Group 15 NCERT 2

Question 1

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 up to arsenic and then decreases up to bismuth.

• Except nitrogen, all the elements show allotropy

Answer 1

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Question 2

Chemical Properties - 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. 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 characterized 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.
• 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,
3HNO2 -> HNO3 + H2O + 2NO

• Similary in the case of Phosophorus, all intermediate oxidation states disproportionate into +5 and –3 both in alkali and acid.
• +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.

Answer 2

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Question 3

Anomalous properties of Nitrogen

• Differs from the rest of the members of this group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d orbitals.
• Nitrogen has unique ability to form p-pi p-pi 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 p-pi p-pi 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–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 d-pi d-pi bond as the heavier elements can e.g., R3P = O or R3P = CH2 (R = alkyl group).
• Phosphorus and arsenic can form d-pi d-pi bond also with transition metals when their compounds like P(C2H5)3 and As(C6H5)3 act as ligands.

Answer 3

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Question 4

Reactivity towards Hydrogen

• All the elements of Group 15 form hydrides of the type EH3 where E = N, P, As, Sb or Bi.
• The hydrides show regular gradation in their properties. The stability of hydrides decreases from NH3 to BiH3 which can be observed from their bond dissociation enthalpy.
• Consequently, the reducing character of the hydrides increases.
• Ammonia is only a mild reducing agent while BiH3 is the strongest reducing agent amongst all the hydrides. Basicity also decreases in the order NH3 > PH3 > AsH3 > SbH3 > BiH3

Answer 4

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Question 5

Reactivity towards Oxygen

• All these elements form two types of oxides: E2O3 and E2O5.
• 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 E2O3 of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth predominantly basic.

Answer 5

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Question 6

Reactivity towards Halogens

• These elements react to form two series of halides: EX3 and EX5.
• 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 NF3 is known to be stable.
• Trihalides except BiF3 are predominantly covalent in nature

Answer 6

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Question 7

Reactivity towards Metals

All these elements react with metals to form their binary compounds exhibiting –3 oxidation state, such as, Ca3N2 (calcium nitride) Ca3P2 (calcium phosphide), Na3As2 (sodium arsenide), Zn3Sb2 (zinc antimonide) and Mg3Bi2 (magnesium bismuthide).

Answer 7

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