Group 15 Elements (Nitrogen Family)

Group 15 elements, alias nitrogen family, are all essentially very important in our lives and their surroundings. Such members include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). These elements have the ability to underpin subjects such as agriculture, medicine, and technology. This is an important topic in Chemistry

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This Story also Contains

1. Group 15 Elements
2. Some Solved Examples
3. Summary

Group 15 Elements

The family of group 15 or pnictogens consists of five electrons within the last shell and has somewhat diversified chemical behaviors. Nitrogen is found most abundantly in the atmosphere of the Earth as a diatomic gas, while Phosphorus and remaining heavier elements—arsenic, antimony, and bismuth—come as solids at room temperature. These diversities in physical state give way to some interesting chemical properties. The general electronic configuration of these elements is ns²np³. A regular trend of variation from non-metallic to metallic character, as we move down a group, is observed. Nitrogen and phosphorus are already established as non-metals whereas arsenic and antimony are metalloids. Bismuth indeed is a metal.

Electronic configuration

The valence shell electronic configuration of these elements is ns²np³. The s orbital in these elements is completely filled and the 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. 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.

Ionisation Enthalpy

Ionization enthalpy decreases down the group due to a gradual increase in atomic size. Because of the extra stable half-filled p orbitals electronic configuration and smaller size, the ionization enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods. The order of successive ionization enthalpies, as expected is ΔH1<ΔH2<ΔH3.

Electronegativity

The electronegativity value, in general, decreases down the group with increasing atomic size. However, amongst the heavier elements, the difference is not that pronounced.

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 a 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. Due to high electronegativity and the small size of nitrogen, NH₃ exhibits hydrogen bonding in the solid as well as the liquid state. Because of this, it has higher melting and boiling points than that of PH₃.

Oxidation States

Group 15 elements exhibit mainly three oxidation states, +3 and +5. Nitrogen has a -3 state in most of its compounds including ammonia NH₃ used in fertilisers. Phosphorus, arsenic and antimony have two states, +3 and +5. Thus phosphoric acid is a substance of composition H₃PO₄ and arsenic pentoxide is As₂O₅. The more metallic element however is bismuth; As a result, the +3 state predominates with bismuth compared with the others. This phenomenon could be attributed to the inert pair effect, whereby the s² electron pair in the valence shell is stabilized and the +5 is less stable.

Reactivity with Hydrogen

Group 15 elements form hydrides with hydrogen: XHz. Their stability decreases when moving down the group. Notable hydrides are ammonia NH₃, phosphine PH₃, arsine AsH₃, stibine SbH₃, and bismuthine BiH₃. The first, NH₃ is very stable and is applied to a lot of industrial processes, the next, PH₃ is less stable and highly toxic; subsequently, AsH₃, SbH₃, and BiH₃ are increasingly less stable and of less commercial significance.

Reactivity towards halogens

These elements react to form two series of halides: EX₃ and EX₅. Nitrogen does not form pentahalide due to the non-availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides. This is due to the fact that in pentahalides +5 oxidation state exists while in the case of trihalides +3 oxidation state exists. Since elements in +5 oxidation state will have more polarising power than in +3 oxidation state, the covalent character of bonds is more in pentahalides. All the trihalides of these elements except those of nitrogen are stable. In the case of nitrogen, only NF₃ is known to be stable. Trihalides except BiF₃ are predominantly covalent in nature

Reactivity with Oxygen

These elements also occur as oxides in different oxidation states. Nitrogen occurs as N₂O, NO, NO₂, and N₂O₅ from anesthetics to pollutants. Phosphorous occurs as P₄O₆ and P₄O₁₀, in fertilizers and flame-retardants. Arsenic, antimony, and bismuth form their respective oxides like As₂O₃, Sb₂O₃, and Bi₂O₃ which find effective use in glass and pigment industries.

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), Na₃As (sodium arsenide), Zn₃Sb₂ (zinc antimonide) and Mg₃Bi₂ (magnesium bismuthide).

Anomalous properties of nitrogen

Nitrogen differs from the rest of the members of this group due to its small size, high electronegativity, high ionization enthalpy, and non-availability of d orbitals. Nitrogen has the 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 seldom form pπ-pπ bonds as their atomic orbitals are so large and diffused that they cannot have effective overlapping. Thus, nitrogen exists as a diatomic molecule with a triple bond (one sigma and two pi) 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 an elemental state. However, the single N–N bond is weaker than the single P–P bond because of the 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 that 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., R₃P = O or R₃P = CH₂ (R = alkyl group). Phosphorus and arsenic can form dπ – dπ bonds also with transition metals when their compounds like P(C₂H₅)₃ and As(C₆H₅)₃ act as ligands.

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Some Solved Examples

Example 1
Question: The correct statement among the following is:
1)(SiH₃)₃N is pyramidal and less basic than (CH₃)₃N.
2)(SiH₃)₃N is planar and less basic than (CH₃)₃N.
3) (SiH₃)₃N is planar and more basic than(CH₃)₃N.
4) (SiH₃)₃N is pyramidal and more basic than (CH₃)₃N.

Solution: The correct answer is option (2). (SiH₃)₃N is planar and less basic than (CH₃)₃N because the lone pair of the nitrogen atom is in an unhybridized p-orbital and it overlaps with the d-orbital of the silicon atom. This makes the structure planar in nature. Additionally, (SiH₃)₃N is less basic than (CH3)3 N because the lone pair of electrons on the nitrogen atom is in conjugation with all the silicon atoms, reducing its availability for donation.

Example 2
Question: Which of the following has the highest metallic character?
1) P
2) As
3) Sb
4) Bi

Solution: The correct answer is option (4). Bismuth (Bi) has the highest metallic character among the given elements. As we move down Group 15 in the periodic table, the metallic character increases. Nitrogen and phosphorus are non-metals, arsenic, and antimony are metalloids, and bismuth is a metal. This trend is due to the decrease in ionization energy and the increase in atomic size down the group, making bismuth the most metallic element among the given options.

Example 3
Question: Which of the following has the highest melting point?
1) P
2) As
3) Sb
4) Bi

Solution: The correct answer is option (3). Antimony (Sb) has the highest melting point among the given elements. The melting points of Group 15 elements do not show a regular trend due to differences in bonding types among metals and non-metals. However, the melting point increases up to arsenic and then decreases as we move down to bismuth. This is because arsenic has a metallic nature, and due to the increase in metallic nature going down the group, the interactions become weaker, causing the melting point to increase up to arsenic and then decrease.

Summary

The elements from nitrogen to bismuth offer a wide range of interesting chemical properties related to many industries and areas of academic study. Their oxidation states, their reactivity with hydrogen and oxygen, and their applications make them very important elements financial. Nitrogen finds application in agriculture, phosphorus in biological systems, arsenic in electronics, antimony in safety materials, and bismuth in medicine—some of the quite diverse ways through which they contribute to modern life.