Group 15 Elements - Nitrogen Family

GROUP 15 ELEMENTS - NITROGEN FAMILY 

 

Group 15 includes nitrogen, phosphorus, arsenic, antimony, bismuth and moscovium. 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, bismuth and moscovium are typical metals.

Occurrence of group 15 elements

 

Molecular nitrogen comprises 78% by volume of the atmosphere. 

In the earth’s crust, it occurs as sodium nitrate, NaNO3 (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, Ca9(PO4)6. CaX2 (X = F, Cl or OH) (e.g., fluorapatite Ca9(PO4)6. CaF2) 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.

Moscovium is a synthetic radioactive element. Its symbol is Mc, atomic number 115, atomic mass 289 and electronic configuration [Rn] 5f146d107s27p3. Due to very short half life and availability in very little amount, its chemistry is yet to be established.

 

Atomic and Physical Properties of Group 15 Elements

 

Electronic Configuration of group 15 elements

 

The valence shell electronic configuration of these elements is ns2np3. 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 of group 15 elements        

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. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled and/or f orbitals in heavier members.

 

Ionisation enthalpy of group 15 elements  

Ionisation enthalpy decreases down the group due to a gradual increase in atomic size. Because of the extra stable half-filled 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 ∆iH< ∆iH< ∆iH3 .

 

Electronegativity of group 15 elements      

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.

 

Physical Properties of group 15 elements  

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 of group 15 elements

Oxidation states and trends in chemical reactivity of group 15 elements  

 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. In fact 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 BiF5. The stability of +5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group. Besides +5 oxidation state, nitrogen exhibits + 1, + 2, + 4 oxidation states also when it reacts with oxygen. However, it does not form compounds in +5 oxidation state with halogens as nitrogen does not have d-orbitals to accommodate electrons from other elements to form bonds. 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,

3HNO→ HNO3 + H2O + 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 becomes 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 PF6.

 

Anomalous properties of nitrogen    

  • Nitrogen has a small size, high electronegativity, high ionisation enthalpy, and no availability of d orbitals, making it different from the rest of the members of its group.
  • Nitrogen can form pπ-pπ multiple bonds with itself and other small-sized and highly electronegative elements like carbon and oxygen, which is not possible for heavier elements due to the large and diffuse atomic orbitals.
  • Nitrogen exists as a diatomic molecule with a triple bond between the two atoms due to the unique ability to form pπ-pπ bonds, and the bond enthalpy is high (941.4 kJ/ mol).
  • Phosphorus, arsenic, and antimony form single bonds, while bismuth forms metallic bonds in the elemental state.
  • The single N–N bond is weaker than the single P–P bond due to high interelectronic repulsion of non-bonding electrons, which restricts nitrogen's catenation tendency.
  • The absence of d orbitals in nitrogen's valence shell restricts its covalency to four and prevents it from forming dπ–pπ bonds, which heavier elements can form.
  • Phosphorus and arsenic can form dπ–dπ bonds with transition metals when their compounds.

 

Reactivity of group 15 elements towards hydrogen       

Group 15 elements are also known as the Nitrogen family. The reactivity of group 15 elements towards hydrogen varies depending on the element in the group. Here are the reactivities of group 15 elements towards hydrogen:

1.     Nitrogen (N): Nitrogen is relatively unreactive towards hydrogen, even at high temperatures. Nitrogen and hydrogen can react to form ammonia (NH3), but this reaction requires high pressure, high temperature, and the presence of a catalyst.

2.     Phosphorus (P): Phosphorus reacts with hydrogen to form phosphine (PH3) at room temperature and pressure. The reaction is exothermic, and the energy released can ignite the phosphine gas.

3.     Arsenic (As): Arsenic reacts with hydrogen to form arsine (AsH3) at high temperatures. The reaction is exothermic and can produce a flammable gas mixture.

4.     Antimony (Sb): Antimony reacts with hydrogen to form stibine (SbH3) at room temperature and pressure. The reaction is exothermic and the energy released can ignite the stibine gas.

5.     Bismuth (Bi): Bismuth is relatively unreactive towards hydrogen, even at high temperatures. Bismuth and hydrogen can react to form bismuth trihydride (BiH3), but this reaction is not well studied.

Overall, the reactivity of group 15 elements towards hydrogen increases as you move down the group. Nitrogen is the least reactive, while bismuth is the most unreactive towards hydrogen.

 

Reactivity of group 15 elements towards oxygen

Group 15 elements (nitrogen, phosphorus, arsenic, antimony, and bismuth) react with oxygen to form oxides. Nitrogen forms mainly nitrogen monoxide (NO) and nitrogen dioxide (NO2), while phosphorus forms phosphorus pentoxide (P2O5). Arsenic and antimony form their respective trioxides (As2O3 and Sb2O3), and bismuth forms bismuth trioxide (Bi2O3). The reactivity of these elements towards oxygen increases down the group.

 

Reactivity of group 15 elements towards halogens        

Group 15 elements (also known as pnictogens) include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). The reactivity of these elements towards halogens (Group 17 elements) varies depending on the specific element and conditions.

Here are some general trends in the reactivity of group 15 elements towards halogens:

  • Nitrogen (N) is relatively unreactive towards halogens due to its triple bond, which is difficult to break. However, nitrogen can react with highly reactive halogens like fluorine (F) or chlorine (Cl) under certain conditions, such as high temperatures or in the presence of a catalyst.
  • Phosphorus (P) is more reactive towards halogens than nitrogen, and can react with all halogens except fluorine. The reaction between phosphorus and a halogen produces a binary compound known as a halide, such as phosphorus trichloride (PCl3) or phosphorus pentabromide (PBr5).
  • Arsenic (As) is less reactive towards halogens than phosphorus, but can still react with most halogens to form arsenic halides. For example, arsenic trichloride (AsCl3) and arsenic pentafluoride (AsF5) are both known compounds.
  • Antimony (Sb) reacts with halogens to form antimony halides, such as antimony trichloride (SbCl3) and antimony pentachloride (SbCl5). These compounds are important reagents in organic synthesis and other chemical applications.
  • Bismuth (Bi) is the least reactive of the group 15 elements towards halogens, and generally requires high temperatures or other special conditions to react. Bismuth trichloride (BiCl3) and bismuth triiodide (BiI3) are two examples of bismuth halides.

 

Reactivity of group 15 elements towards metals

Group 15 elements are generally not very reactive towards metals. However, they can form binary compounds with metals under certain conditions. For example, nitrogen can react with alkali metals to form nitrides, while phosphorus can react with certain transition metals to form interstitial compounds. The reactivity of group 15 elements towards metals increases as we move down the group.