Group 15 elements – The Nitrogen Family

The contemporary periodic table, devised by Dimitri Mendeleev, lists all known elements according to their atomic number, which is unique to each element. The periodic table was created as a result of such an arrangement. The items with comparable qualities were grouped together in a column.

Nitrogen, phosphorus, arsenic, antimony, and bismuth are all part of Group 15. There is a transition from non-metallic to metallic through metalloidic property as we move down the group. Non-metals are nitrogen and phosphorus, metalloids are arsenic and antimony, while bismuth is a typical metal.

Occurrence of Group 15 Elements

• Molecular nitrogen makes over 78% of the atmosphere’s volume. It is found in the earth’s crust as sodium nitrate, NaNO3 (also known as Chile saltpeter), and potassium nitrate (Indian saltpeter).
• Plants and animals both contain it in the form of proteins. Phosphorus is found in the apatite family of minerals, which are the major constituents of phosphate rocks.
• Phosphorus is a mineral that is found in both animal and plant matter. It can be found in both living and dead cells. Milk and eggs both contain phosphoproteins. Sulphide minerals are the most common form of arsenic, antimony, and bismuth.

Periodic Trends in Group 15 Elements

As you proceed through the Group 15 elements, starting with the lightest and ending with the heaviest, you’ll see a general flow in attributes as you go down the list. For example, nitrogen is a non-metal gas, but as we progress down the group, we meet metalloids, and finally metal, such as Bismuth. These periodic table patterns aid in the understanding of atom behaviour as well as the prediction of new elements.

• Electronic Configuration

The electrical configuration of these elements’ valence shells is ns² np³. These elements have entirely filled s orbitals and half-filled p orbitals, making their electrical structure extremely stable.

• Atomic and Ionic Radii

Radii of covalent and ionic (in a certain state) compounds grow in larger as they progress through the group. From N to P, the covalent radius increases dramatically. However, there is only a modest increase in covalent radius from As to Bi. The presence of totally filled d and/or f orbitals in heavier members explains this. 

• Ionization Enthalpy

Due to the progressive rise in atomic size, the ionization enthalpy drops down the group. The ionization enthalpy of group 15 elements is substantially greater than that of group 14 elements in the equivalent periods due to the extra stable half-filled p orbitals electrical configuration and smaller size. As expected, the order of consecutive ionization enthalpies is 

∆H₁ < ∆H₂ < ∆H₃ 

• Electronegativity

With increasing atomic size, the electronegativity value generally drops down the group. However, the gap is less significant among the heavier elements.

Physical Properties

• This group’s elements are all polyatomic. All other elements are solids except dinitrogen, which is a diatomic gas.
• The group’s metallic aspect grows stronger as it progresses. Non-metals are nitrogen and phosphorus, metalloids are arsenic and antimony, and bismuth is a metal. This is owing to an increase in atomic size and a decrease in ionisation enthalpy.
• In general, boiling points rise from top to bottom in the group, although melting points rise until arsenic and then fall until bismuth.
• All elements, with the exception of nitrogen, exhibit allotropy.

Chemical Properties

• Oxidation states and trends in chemical reactivity

These elements’ most common oxidation states are –3, +3, and +5. Because of the increase in size and metallic nature, the tendency to exhibit –3 oxidation state diminishes down the group. Bismuth, the group’s final component, seldom produces any compounds in the –3 oxidation state. The stability of the +5 oxidation state reduces as you progress through the group. BiF₅ is the only Bi(V) compound that has been thoroughly studied. The stability of the +5 oxidation state reduces as the group progresses, while the stability of the +3 state improves (owing to the inert pair effect). When nitrogen combines with oxygen, it has oxidation states of + 1, + 2, and + 4. In several oxoacids, phosphorus has oxidation states of +1 and +4.

Since only four orbitals (one s and three p) are available for bonding, nitrogen can only have a maximum covalency of four. The heavier elements contain unoccupied d orbitals in their outermost shells that can be utilised for bonding (covalency) and so enlarge their covalence, as seen in PF6^–.

• Anomalous properties of nitrogen

Nitrogen is distinguished from the other members of this group by its small size, high electronegativity, high ionization enthalpy, and lack of d orbital availability. Nitrogen has a unique ability to create p-p multiple bonds with itself and other tiny, electronegativity-rich elements (e.g., C, O). This group’s heavier elements do not form p-p bonds because their atomic orbitals are too vast and diffuse to overlap effectively. Thus, nitrogen is a diatomic molecule having a triple bond between the two atoms (one s and two p). As a result, its bond enthalpy is extremely high. 

Phosphorus, arsenic, and antimony, on the other hand, form single bonds as P–P, As–As, and Sb–Sb, respectively, whereas bismuth forms metallic bonds in its elemental state. However, because of the small bond length, the single N–N bond is weaker than the single P–P bond due to the high interelectronic repulsion of the non-bonding electrons. As a result, the catenation tendency in nitrogen is weaker. Another factor influencing nitrogen chemistry is the lack of d orbitals in its valence shell.

Sample Problems

Question 1: What happens to the size of atoms of elements of p-block as we move from left to right in the same period?

Answer:

In the same time span, the size of the atoms of the elements decreases from left to right. The electrons are added to the same shell because the row is the same. The increase in atomic number, on the other hand, represents the increase in protons, i.e. the positive charge. As a result, the total effective nuclear charge rises. As a result, the electron cloud is dragged even closer to the atom’s nucleus. As a result, the size shrinks.

Question 2: What is covalency?

Answer:

The number of electrons that an atom can share to form chemical bonds is referred to as its covalency. It is usually the number of bonds formed by the atom. 

Question 3: What is the maximum covalency of the nitrogen atom?

Answer:

Nitrogen atoms can share up to four electrons, one in the s-subshell and three in the p-subshell. Furthermore, the lack of d-orbitals limits its covalency to four.

Question 4: Why does nitrogen show a poor tendency towards catenation?

Answer:

The N – N single bond is extremely weak and unstable due to the high magnitude of inter-electronic repulsions of non-bonding electrons, which is caused by the short bond length of the single bond. As a result of the aforementioned factors, the catenation tendency weakens, resulting in instability.

Question 5: What is the primary product of the Haber-Bosch process?

Answer:

Ammonia is the primary byproduct of the Haber-Bosch process.