10.1 Group 13 Elements: The Boron Family

This group elements show a wide variation in properties. Boron is a typical non-metal, aluminium is a metal but shows many chemical similarities to boron, and gallium, indium, thallium and nihonium are almost exclusively metallic in character.

Boron is a fairly rare element, mainly occurs as orthoboric acid, \(\left(\mathrm{H}_3 \mathrm{BO}_3\right)\), borax, \(\mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7 \cdot 10 \mathrm{H}_2 \mathrm{O}\), and kernite, \(\mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7 \cdot 4 \mathrm{H}_2 \mathrm{O}\). In India borax occurs in Puga Valley (Ladakh) and Sambhar Lake (Rajasthan). The abundance of boron in earth crust is less than \(0.0001 \%\) by mass. There are two isotopic forms of boron \({ }^{10} \mathrm{~B}\) (19%) and \({ }^{11} \mathrm{~B}\) (81%). Aluminium is the most abundant metal and the third most abundant element in the earth’s crust ( \(8.3 \%\) by mass) after oxygen ( \(45.5 \%\) ) and Si (27.7%). Bauxite, \(\mathrm{Al}_2 \mathrm{O}_3 \cdot 2 \mathrm{H}_2 \mathrm{O}\) and cryolite, \(\mathrm{Na}_3 \mathrm{AlF}_6\) are the important minerals of aluminium. In India it is found as mica in Madhya Pradesh, Karnataka, Orissa and Jammu. Gallium, indium and thallium are less abundant elements in nature. Nihonium has symbol Nh , atomic number 113 , atomic mass \(286 \mathrm{~g} \mathrm{~mol}^{-1}\) and electronic configuration [Rn] \(5 f^{14} 6 d^{10} 7 s^2 7 p^2\). So far it has been prepared in small amount and half life of its most stable isotope is 20 seconds. Due to these reasons its chemistry has not been established.

Nihonium is a synthetically prepared radioactive element. Here atomic, physical and chemical properties of elements of this group leaving nihonium are discussed below.

Electronic Configuration

The outer electronic configuration of these elements is \(n s^2 n p^1\). A close look at the electronic configuration suggests that while boron and aluminium have noble gas core, gallium and indium have noble gas plus \(10 d\) electrons, and thallium has noble gas plus \(14 f\)-electrons plus \(10 d\)-electrons cores. Thus, the electronic structures of these elements are more complex than for the first two groups of elements discussed in unit 10. This difference in electronic structures affects the other properties and consequently the chemistry of all the elements of this group.

Atomic Radii

On moving down the group, for each successive member one extra shell of electrons is added and, therefore, atomic radius is expected to increase. However, a deviation can be seen. Atomic radius of Ga is less than that of Al. This can be understood from the variation in the inner core of the electronic configuration. The presence of additional \(10 d\)-electrons offer only poor screening effect (Unit 2) for the outer electrons from the increased nuclear charge in gallium. Consequently, the atomic radius of gallium ( 135 pm ) is less than that of aluminium ( 143 pm ).

Ionization Enthalpy

The ionisation enthalpy values as expected from the general trends do not decrease smoothly down the group. The decrease from B to Al is associated with increase in size. The observed discontinuity in the ionisation enthalpy values between Al and Ga , and between In and Tl are due to inability of \(d\)-and \(f\)-electrons, which have low screening effect, to compensate the increase in nuclear charge.

The order of ionisation enthalpies, as expected, is \(\Delta_i \mathrm{H}_1<\Delta_i \mathrm{H}_2<\Delta_i \mathrm{H}_3\). The sum of the first three ionisation enthalpies for each of the elements is very high. Effect of this will be apparent when you study their chemical properties.

Electronegativity

Down the group, electronegativity first decreases from B to Al and then increases marginally (Table 10.2). This is because of the discrepancies 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. Rest of the members are soft metals with low melting point and high electrical conductivity. It is worthwhile to note that gallium with unusually low melting point ( 303 K ), could exist in liquid state during summer. Its high boiling point ( 2676 K ) makes it a useful material for measuring high temperatures. Density of the elements increases down the group from boron to thallium.

Chemical Properties

Oxidation state and trends in chemical reactivity

Due to small size of boron, the sum of its first three ionization enthalpies is very high. This prevents it to form +3 ions and forces it to form only covalent compounds. But as we move from B to Al , the sum of the first three ionisation enthalpies of Al considerably decreases, and is therefore able to form \(\mathrm{Al}^{3+}\) ions. In fact, aluminium is a highly electropositive metal. However, down the group, due to poor shielding effect of intervening \(d\) and \(f\) orbitals, the increased effective nuclear charge holds \(n s\) electrons tightly (responsible for inert pair effect) and thereby, restricting their participation in bonding. As a result of this, only \(p\)-orbital electron may be involved in bonding. In fact in Ga , In and Tl , both +1 and +3 oxidation states are observed. The relative stability of +1 oxidation state progressively increases for heavier elements: \(\mathrm{Al}<\mathrm{Ga}<\mathrm{In}<\mathrm{Tl}\). In thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly oxidising in character. The compounds in +1 oxidation state, as expected from energy considerations, are more ionic than those in +3 oxidation state.

