GROUP 15 ELEMENTS
1. Atomic and ionic radii: Covalent and ionic radii increase down the group.
There is appreciable increase in covalent radii from N to P. There is small
increase from As to Bi due to presence of completely filled d or f orbitals in
heavy elements.
2. Ionisation energy: It goes on decreasing down the group due to increase in
atomic size. Group 15 elements have higher ionisation energy than group 14
elements due to smaller size of group 15 elements. Group 15 elements have
higher ionization energy than group 16 elements because they have stable
electronic configuration i.e., half filled p-orbitals.
3. Allotropy: All elements of Group 15 except nitrogen show allotropy.
Catenation: Nitrogen shows catenation to some extent due to triple bond but
phosphorus shows catenation to maximum extent. The tendency to show
catenation decreases down the group. 13. Oxidation states: The common
oxidation states are +3, +5, –3. The tendency shows –3 oxidation state decreases
down the group due to decrease in electronegativity which is due to increase in
atomic size. The stability of +5 oxidation state decreases whereas stability of +3
oxidation state increases due to inert pair effect. Nitrogen shows oxidation states
from –3 to +5. Nitrogen and phosphorus with oxidation states from +1 to +4
undergo oxidation as well as reduction in acidic medium. This process is called
disproportionate.
3 HNO2 → HNO3 + H2O + 2 NO
4. Reactivity towards hydrogen: All group 15 elements from trihydrides,
MH3. Hybridisation - sp3
The stability of hydrides decrease down the group due to decrease in bond
dissociation energy down the group.
NH3 > PH3 > AsH3 > SbH3 > BiH3
Boiling point: PH3 < AsH3 < NH3 < SbH3 < BiH3
Boiling point increases with increase in size due to increase in van der Waals
forces. Boiling point of NH3 is more because of hydrogen bonding.
Bond angle: NH3 (107.8°) > PH3 (99.5°) > AsH3 (91.8°) ≈ SbH3 (91.3°) > BiH3
(90°)
Electronegativity of N is highest. Therefore, the lone pairs will be towards
nitrogen and hence more repulsion between bond pairs.
Therefore bond angle is the highest. After nitrogen, the electronegativity
decreases down the group.
Basicity decreases as NH3 > PH3 > AsH3 > SbH3 < BiH3.
This is because the lone pair of electrons are concentrated more on nitrogen and
hence the basicity will be maximum in the case of NH3. It will decrease down
the group as the electronegativity decreases down the group.
The reducing power of hydrides increases down the group due to decrease in
bond dissociation energy down the group.
5. Reactivity towards oxygen: All group 15 elements from trioxides
(M2O3) and pentoxides (M2O5).
Acidic character of oxides decreases and basicity increases down the group.
This is because the size of nitrogen is very small. It has a strong positive field in
a very small area. Therefore, it attracts the electrons of water’s O-H bond to
itself and release H+ ions easily. As we move down the group, the atomic size
increases. Hence, the acidic character of oxides decreases and basicity increases
as we move down the group.
6. Reactivity towards halogen: Group 15 elements form trihalides and
pentahalides.
Trihalides – covalent compounds and become ionic down the group. sp3
hybridisation , pyramidal shape
Pentahalides - sp3d hybridisation, TBP shape
They are lewis acids because of the presence of vacant d – orbitals.
PCl5 + Cl- → [PCl6]-
PCl5 is ionic in solid state and exist as [PCl4] + [PCl6]-
In PCl5, there are three equatorial bonds and two axial bonds.
The axial bonds are longer than equatorial bonds because of greater repulsion
from equatorial bonds.
Nitrogen does not form pentahalides due to absence of d- orbitals.
7. Reactivity towards metals: All elements react with metals to form binary
compounds in –3 oxidation state.
8. Anomalous behaviour of nitrogen: The behaviour of nitrogen differs from
rest of the elements.
