Group 15 Elements (Nitrogen Family)

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:39 PM IST

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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Group 15 Elements
Some Solved Examples
Summary

Group 15 Elements (Nitrogen Family)

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.

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.

Frequently Asked Questions (FAQs)

1. What are the Group 15 elements, and why are they called the nitrogen family?

The Group 15 elements are nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). They are called the nitrogen family because nitrogen is the first and most abundant element in this group. These elements share similar electronic configurations and chemical properties due to their position in the periodic table.

2. How does the electronic configuration of Group 15 elements affect their chemical behavior?

The Group 15 elements have an ns2np3 valence electronic configuration, where n is the highest occupied energy level. This configuration results in three unpaired electrons, allowing these elements to form covalent bonds and exhibit a maximum oxidation state of +5. The presence of a lone pair of electrons also influences their chemical behavior, making them good Lewis bases.

3. Why does the reactivity of Group 15 elements decrease down the group?

The reactivity of Group 15 elements decreases down the group due to increasing atomic size and decreasing electronegativity. As the atomic number increases, the valence electrons are farther from the nucleus and more shielded, making them less available for chemical reactions. This trend results in a gradual change from non-metallic to metallic character down the group.

4. How does the melting point of Group 15 elements vary, and why?

The melting point of Group 15 elements generally increases down the group, with some exceptions. This trend is due to increasing atomic size and stronger metallic bonding in heavier elements. However, nitrogen has a very low melting point because it exists as diatomic molecules (N2) with weak intermolecular forces, while phosphorus has a higher melting point due to its tetrahedral structure (P4).

5. What is the inert pair effect, and how does it affect the chemistry of Group 15 elements?

The inert pair effect is the tendency of the two s electrons in the outermost shell to remain unionized or unshared in heavier elements. This effect becomes more pronounced for elements in lower periods, particularly affecting bismuth in Group 15. As a result, bismuth predominantly exhibits a +3 oxidation state rather than +5, unlike lighter elements in the group.

6. How do the physical states of Group 15 elements change down the group, and why?

The physical states of Group 15 elements change from gaseous (nitrogen) to solid (bismuth) down the group. This transition occurs due to increasing atomic size and stronger interatomic forces. Nitrogen exists as a diatomic gas (N2) with weak intermolecular forces, while heavier elements form more complex structures with stronger bonding, resulting in higher melting and boiling points.

7. Why does nitrogen show a strong tendency to form multiple bonds?

Nitrogen shows a strong tendency to form multiple bonds due to its small atomic size and high electronegativity. The small size allows for effective orbital overlap, while the high electronegativity promotes the sharing of electrons. These factors, combined with nitrogen's need to complete its octet, result in the formation of strong triple bonds in N2 molecules and multiple bonds in other nitrogen compounds.

8. How does the ability to form hydrogen bonds affect the properties of nitrogen compounds compared to other Group 15 elements?

Nitrogen's ability to form hydrogen bonds significantly affects the properties of its compounds. Hydrogen bonding in nitrogen compounds leads to higher boiling points, increased solubility in water, and unique structural properties compared to similar compounds of other Group 15 elements. This is evident in the properties of ammonia (NH3) and its importance in biological systems.

9. Why is phosphorus more reactive than nitrogen, despite being lower in the group?

Phosphorus is more reactive than nitrogen because of its larger atomic size and lower electronegativity. These factors make phosphorus atoms less tightly held and more easily shared in chemical reactions. Additionally, the strong triple bond in N2 molecules makes nitrogen less reactive, while phosphorus exists in more reactive allotropic forms like white phosphorus (P4).

10. How do the oxidation states of Group 15 elements vary, and what causes this variation?

Group 15 elements can exhibit oxidation states from -3 to +5. The most common oxidation states are -3, +3, and +5. However, the stability of higher oxidation states decreases down the group due to the inert pair effect. Nitrogen commonly shows -3 and +5 states, while heavier elements like bismuth prefer the +3 state. This variation is caused by the increasing difficulty in removing all five valence electrons as atomic size increases.

11. What is the significance of the lone pair of electrons in Group 15 elements?

The lone pair of electrons in Group 15 elements plays a crucial role in their chemical behavior. It makes these elements good Lewis bases, allowing them to donate electrons and form coordinate covalent bonds. The lone pair also influences the geometry of molecules containing these elements, often leading to pyramidal structures (e.g., NH3) and affecting their polarity and reactivity.

12. How does the electronegativity of Group 15 elements change down the group, and what are its implications?

The electronegativity of Group 15 elements decreases down the group. Nitrogen is highly electronegative, while bismuth is the least electronegative. This trend affects the nature of bonds formed by these elements. Nitrogen tends to form more polar covalent bonds, while heavier elements form more metallic bonds. This change in bonding character influences the physical and chemical properties of compounds formed by these elements.

