Describe the general trends in the following properties of the elements in Groups 13 and 14.
(i) Atomic size
(ii) Ionisation enthalpy
(iii) Metallic character
(iv) Oxidation states
(v) Nature of halides
(i) ATOMIC SIZE
In group 13
Down the group, atomic radius increases and every successive member increases 1 extra shell of the electron while going down the group, but some deviation is seen where atomic radius of gallium is less than that of aluminium, this can be understood by inner core of electronic configuration which says d orbital does not screen effectively whereas s and p orbital can. Gallium has 10 electrons in 3d subshell and each shell of d orbit does the poor screening of nuclear charge than s and p orbital. Therefore nuclear charge increase and atomic radius of gallium which is 135 pm less than that of aluminium which is 143 pm
B < Ga < Al < In < Tl
In group 14
Down the group, covalent radius increases from carbon to silicon and then from Silicon to lead a small increase is observed due to presence of completely filled d and f orbital in heavy members.
C < Si < Ge < Sn < Pb
(ii) IONISATION ENTHALPY
In group 13
Ionisation enthalpy does not decrease down the group as expected. There is a decreasing trend from B to Al with increase in size. The discontinuity occur in Al to Ga and In to Tl due to inability of d and f orbital to screen the nuclear charge. They have low screening effect.
B > TI > Ga > Al > In
In group 14
First Ionisation enthalpy decreases from silicon to tin then show a slight increase in ionisation enthalpy from tin to lead due to the poor shielding effect of d and f orbital as there is a poor screening of nuclear charge in d and f orbital in comparison to s and p orbital. C > Si > Ge > Sn < Pb
(iii) METALLIC CHARACTER
In group 13
Boron is non-metallic in nature and is very hard and is black in colour. Boron also consists of allotropes like carbon. Due to very strong crystal lattice boron has a high melting point, rest are soft metals with less melting point.
In group 14
Carbon and silicons are non-metals, germanium is metalloid and tin and lead are soft metals with less melting point
(iv) OXIDATION STATE
In group 13
Boron has a small size and first 3 ionisation enthalpy are very high, which prevent boron to form +3 oxidation state and force boron to form only covalent compounds. Now when we move down the group ionisation enthalpy drastically decrease and aluminium forms Al3+ and due to poor shielding effect of d and f orbitals, the increase in nuclear charge hold ns electron tightly which shows inert pair effect (effect which shows oxidation state reduce by 2, which is more stable than other oxidation states) and restrict them to participate in bonding and as a result p orbital electron participate in bonding. Ga, In, and Tl shows both +1 and +3 oxidation state. The compound in +1 oxidation state is more ionic than +3 oxidation state.
In group 14
Oxidation state is +2 and +4. +2 oxidation state is common in heavy elements Ge< Sn<Pb because the ns2 electrons of valence shells are unable to participate in bonding. Carbon and Silicon consist of a +4 oxidation state. Germanium is stable in +4 oxidation state and some compound of germanium in +2 states. Tin form compound in both +2 and +4 state and Tin is a reducing agent in +2 states. Lead is a strong oxidising agent in +4 oxidation state and very stable in +2 oxidation state.
(v) NATURE OF HALIDES
In group 13
Element react with halogens to form trihalides except TlI3
2E + 3X2 →2EX3
X can be F, Cl, Br, I
The trichloride, tribromide, triiodide are covalent in nature and hydrolysed in water. The monomeric trihalides are electron deficient so-called as strong Lewis acid.
In group 14
The element can form halides of MX2 and MX4, X can be F, Cl, Br, I.Except carbon all other member react directly with halogen under some condition forming halides. Mostly MX4 is covalent in nature. Central metal goes in sp3 hybridisation forming the tetrahedral shape. SnF4 and PbF4 are exceptions as they are ionic in nature. PbI4 does not exist as it form initially and does not releaseenergy for 6s2 electron and excite one of them to higher orbital so that lead has 4 unpaired electrons around it. Ge and Pb make the formula of MX2. Down the group stability of dihalides increases.