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 covalence. The tendency to show +3 covalence however decreases down the group Even Al shows +3 covalence 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.
(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 (table) 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
E0op 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.