13. Why do Group 15 elements form covalent hydrides, and how do these hydrides differ down the group?

Group 15 elements form covalent hydrides (EH3) due to their ability to share electrons and complete their octet. The properties of these hydrides change down the group. NH3 (ammonia) is a strong base and highly soluble in water due to hydrogen bonding, while PH3 (phosphine) is a weaker base with lower solubility. The thermal stability and reducing power of these hydrides generally increase down the group.

14. How does the atomic radius of Group 15 elements compare to elements in the same period?

The atomic radius of Group 15 elements is generally smaller than that of elements in the same period to their left (Groups 13 and 14) but larger than elements to their right (Groups 16, 17, and 18). This trend is due to the increased nuclear charge and electron shielding effects. Within Group 15, the atomic radius increases down the group due to the addition of new electron shells.

15. What is the nitrogen cycle, and why is it important?

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. It involves processes such as nitrogen fixation, nitrification, denitrification, and ammonification. The nitrogen cycle is crucial for life on Earth as it makes nitrogen available to living organisms in usable forms, playing a vital role in the growth and development of plants and other organisms.

16. How do the allotropes of phosphorus differ, and why are they important?

Phosphorus exists in several allotropic forms, including white, red, and black phosphorus. White phosphorus (P4) is highly reactive and poisonous, red phosphorus is less reactive and more stable, while black phosphorus is the most stable form. These allotropes have different structures and properties, leading to varied applications. Understanding phosphorus allotropes is important for their safe handling and use in industries such as match production, fertilizers, and electronics.

17. Why is nitrogen gas (N2) considered inert, and how does this affect its applications?

Nitrogen gas (N2) is considered inert due to the strong triple bond between nitrogen atoms, which requires significant energy to break. This stability makes N2 unreactive under normal conditions, allowing it to be used as an inert atmosphere in various industrial processes, food packaging, and as a coolant. The inertness of N2 also explains why nitrogen fixation (the conversion of N2 to biologically usable forms) is a crucial process in nature and agriculture.

18. How do the oxides of Group 15 elements vary in their acid-base properties?

The oxides of Group 15 elements show a gradual change from acidic to basic character down the group. Nitrogen oxides (e.g., N2O5) are strongly acidic, phosphorus oxides (e.g., P4O10) are acidic, arsenic and antimony oxides are amphoteric, while bismuth oxide (Bi2O3) is basic. This trend is due to the decreasing electronegativity and increasing metallic character of the elements down the group, affecting their ability to donate or accept electrons in acid-base reactions.

19. What is the biological significance of nitrogen and phosphorus in Group 15?

Nitrogen and phosphorus are essential elements for life. Nitrogen is a key component of amino acids, nucleic acids (DNA and RNA), and many other biological molecules. Phosphorus is crucial for energy storage and transfer in the form of ATP (adenosine triphosphate), and it's a major component of DNA, RNA, and cell membranes (as phospholipids). Both elements are vital for plant growth, making them important in agriculture as components of fertilizers.

20. How does the ability to catenate (form chains) vary among Group 15 elements?

The ability to catenate (form chains or rings of atoms of the same element) decreases down Group 15. Nitrogen shows limited catenation, forming small chains or rings in some compounds like hydrazine (N2H4). Phosphorus exhibits stronger catenation, forming various allotropes with complex structures. Arsenic and antimony show some catenation in their elemental forms, while bismuth rarely catenates. This trend is due to decreasing bond strength between atoms of the same element down the group.

21. Why are nitrogen compounds often used as explosives?

Nitrogen compounds are often used as explosives due to their ability to rapidly decompose, releasing large volumes of gas and energy. Many nitrogen-based explosives contain nitrogen-oxygen bonds, which are relatively weak. When triggered, these compounds quickly break down into more stable molecules (often N2 gas), releasing energy and expanding gases. Examples include TNT (trinitrotoluene) and nitroglycerin. The stability of N2 as a product contributes to the effectiveness of these explosives.

22. How does the reactivity of Group 15 elements with oxygen change down the group?

The reactivity of Group 15 elements with oxygen generally decreases down the group. Nitrogen reacts with oxygen only at high temperatures or in the presence of electric sparks, forming various oxides. Phosphorus is highly reactive, spontaneously igniting in air to form oxides. Arsenic and antimony oxidize more slowly, while bismuth forms a protective oxide layer on its surface. This trend is due to decreasing electronegativity and increasing metallic character down the group.

23. What is the role of phosphorus in energy transfer in living organisms?

Phosphorus plays a crucial role in energy transfer in living organisms through the molecule adenosine triphosphate (ATP). ATP stores energy in its phosphate bonds, which can be broken to release energy for cellular processes. When ATP is hydrolyzed to ADP (adenosine diphosphate), energy is released and can be used for various biological functions. The ability of phosphorus to form these high-energy bonds makes it essential for energy metabolism in all living organisms.

