Group 15 Elements (Nitrogen Family)

Introduction
A world without nitrogen, the fertilizing nutrient; phosphorus, the rigid structural material of bones; or arsenic, the semiconducting material of great importance to electronics—would indeed be different from today. 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.

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Beginning with nitrogen, it turns out that one of the central constituents of amino acids and nucleic acids is nitrogen—the very building blocks of life. It is also required in fertilizers, increasing the yield of farming to ensure the food security of the human population worldwide. Another is phosphorus, found in DNA and in ATP—the energy currency of cells—a fact that underlines its importance to biological systems themselves. Although quite a poisonous element, arsenic still has major uses in semiconductors—the core of most modern electronics. Antimony is used in flame retardants, providing safety to many everyday materials. Bismuth has medical uses because it is nontoxic.

This paper gives an overview with regard to the chemical properties of the Group 15 elements, particularly their oxidation states and reactivity, which have some special behaviors. We then link such properties to applications in respective fields. This is going to help give us insight into why these elements are generally so crucial in our world today, showing us what many roles they play although hidden by the technology and science of how they function. By the completion of this paper, a reader will have grown an appreciation for the nitrogen family and its deep impact on both natural and technological environment contexts.

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 $\Delta H_1<\Delta H_2<\Delta H_3$.

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.

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 the 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 that of bismuth predominantly basic.
Relevance and Applications
The chemical properties of group 15 extend to real life as much as in the academic field. Nitrogen is used to prepare fertilizers, thus the basis for agriculture and therefore the food industry. Phosphorus is a part of DNA; hence, it is indispensable in life sciences and medical research. Arsenic, although very poisonous, is used in semiconductor technology to improve electronic gadgets. Most of the applications of antimony are found in flame retardants that ensure safety from flammability in various materials. On the other hand, the non-toxic feature of bismuth makes it a potential material for medical applications. Bismuth subsalicylate is used in treatments regarding the gastrointestinal tract. Precisely, knowledge about these elements from an academic perspective is essentially about
Inorganic chemistry must therefore rely on the chemical properties of these elements. More important, however, are their diversified oxidation states, patterns of reactivity, and compound formation, which give insight into periodic trends and theories developed for chemical bonding. Studies on the unusual properties of these elements also further develop areas such as materials science, environmental chemistry, and nanotechnology.

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Some Solved Examples

Example 1
Question: The correct statement among the following is:
1)$\left(\mathrm{SiH}_3\right)_3 \mathrm{~N}$ is pyramidal and less basic than $\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$.
2)$\left(\mathrm{SiH}_3\right)_3 \mathrm{~N}$ is planar and less basic than $\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$.
3) $\left(\mathrm{SiH}_3\right)_3 \mathrm{~N}$ is planar and more basic than$\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$.
4) $\left(\mathrm{SiH}_3\right)_3 \mathrm{~N}$ is pyramidal and more basic than $\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$.

Solution: The correct answer is option (2). $\left(\mathrm{SiH}_3\right)_3 \mathrm{~N}$ is planar and less basic than $\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$ 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, $\left(\mathrm{SiH}_3\right)_3 \mathrm{~N}$ is less basic than $\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$ 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. These enrich one's knowledge about chemistry and how it works in the living world that surrounds a person.