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Introduction to Chemistry for JEE Main:
Importa...
Syllabus: Isolation/preparation and properties of the following non-metals: Boron, silicon, nitrogen,
phosphorous, oxygen, sulphur. Properties of allotropes of carbon (only diamond and graphite),
phosphorus and sulphur. Preparation and properties of the following compounds: Boron: Diborane,
boric acid and borax; Aluminium: alumina, aluminium chloride and alums; Carbon: oxides and oxyacid
(carbonic acid); Silicon: silicones, silicates and silicon carbide; Nitrogen: oxides, oxyacids and
ammonia; Phosphorus: oxides, oxyacids (phosphorous acid, phosphoric acid) and phosphine; Oxygen:
ozone and hydrogen peroxide; Sulphur: hydrogen sulphide, oxides, sulphurous acid, sulphuric acid and
sodium thiosulphate.
1. I N ER T PAIR EFFE CT
In the sblock, group I elements are univalent and group II elements are divalent. In group III we would
expect the elements to be trivalent. In most of their compounds this is the case, but some of the elements
show lower valency states as well. There is an increasing tendency to form univalent compounds on
descending the group. Compounds with Ga(I), In(I) and Tl(I) are known. With Ga and In the (I) oxidation
state is less stable than the (III) state. However, the stability of the lower oxidation state increases on
descending the group. Tl(I) thallous compounds are more stable than Tl(III) thallic compounds.
How and why does monovalency occur in group III? The atoms in this group have an outer electronic
configuration of s2p1. Monovalency is explained by the selectrons in the outer shell remaining paired and
not participating in bonding. This is called the ‘inert pair effect’. If the energy required to unpair them
exceeds the energy evolved when they form bonds, then the selectrons will remain paired. The strength
of the bonds in MX3 compounds decreases down the group. The mean bond energy for chlorides are
GaCl3 = 242, InCl3 = 206 and TlCl3 = 153 kJ mol1. Thus the selectrons are most likely to be inert in
thallium.
The inert pair effect is not the explanation of why monovalency occurs in group III.
It merely describes what happens, i.e. two electrons do not participate in bonding. The reason that they do
not take part in bonding is energy. The univalent ions are much larger than the trivalent ions and (I)
compounds are ionic and are similar in many ways to group I elements.
The inert pair effect is not restricted to group III, but also occurs among the heavier elements in
other groups in the pblock. Examples from group IV are Sn2+ and Pb2+ and examples from group V are
Sb3+ and Bi3+. The lower oxidation state becomes more stable on descending the group. Thus, Sn2+ is a
reducing agent but Pb2+ is stable and Sb3+ is a reducing agent but Bi3+ is stable. When the selectrons
remain paired, the oxidation state is always two lower than the usual oxidation state for the group.
Thus, in the sblock, groups I and II show only the group valency. Groups in the pblock show
variable valency, differing in steps of two. Variable valency also occurs with elements in the dblock.
This arises from using different number of delectrons for bonding, so in this case the valency can change
in steps of one (e.g. Cu+ and Cu2+, Fe2+ and Fe3+).
Ashwani Tyagi Sir 2 Code: ATJEE
,Chemistry p-Block Elements
2. TH E G RO U P 13 E LEM EN TS (BO RO N G RO U P)
2.1 GENERAL PROPERTIES
2.1.1. Electronic configuration
Element Electronic configuration ( ns 2 np1 )
5B 1s 2 ,2s 2 2 p 1 or [He] 2s 2 2 p 1
13 Al 1s 2 ,2s 2 2 p 6 ,3 s 2 3 p 1 or [ Ne] 3 s 2 3 p 1
31 Ga 1s 2 ,2s 2 2 p 6 ,3 s 2 3 p 6 3d 10 ,4 s 2 4 p 1 or [ Ar] 3d 10 4 s 2 4 p 1
49 In 1s 2 ,2s 2 2 p 6 ,3 s 2 3 p 6 3d 10 ,4 s 2 4 p 6 4 d 10 ,5 s 2 5 p 1 or [Kr] 4 d 10 5 s 2 5 p1
81 Tl 1s 2 ,2s 2 2 p 6 ,3s 2 3 p 6 3d 10 ,4 s 2 4 p 6 4 d 10 4 f 14 ,5 s 2 5 p 6 5d 10 ,6 s 2 6 p1 or [ Xe] 4 f 14 5d 10 6 s 2 6 p 1
2.1.2. Physical properties
(1) A regular increasing trend in density down the group is due to increase in size.