24. How do the boiling points of the hydrides of Group 15 elements compare, and why?

The boiling points of Group 15 hydrides (EH3) generally increase down the group, with some exceptions. NH3 (ammonia) has an unusually high boiling point due to hydrogen bonding. PH3 (phosphine) has a lower boiling point than NH3 but higher than AsH3 (arsine). This trend is primarily due to increasing molecular mass down the group. However, the absence of strong hydrogen bonding in hydrides other than NH3 results in lower boiling points than might be expected based solely on molecular mass.

25. Why is the +3 oxidation state more stable for heavier Group 15 elements?

The +3 oxidation state becomes more stable for heavier Group 15 elements due to the inert pair effect. As the atomic number increases, the 's' electrons in the outermost shell become more difficult to remove or share. This effect is particularly noticeable in bismuth, where the +3 state is much more common than the +5 state. The stability of the +3 state in heavier elements is also related to the increasing energy required to remove all five valence electrons to achieve the +5 state.

26. How does the structure of white phosphorus (P4) compare to that of nitrogen (N2)?

White phosphorus (P4) has a tetrahedral structure with four phosphorus atoms at the corners of a tetrahedron, while nitrogen exists as diatomic N2 molecules with a triple bond. This difference arises because phosphorus atoms are larger and can accommodate more atoms in a stable structure. The P-P single bonds in P4 are much weaker than the N≡N triple bond, making white phosphorus highly reactive compared to the stable N2 molecule.

27. What is the significance of nitrogen fixation, and how does it occur?

Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into biologically usable forms like ammonia (NH3). It's crucial because most organisms cannot directly use N2 gas. Nitrogen fixation occurs naturally through lightning and certain bacteria (like those in root nodules of legumes) or can be done industrially through the Haber process. This process is essential for the nitrogen cycle and for producing fertilizers, playing a vital role in agriculture and ecosystem health.

28. How do the magnetic properties of Group 15 elements change down the group?

The magnetic properties of Group 15 elements change from diamagnetic to paramagnetic down the group. Nitrogen and phosphorus are diamagnetic due to their paired electrons in molecular or solid forms. Arsenic, antimony, and bismuth show increasing paramagnetic behavior due to unpaired electrons in their atomic or molecular orbitals. This trend is related to the increasing atomic size and the presence of more unpaired electrons in the heavier elements.

29. Why is phosphorus used in matches, and how does it work?

Phosphorus is used in matches due to its low ignition temperature and high reactivity with oxygen. In safety matches, red phosphorus is on the striking surface, while the match head contains an oxidizing agent. When struck, the friction converts some red phosphorus to white phosphorus, which ignites easily and starts the combustion process. This property of phosphorus allows for the controlled and reliable ignition of matches.

30. How does the tendency to form coordination compounds vary among Group 15 elements?

The tendency to form coordination compounds increases down Group 15. Nitrogen forms relatively few coordination compounds due to its small size and high electronegativity. Phosphorus, arsenic, antimony, and bismuth show an increasing ability to form coordination compounds, with bismuth forming the most stable complexes. This trend is due to the increasing availability of d-orbitals for bonding and the decreasing electronegativity down the group, allowing these elements to act as better Lewis acids.

31. What is the environmental impact of excessive nitrogen and phosphorus in ecosystems?

Excessive nitrogen and phosphorus in ecosystems, often from agricultural runoff or sewage, can lead to eutrophication of water bodies. This process causes algal blooms, which deplete oxygen in the water, leading to fish kills and ecosystem imbalance. Excess nitrogen can also lead to soil acidification and contribute to air pollution through the formation of nitrogen oxides. Understanding and managing the environmental impacts of these elements is crucial for maintaining ecological balance and water quality.

32. How does the ability to form pi bonds change among the Group 15 elements?

The ability to form pi bonds decreases down Group 15. Nitrogen readily forms strong pi bonds, as seen in the triple bond of N2 and in many organic compounds. Phosphorus can form pi bonds but less easily than nitrogen, while heavier elements (As, Sb, Bi) have a much lower tendency to form pi bonds. This trend is due to the increasing atomic size and decreasing orbital overlap efficiency down the group, making it harder for the larger atoms to form effective pi bonds.

33. Why is bismuth considered the least toxic of the heavy metals?

Bismuth is considered the least toxic of the heavy metals due to its low solubility in water and bodily fluids, which limits its absorption and bioaccumulation. Unlike other heavy metals, bismuth compounds are poorly absorbed in the digestive tract and are quickly eliminated from the body. Additionally, bismuth's stable +3 oxidation state makes it less likely to participate in harmful redox reactions in biological systems. These properties make bismuth relatively safe for use in medicines and cosmetics.

34. How does the strength of the E-H bond (where E is a Group 15 element) change down the group, and why?

The strength of the E-H bond (where E is a Group 15 element) generally decreases down the group. The N-H bond is the strongest, while the Bi-H bond is the weakest. This trend is due to increasing atomic size down the group, which leads to less effective orbital overlap between the element and hydrogen. The