In trivalent state, the number of electrons around the central atom in a molecule of the compounds of these elements (e.g., boron in \(\mathrm{BF}_3\) ) will be only six. Such electron deficient molecules have tendency to accept a pair of electrons to achieve stable electronic configuration and thus, behave as Lewis acids. The tendency to behave as Lewis acid decreases with the increase in the size down the group. \(\mathrm{BCl}_3\) easily accepts a lone pair of electrons from ammonia to form \(\mathrm{BCl}_3 \cdot \mathrm{NH}_3\).

In trivalent state most of the compounds being covalent are hydrolysed in water. For example, the trichlorides on hyrolysis in water form tetrahedral \(\left[\mathrm{M}(\mathrm{OH})_4\right]^{-}\)species; the hybridisation state of element M is \(s p^3\). Aluminium chloride in acidified aqueous solution forms octahedral \(\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}\) ion. In this complex ion, the \(3 d\) orbitals of Al are involved and the hybridisation state of Al is \(s p^3 d^2\).

Example 10.1: Standard electrode potential values, \(\mathrm{E}^{\ominus}\) for \(\mathrm{Al}^{3+} / \mathrm{Al}\) is -1.66 V and that of \(\mathrm{Tl}^{3+} / \mathrm{Tl}\) is +1.26 V . Predict about the formation of \(\mathrm{M}^{3+}\) ion in solution and compare the electropositive character of the two metals.

Solution: Standard electrode potential values for two half cell reactions suggest that aluminium has high tendency to make \(\mathrm{Al}^{3+}(\mathrm{aq})\) ions, whereas \(\mathrm{Tl}^{3+}\) is not only unstable in solution but is a powerful oxidising agent also. Thus \(\mathrm{Tl}^{+}\)is more stable in solution than \(\mathrm{Tl}^{3+}\). Aluminium being able to form +3 ions easily, is more electropositive than thallium.

(i) 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 aluminium metal on heating in air form \(\mathrm{B}_2 \mathrm{O}_3\) and \(\mathrm{Al}_2 \mathrm{O}_3\) respectively. With dinitrogen at high temperature they form nitrides.
\(
\begin{aligned}
& 2 \mathrm{E}(\mathrm{~s})+3 \mathrm{O}_2(\mathrm{~g}) \xrightarrow{\Delta} 2 \mathrm{E}_2 \mathrm{O}_3(\mathrm{~s}) \\
& 2 \mathrm{E}(\mathrm{~s})+\mathrm{N}_2(\mathrm{~g}) \xrightarrow{\Delta} 2 \mathrm{EN}(\mathrm{~s}) \quad \text { ( } \mathrm{E}=\text { element) }
\end{aligned}
\)
The nature of these oxides varies down the group. Boron trioxide is acidic and reacts with basic (metallic) oxides forming metal borates. Aluminium and gallium oxides are amphoteric and those of indium and thallium are basic in their properties.

(ii) Reactivity towards acids and alkalies

Boron does not react with acids and alkalies even at moderate temperature; but aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character.

Aluminium dissolves in dilute HCl and liberates dihydrogen.
\(
2 \mathrm{Al}(\mathrm{~s})+6 \mathrm{HCl}(\mathrm{aq}) \rightarrow 2 \mathrm{Al}^{3+}(\mathrm{aq})+6 \mathrm{Cl}^{-}(\mathrm{aq})+3 \mathrm{H}_2(\mathrm{~g})
\)
However, concentrated nitric acid renders aluminium passive by forming a protective oxide layer on the surface.

Aluminium also reacts with aqueous alkali and liberates dihydrogen.
\(
\begin{aligned}
& \quad 2 \mathrm{Al}(\mathrm{~s})+2 \mathrm{NaOH}(\mathrm{aq})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \\
& \quad \quad \quad \quad \quad \quad \downarrow \\
& \quad \quad 2 \mathrm{Na}^{+}\left[\mathrm{Al}(\mathrm{OH})_4\right]^{-}(\mathrm{aq})+3 \mathrm{H}_2(\mathrm{~g}) \\
& \quad \quad \text { Sodium } \\
& \text { tetrahydroxoaluminate(III) }
\end{aligned}
\)

(iii) Reactivity towards halogens

These elements react with halogens to form trihalides (except \(\mathrm{TlI}_3\) ).
\(
2 \mathrm{E}(\mathrm{~s})+3 \mathrm{X}_2(\mathrm{~g}) \rightarrow 2 \mathrm{EX}_3(\mathrm{~s}) \quad(\mathrm{X}=\mathrm{F}, \mathrm{Cl}, \mathrm{Br}, \mathrm{I})
\)

Example 10.2: White fumes appear around the bottle of anhydrous aluminium chloride. Give reason.

Solution: Anhydrous aluminium chloride is partially hydrolysed with atmospheric moisture to liberate HCl gas. Moist HCl appears white in colour.