(2) Melting points do not vary regularly and decrease from B to Ga and then increase.
(3) Boron has very high m.pt because it exist as giant covalent polymer in both solid and liquid
state.
(4) Low m.pt of Ga (29.80C) is due to the fact that consists of only Ga2 molecule; it exist as liquid
upto 20000C and hence used in high temperature thermometry.
(5) Boiling point of these elements however show a regular decrease down the group.
(6) The abrupt increase in the atomic radius of Al is due to greater screening effect in Al (it has 8
electrons in its penultimate shell ) than in B (it has 2 electrons in its penultimate shell)
(7) The atomic radii of group 13 elements are smaller than the corresponding s-block elements.
This is due to the fact that when we move along the period, the new incoming electron occupy
the same shell whereas the nuclear charge increases regularly showing more effective pull of
nucleus towards shell electrons. This ultimately reduces the atomic size.
(8) The atomic radius of Ga is slightly lesser than of Al because in going from Al to Ga, the
electrons have already occupied 3d sub shell in Ga. The screening effect of these intervening
electrons being poor and has less influence to decrease the effective nuclear charge, therefore
the electrons in Ga experience more forces of attractions towards nucleus to result in lower size
of Ga than Al
(9) Oxidation state
(i) All exhibit +3 oxidation state and thus complete their octet either by covalent or ionic union.
(ii) Boron being smaller in size cannot lose its valence electrons to form B3+ ion and it usually
show +3 oxidation no. The tendency to show +3 oxidation no. however decreases down the
group Even Al shows +3 oxidation no. in most of its compounds.
(iii) Lower elements also show +1 ionic state e.g Tl +, Ga+. This is due to inert pair effect. The
phenomenon in which outer shell ‘s’ electrons (ns2) penetrate to (n-1) d electrons and thus
become closer to nucleus and are more effectively pulled the nucleus. This results in less
availability of ns2 electrons pair for bonding or ns2 electron pair becomes inert. The inert pair
effect begins after n 4 and increases with increasing value of n.
Ashwani Tyagi Sir 3 Code: ATJEE
, Chemistry p-Block Elements
(iv) The tendency to form M+ ion increases down the gp. Ga+1 < Tl+1
(10) Hydrated ions : All metal ions exist in hydrated state.
(11) Ionisation energy
(i) Inspite of the more charge in nucleus and small size, the first ionisation energies of this
group elements are lesser than the corresponding elements of s block. This is due to the fact
that removal of electron from a p-orbitals (being far away from nucleus and thus less
effectively held than s-orbitals) is relatively easier than s-orbitals.
(ii) The ionisation energy of this group element decrease down the group due to increases in
size like other group elements.
(iii) However, ionisation energy of Ga are higher than that of Al because of smaller atomic
size of Ga due to less effective shielding of 3d electrons in Ga. Thus valence shell exert
more effective nuclear charge in Ga to show higher ionisation energies.
(12) Electropositive character
(i) Electropositive character increases from B to Tl.
(ii) Boron is semi metal, more closer to non-metallic nature whereas rest all members are
pure metals.
(iii) Furthermore, these elements are less electropositive than s-block elements because of
smaller size and higher ionisation energies.
(13) Oxidation potential
(i) The standard oxidation potentials of these element are quite high and are given below,
B Al Ga In Tl
E0op for M M3++ 3e – +1.66 +0.56 +0.34 +1.26
0 +
E op for M M + e – +0.55 – +0.18 +0.34
(ii) However Boron does not form positive ions in aqueous solution and has very low
oxidation potential.
(iii) The higher values of standard oxidation potentials are due to higher heats of hydration
on account of smaller size of trivalent cations.
(iv) Aluminium is a strong reducing agent and can reduce oxides which are not reduced even
by carbon. This is due to lower ionisation energy of aluminium than carbon. The reducing
character of these elements is Al > Ga > In > Tl.
(14) Complex formation : On account of their smaller size and more effective nuclear charge as
well as vacant orbitals to accept elements, these elements have more tendency to form
complexes than-s block elements.
2.1.3. Chemical Properties
(1) Occurrence: The important of this group elements are given below,
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