General Introduction (Group 15 Elements)
Introduction to Group 15 Elements
Also known as Pnictogens.
Elements include: Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), Bismuth (Bi) and Moscovium (Mc)
Non-metals – Nitrogen and phosphorus
Metalloids – Arsenic and antimony
Metals – Bismuth and Moscovium
Electronic configuration and Oxidation States
Valence shell electronic configuration: (ns^2np^3). s-orbital is completely filled while p-orbitals are half filled making them extra stable.
| Element | Atomic Number | Electronic Configuration | Common Oxidation States |
|---|---|---|---|
| Nitrogen (N) | 7 | ([He]2s^22p^3) | -3, -2, -1, 0, 1, 2, 3, 4, 5 |
| Phosphorus (P) | 15 | ([Ne]3s^23p^3) | -3, 3, 5 |
| Arsenic (As) | 33 | ([Ar]3d^{10}4s^24p^3) | 3, 5 |
| Antimony (Sb) | 51 | ([Kr]4d^{10}5s^25p^3) | 3, 5 |
| Bismuth (Bi) | 83 | ([Xe]4f^{14}5d^{10}6s^26p^3) | 3, 5 |
| Moscovium (Mc) | 115 | ([Rn] 5f^{14}6d^{10}7s^27p^3) | 3, 5 |
The tendency to exhibit oxidation state -3 decreases down the group as metallic character increases.
All the elements have five electrons in their outermost shell. They show maximum oxidation state of five while forming bonds with oxygen. However, the tendency to form compounds with oxidation state of five decreases down the group due to inert pair effect. Inertness of s electrons lead to involvement of only p electrons in bond formation and favors the trivalency. The only bismuth compound with oxidation state +5 is (ce{BiF5}). Oxidation states of Nitrogen in their oxides include : +1 in (ce{N2O}), +2 in (ce{NO}), +3 in (ce{HNO3}), +4 in (ce{NO2}) and +5 in (ce{HNO3}). However, oxidation state of nitrogen is not +5 for its compounds with halogens. It is so because it does not have d-orbitals to exhibit covalency more than 4.
Intermediate oxidation states (+1 to +4) of Nitrogen tend to disproportionate in acidic medium. Intermediate oxidation states of Phosphorous also disproportionate into +5 and -3 oxidation states in acidic as well as basic medium.
Except Nitrogen, all elements in the group have covalency more than four like (ce{PF^-6}). The reason is presence of d-orbitals in addition to one s and three p orbitals.
Occurrence
Nitrogen
Molecular Nitrogen – 78% by volume of atmosphere
Sodium nitrate (ce{NaNO3}) (Chile Saltpetre) and Potassium nitrate (ce{KNO3}) (Indian Saltpetre) – Nitrogen compounds on earth’s crust
Proteins – Nitrogen compounds in plants and animals
Phosphorous
Minerals of Apatite family ((ce{Ca9(PO4)6.CaX2}; X = F, Cl, or OH) ) found in Phosphate rocks
Bones and living cells ; Phosphoproteins in milk and eggs.
Arsenic, Antimony and Bismuth: found in sulphide minerals.
Moscovium: synthetic radioactive element; very short half life and availability in very little amount
Trends in physical properties
Atomic radius and Ionic Radius:
The radius increases down the group as new shell is added. Considerable increase in radius from N (70 pm) to P (110 pm), but comparatively lower increase from P to As (121 pm) as d-orbitals are filled. Similarly, a lower increase from As to Sb(141 pm) as d orbitals are filled. The decrease is much smaller from As to Bi (148 pm) as d and f both orbitals are filled.
Similar impact of presence of filled d and f orbitals are observed in their ionic radii too.
Ionization Enthalpy
Ionization enthalpy decreases down the group as atomic size increases. In the corresponding periods, the 15th group elements have higher ionization enthalpy as compared to the 14th group elements due to smaller radius and half filled extra stable electronic configuration. e.g. (Delta_i H) for N > (Delta H_i) for C.
