitrogen makes up about 0.002% of the earth’s crust; however, it constitutes 78% of the earth’s atmosphere by volume. Nitrogen is found naturally in animal and plant proteins and in fossilized remains of ancient plant life. Important nitrogen-containing minerals are niter, KNO3, and soda niter, NaNO3​, which are found in desert regions and are important components of fertilizers.

Phosphorus is the eleventh most abundant element, making up 0.11% of the earth’s crust. The main source of phosphorus compounds is phosphorus rocks. Phosphorous is not found pure in nature, but in the form of apatite ores. These include compounds such as fluorapatite (Ca5(PO4)3F), which in fluoridated water is used to strengthen teeth, and hydroxylapatite (Ca10(OH)2(PO4)6), a major component of tooth enamel and bone material. Phosphorus has many applications: phosphorus trichloride (PCl3) is used in soaps, detergents, plastics, synthetic rubber nylon, motor oils, insecticides and herbicides; phosphoric acid, H3PO4​, is used in fertilizers; phosphorus is also prevalent in the food industry, used in baking powders, instant cereals, cheese, the curing of ham, and in the tartness of soft drinks.

Arsenic is a highly poisonous metalloid. Because it is a metalloid, arsenic has a high density, moderate thermal conductivity, and a limited ability to conduct electricity. Compounds of arsenic are used in insecticides, weed killers, and alloys. 

Antimony is obtained mainly from its sulfide ores, and it vaporizes at low temperatures. Along with arsenic, antimony is commonly used in alloys. Arsenic, antimony, and lead form an alloy with desirable properties for electrodes in lead-acid batteries. Arsenic and antimony are also used to produce semiconductor materials such as GaAs, GaSb, and InSb in electronic devices.

Bismuth is a poor metal (one with a significant covalent character) that is similar to both arsenic and antimony. Bismuth is commonly used in cosmetic products and medicine. Bismuth is obtained as a by-product of the refining of other metals, allowing other metals to recycle their by-products into bismuth.

(i) 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.

(ii) Ionisation Enthalpy:  It goes on decreasing down the group due to an increase in atomic size. Group 15 elements have higher ionization energy than group 14 elements due to the 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 ionization enthalpies, as expected is Δi H1 < Δi H2 < Δi H3.

(iii) 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.

General Properties of Group 15 p – Block elements

Chemical Properties:
(i) 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 the 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.

(ii) Anomalous Properties of Nitrogen: Nitrogen differs from the rest of the members of this group due to its smaller size, high electronegativity, high ionization enthalpy, and non-availability of d orbitals. Some of the anomalous properties shown by nitrogen are:

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

1. 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 polyatomic with single bonds.

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

3. Due to the absence of d orbitals in its valence shell, the maximum covalency of nitrogen is four. N cannot form dπpπ bond. While Phosphorus and arsenic can form dπdπ bond with transition metals and with C and O.

(iii) Reactivity Towards Hydrogen: All elements of group 15 react with Hydrogen to form Hydrides of the type EH3 (where E = N, P, As, Sb or Bi). They belong to sp3 hybridization. The stability of hydrides decreases down the group due to a decrease in bond dissociation energy down the group. NH3 > PH3 > AsH3 > SbH3 > BiH3.   

iv) Bond angles: NH3 > PH3 > AsH3 > SbH3 > BiH3
Electronegativity of N is highest. Therefore, the lone pairs will be towards nitrogen and hence more repulsion between bond pairs. Therefore, the bond angle is the highest. After nitrogen, the electronegativity decreases down the group.

v) Bond Dissociation Enthalpy: NH3 > PH3 > AsH3 > SbH3 > BiH3
Bond dissociation enthalpy of E – H decreases from NH3 to BiH3 due to an increase in size and E – H bond length. So, the thermal stability also decreases from NH3 to BiH3.

vi) Boiling Point: PH3 < AsH3 < NH3 < SbH3 < BiH3
Boiling point increases with an increase in size due to an increase in the extent of van der Waal’s forces. The boiling point of NH3 is more because of hydrogen bonding.

vii) Basic Nature: Basic nature depends on the availability of lone pair of electrons. NH3 > PH3 > AsH3 > SbH3 > BiH3.
Size of the central atom increases and the availability of lp of e− for protonation decreases. So, the basic nature decreases.

viii) Reducing Nature: NH3 < PH3 < AsH3 < SbH3 < BiH3
Size of the central atom increases and the thermal stability decreases. Ease in the availability of hydrogen increases.

Ammonia forms hydrogen bonding with water molecules, therefore it is soluble in water, while other hydrides are insoluble in water.

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