Reason:
i. It has a small size.
ii. It does not have d – orbitals
iii. It has high electronegativity
iv. It has high ionization enthalpy
1. Atomic and ionic radii: Covalent and ionic radii increase down the group.
There is appreciable increase in covalent radii from N to P. There is small
increase from As to Bi due to presence of completely filled d or f orbitals in
heavy elements.
2. Ionisation energy: It goes on decreasing down the group due to increase in
atomic size. Group 15 elements have higher ionisation energy than group 14
elements due to smaller size of group 15 elements. Group 15 elements have
higher ionization energy than group 16 elements because they have stable
electronic configuration i.e., half filled p-orbitals.
3. Allotropy: All elements of Group 15 except nitrogen show allotropy.
Catenation: Nitrogen shows catenation to some extent due to triple bond but
phosphorus shows catenation to maximum extent. The tendency to show
catenation decreases down the group. 13. Oxidation states: The common
oxidation states are +3, +5, –3. The tendency shows –3 oxidation state decreases
down the group due to decrease in electronegativity which is due to increase in
atomic size. The stability of +5 oxidation state decreases whereas stability of +3
oxidation state increases due to inert pair effect. Nitrogen shows oxidation states
from –3 to +5. Nitrogen and phosphorus with oxidation states from +1 to +4
undergo oxidation as well as reduction in acidic medium. This process is called
disproportionate.
3 HNO2 → HNO3 + H2O + 2 NO
4. Reactivity towards hydrogen: All group 15 elements from trihydrides,
MH3. Hybridisation - sp3
The stability of hydrides decrease down the group due to decrease in bond
dissociation energy down the group.
NH3 > PH3 > AsH3 > SbH3 > BiH3
Boiling point: PH3 < AsH3 < NH3 < SbH3 < BiH3
Boiling point increases with increase in size due to increase in van der Waals
forces. Boiling point of NH3 is more because of hydrogen bonding.
Bond angle: NH3 (107.8°) > PH3 (99.5°) > AsH3 (91.8°) ≈ SbH3 (91.3°) > BiH3
(90°)
Electronegativity of N is highest. Therefore, the lone pairs will be towards
nitrogen and hence more repulsion between bond pairs.
Therefore bond angle is the highest. After nitrogen, the electronegativity
decreases down the group.
Basicity decreases as NH3 > PH3 > AsH3 > SbH3 < BiH3.
This is because the lone pair of electrons are concentrated more on nitrogen and
hence the basicity will be maximum in the case of NH3. It will decrease down
the group as the electronegativity decreases down the group.
The reducing power of hydrides increases down the group due to decrease in
bond dissociation energy down the group.
5. Reactivity towards oxygen: All group 15 elements from trioxides
(M2O3) and pentoxides (M2O5).
Acidic character of oxides decreases and basicity increases down the group.
This is because the size of nitrogen is very small. It has a strong positive field in
a very small area. Therefore, it attracts the electrons of water’s O-H bond to
itself and release H+ ions easily. As we move down the group, the atomic size
increases. Hence, the acidic character of oxides decreases and basicity increases
as we move down the group.
6. Reactivity towards halogen: Group 15 elements form trihalides and
pentahalides.
Trihalides – covalent compounds and become ionic down the group. sp3
hybridisation , pyramidal shape
Pentahalides - sp3d hybridisation, TBP shape
They are lewis acids because of the presence of vacant d – orbitals.
PCl5 + Cl- → [PCl6]-
PCl5 is ionic in solid state and exist as [PCl4] + [PCl6]-
In PCl5, there are three equatorial bonds and two axial bonds.
The axial bonds are longer than equatorial bonds because of greater repulsion
from equatorial bonds.
Nitrogen does not form pentahalides due to absence of d- orbitals.
7. Reactivity towards metals: All elements react with metals to form binary
compounds in –3 oxidation state.
8. Anomalous behaviour of nitrogen: The behaviour of nitrogen differs from
rest of the elements.
Reason:
i. It has a small size.
ii. It does not have d – orbitals
iii. It has high electronegativity
iv. It has high ionization enthalpy
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