The order of successive ionization enthalpies follows the order; (Delta_i H_3) < (Delta_i H_2) < (Delta_i H_1).
Electronegativity
Electronegativity of the elements decreases down the group due to increase in the atomic size. However, the difference in electronegativity for heavier elements is not pronounced.
Other Physical Properties
Due to increase in atomic size and decrease in ionization enthalpy, the metallic character increases down the group. All the elements are polyatomic. Nitrogen is diatomic gas while others are solid. Melting point increases down the group but up to Arsenic only. It then decreases up to Bismuth. Boiling point increases down the group. All the elements except Nitrogen show allotropy.
Chemical properties with hydrogen, oxygen and halogens.
Anomalous properties of nitrogen
Nitrogen has small size, high ionization enthalpy, high electronegativity and lacks d-orbitals. This, in total, make it anomalous as compared to other members of the 15th group. Small sized p-orbitals undergo effective overlapping resulting in (p pi ) – (p pi ) multiple bonds with itself and other small and highly electronegative elements like C and O. p-orbitals in heavier elements of the group are too large in size and diffuse to have effective overlapping. The nitrogen exists as diatomic molecules (ce{N2} ) having triple bond between them. As a result, the bond strength is too high for nitrogen and it makes it less reactive at room temperature. Phosphorus, arsenic and antimony forms single bond while bismuth forms metallic bond in elemental state.
However, bond strength of N-N single bond is weaker as compared to that of P-P bond. This is attributed to strong interelectronic repulsion of non-bonding electron pairs in nitrogen which is an outcome of small bond length. This is also the reason that catenation tendency is weaker in nitrogen.
Moreover, nitrogen cannot make (d pi ) – (p pi ) bonds or (d pi ) – (d pi ) with other elements or transition elements as heavier elements of the group can do. For example, (ce{R3P = O} ), (ce{R3P = CH2}) (R = alkyl group), (ce{P(C2H5)3}), (ce{As(C6H5)3}) etc. This is due to lack of d-orbitals in nitrogen.
Reactivity towards Hydrogen
- The elements form hydrides of type (ce{EH3}) where E = N, P, As, Sb, or Bi.
- Bond dissociation enthalpies of the hydrides decrease down the group. This results in decrease in stability of the hydrides. As a consequence, reducing character of hydrides also increases. Ammonia is mild reducing agent while (ce{BiH3}) is strongest reducing agent. Basic character
- Basic character of hydriides follow the order (ce{NH3 > PH3 > AsH3 > SbH3 geq BiH3})
- (ce{NH3}) exihibits H-bonding in its liquid as well as solid state due to high electronegativity and small atomic size. Thus, it has higher melting and boiling point as compared to (ce{PH3})
Reactivity towards oxygen
- Group 15 elements form oxides of type (ce{E2O3}) and (ce{E2O5}).
- Oxides (ce{E2O5}) are more acidic than (ce{E2O3})
- Acidic characteristics of the oxides decrease down the group.
- (ce{E2O3}) oxides of nitrogen and phosphorus are purely acidic, those of arsenic and antimony are amphoteric and those of bismuth are basic.
Reactivity towards halogens
- All pnictogens except nitrogen form halides of types (ce{EX3}) and (ce{EX5}). Nitrogen forms only trivalent halide as it does not have d orbitals in its valence shell
- Pentahalides are more covalent than trihalides. This is due to higher polarizing power of element in +5 oxidation state as compared to that in +3 oxidation state.
- Nitrogen halides except (ce{NF3}) are unstable.
- Trihalides of other elements are stable,
- (ce{BiF3}) is ionic in nature. All other trihalides are predominantly covalent.
Reactivity towards metals
- They show -3 oxidation states in binary compounds with metals.
- Examples: (ce{Ca3N2})(calcium nitride) (ce{Ca3P2})(calcium phosphide), (ce{Na3As})(sodium arsenide), (ce{Zn3Sb2})(zinc antimonide) and (ce{Mg3Bi2})(magnesium bismuthide)
() (ce{})
