1. p-Block
elements: Elements belonging to groups
13 to 18 of the periodic table are called p-block elements.
2. General
electronic configuration of p-block elements: The p- block elements are
characterized by the ns2np1-6 valence shell electronic configuration.
3. Representative
elements: Elements belonging to the s and p-blocks in the periodic table
are called the representative elements or main group elements.
4. Inert
pair effect: The tendency of ns2 electron
pair to participate in bond formation decreases with the increase in atomic
size. Within a group the higher oxidation state becomes less stable with
respect to the lower oxidation state as the atomic number increases. This trend
is called ‘inert pair effect’. In other
words, the energy required to unpair the electrons is more than energy released
in the formation of two additional bonds.
5. Nitrogen
family: The elements of group 15 – nitrogen (N), phosphorus (P), arsenic
(As), antimony (Sb) and bismuth (Bi) belong to configuration is ns2np3.
6. Oxygen
family: Group 16 of periodic table consists of five elements – oxygen (O),
sulphur (S), selenium (Se), tellurium (Te) and polonium (Po). Their general
electronic configuration is ns2np4.
7. The
halogen family: Group 17 elements, fluorine (F), chlorine (Cl), bromine
(Br), iodine (I) and astatine (At),
belong to halogen family. Their general electronic configuration is ns2np5.
8. Group 18
elements: Helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and
radon (Rn) are Group 18 elements. They are also called noble gases. Their
general electronic configuration is ns2np6 except helium which has electronic
configuration 1s2.
They are called noble gases because they show very low chemical reactivity.
GROUP 15 ELEMENTS
9. 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.
10. 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.
11.
Allotropy:
All elements of Group 15 except nitrogen show allotropy.
12.
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 to show –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 disproportionation.
3 HNO2 ® HNO3 + H2O
+ 2 NO
14. 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.
15.
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 and hence the acidic character of oxides
decreases and basicity increases down the group.
16.
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.
17.
Reactivity
towards metals: All elements react with metals to form binary compounds in
–3 oxidation state.
18. Anomalous
behaviour of nitrogen: The behaviour
of nitrogen differs from rest of the elements.
Reason:
i.
It has
a small size.
i. It does not have d – orbitals
ii. It has high electronegativity
iii. It has high ionization enthalpy.
Click here for Online Notes on "p-Block elements" (Chapter=>7 ) Part 1
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