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Class notes chemistry Chemistry Simplified NCERT Class 11
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Class notes chemistry Chemistry Simplified NCERT Class 11
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UNIT 11 THE P-BLOCK ELEMENTS The elements in which the last electron enters the p-orbitals of their valence shell are
called P-block elements.
It consists of group 13 to 18 except He.
The valence shell configuration of these elements is ns2 np1―6.
The maximum oxidation state shown by a P-block element is equal to the total number of
valence electrons (i.e., the sum of s and p electrons).
ANOMALOUS BEHAVIOUR OF THE FIRST MEMBER OF A GROUP
The first member in each group differ from that of the other elements in their respective
groups.
This anomalous behaviour is due to
The very small size of the atoms
High Ionisation energies
High electronegativities
Absence of vacant d orbitals
INERT PAIR EFFECT
It is the reluctance of s-electrons to participate in chemical bonding.
It is commonly seen in the elements of groups 13, 14 & 15.
Down the group, due to the poor shielding effect of inner d and f orbitals, the effective
nuclear charge is greater, which holds the s-electrons tightly. So they cannot participate
in bonding.
Due to the above reason, TlCl is more stable than TlCl 3.
Lead mainly forms PbCl2 than PbCl4.
Tl+ is stabler than Tl 3+ and Pb2+ is stabler than Pb4+
GROUP 13 ELEMENTS: THE BORON FAMILY
It includes Boron, Aluminium, Gallium, Indium and Thallium.
Boron is a typical non metal.
Al is a metal.
Ga, In and Tl are almost metallic in character.
Boron mainly occurs as orthoboric acid (H3BO3), borax (Na2B4O7.10H2O) and Kernite
(Na2B4O7.4H2O).
The two isotopic forms of Boron are 10
B and 11
B.
Aluminium is the most abundant metal and the third most abundant element in
the earth’s crust.
Bauxite (Al2O3.2H2O) and Cryolite (Na3AlF6) are the important minerals of aluminium.
Ga, In and Tl are less abundant elements in nature.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM Page | 1
, ATOMIC PROPERTIES
1. ELECTRONIC CONFIGURATION
The outer electronic configuration of these elements is ns2np1.
Boron and Aluminium have noble gas core.
Ga and In have noble gas core plus 10 d electrons.
Thallium has noble gas core plus 14 f electrons plus 10 d electron cores.
2. ATOM RADII
On moving down the group, atomic radius increases.
Atomic radius of Ga is less than that of Al.
This is due to the presence of completely filled d orbitals in Ga.
The presence of 10 d electrons offers only very poor shielding effect for the outer
electrons.
Consequently, the atomic radius of gallium (135 pm) is less than that of Al (143 pm).
3. IONISATION ENTHALPY
Ionisation enthalpy does not decrease smoothly down the group.
This is due to the poor shielding effect of the completely filled inner d and f electrons.
4. ELECTRONEGATIVITY
Down the group, electronegativity first decreases from B to Al and then increases
marginally.
This is because of the variation in atomic size of the elements.
PHYSICAL PROPERTIES
Boron is non metallic in nature.
It is extremely hard and black coloured solid.
It exists in many allotropic forms.
Due to very strong crystalline lattice, boron has unusually high melting point.
The other members of this group are soft metals.
They have low melting point and high electrical conductivity.
Ga with very low melting point (303 K) could exist in liquid state during summer.
Its boiling point is very high. (2676 K).
Therefore, it is used in high temperature thermometers.
CHEMICAL PROPERTIES
1. REACTIVITY TOWARDS AIR
Boron is unreactive in crystalline form.
Aluminium forms a very thin oxide layer on the surface which protects the metal from
further attack.
Amorphous boron and Al metal on heating in air form B2O3 and Al2O3 respectively.
With dinitrogen at high temperature they form nitrides.
4E + 3O2
2E2O3
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM Page | 2
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