An Element is one of a limited class of substances encompasses completely of atoms that have an invariant nuclear charge and which cannot be further divided by ordinary chemical methods. An Isotope is a set of atoms that have the same number of protons and neutrons in the nucleus. Elements are divided into various groups. In this section, we are focussing on group 3A elements. Boron is the first element in group 3A and it occurs naturally in borate minerals. Although not widespread, large deposits of the borate minerals are found in the arid regions of the world. Purified borax or sodium tetraborate dehydrate, Na2B4O7 .10H2O, is soft, colourless, and used in fertilizers.
Group 3A elements have the outer electron configuration of ns2np1.
The atomic radii of group 3A elements increase down the group due to the addition of one extra shell. The atomic radius of aluminium is greater than the atomic radius of gallium because of the additional 10d-electrons which offers weak screening effect for the outer electrons from the resulting increased nuclear charge in gallium.
There is a gradual change in the ionization energy of group 13A. Ionization energy decreases from boron to aluminium due to the increase in size. However, the trend changes from indium to thallium due to inability of the d and f electrons which have low screening effect that aims at compensating the increase in nuclear energy.
As you move down the group, the electronegativity of group 13A decreases from boron to aluminium. From gallium to thallium, electronegativity increases due to the discrepancies in the size of group 13A elements.
Physical properties of group 3A elements
Pure boron is black, lustrous, and extremely hard but brittle.
Boron is a metalloid/ semiconductor
Boron has a unique high melting point due to the presence of strong crystalline lattice in its structure.
Exists in different forms of an element.
The remaining members of group 13A exists as soft metals which contain delocalized electrons that help them in conducting electricity. At temperatures below 303K, gallium exists as a liquid. It has a high melting point of approximately 2676K, making it a useful substance that can be used to measure high temperatures. The density of group 13A increases as one moves down the group.
Chemical properties of group 3A
Oxidation state and trends in reactivity of group 3A
The 1st, 2nd and 3rd ionization energy of boron is very high because of its small size. This inhibits boron from forming a cation with +3 charge but allows it to form covalent substances. However, as you move from boron to aluminium, the ionization energy decreases making aluminium to form a cation of +3. The ionization energy further decreases as you move down the group, from aluminium to thallium. This is due to the poor shielding effect on the positive nucleus. Gallium, indium and thallium have both +1 and +3 oxidation states. The stability of the +1 oxidation state mainly increases in larger elements like Gallium, indium and thallium. (Al < Ga < In < Tl).
In thallium +1 oxidation state is predominant and + 3 oxidation state highly oxidizing in character. The compound in +1 oxidation state, as expected from energy considerations, is more ionic than those in + 3 oxidations state. In trivalent state, the number of electrons around the central atom in a molecule of the compounds of these elements will be only six. The possibility of such electron-deficient molecules to receive/accept the lone pair of electrons is very high. After accepting the lone pair electron(s), the molecules achieve stable electronic configuration and thus, behave as Lewis acids. The tendency to behave as Lewis acid decreases with the increase in the atomic size as one moves down the group. Boron chloride easily receives unpaired electronsfrom ammonia to form a complex BCl3. NH3In trivalent state, most of the compounds being covalent are hydrolysed in water. The trichloride on hydrolysis in water form tetrahedral[M(OH)4]- species; Aluminium chloride in acidified aqueous solution form octahedral [ Al (H2O)6]3+ ion.
Reaction of group3A elements with air
Boron is unreactive in crystalline form while aluminium reacts with air to form a very thin oxide layer on the surface of the metal which protects the metal from further chemical reaction. When heated in air, both amorphous boron and aluminium metal forms B2O3 and Al2O3 respectively. They react with dinitrogen at high temperature they form nitrides. The nature of these oxides varies down the group. Boron trioxide is acidic and reacts with basic oxides forming metal borates. Both aluminium and gallium oxides are amphoteric in nature and those of indium and thallium are basic in their properties.
Reaction of group13A with acids and alkalies
Boron doesn’t react with acids and bases. On the other hand, aluminium reacts with mineral acids and aqueous alkalies and thus shows amphoteric character. Aluminium reacts with dilute hydrochloric acid to produce hydrogen gas. However, the reaction between nitric acid starts but stops after sometimes due to the formation of a protective layer of aluminium oxide that prevents further reaction. Aluminium can also react with aqueous base and produces hydrogen gas.