Group 13 Elements - Boron Family

GROUP 13 ELEMENTS: THE BORON FAMILY

Boron is a typical non-metal, aluminium is a metal but shows many chemical similarities to boron, and gallium, indium and thallium are almost exclusively metallic in character.

Boron is a fairly rare element, mainly occurs as orthoboric acid, (H₃BO₃), borax, Na₂B₄O₇·10H₂O, and kernite, Na₂B₄O₇·4H₂O.

There are two isotopic forms of boron 10B (19%) and 11B (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, Al₂O₃. 2H₂O and cryolite, Na₃AlF₆are 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.

Electronic Configuration of boron family

The elements of group 13 belong to p-block of the periodic table and these elements contain three electrons in the valence shell, therefore, their valence shell electronic configuration is ns²np¹.

Atomic Radii of boron family

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 10d-electrons offer only poor screening effect 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 of boron family

The first I.E. values of group 13 elements are lower than the corresponding values of the alkaline earth metals, due to the fact that removal of electron is easy in former case (p-electron) than latter (s-electron).

This results in the increase of nuclear charge. Consequently the valence electrons are more tightly held leading to high I.E. Similarly we can explain the irregularity in case of Tl on the basis of ineffective shielding of intervening electrons

On moving down the group IE, decreases from B to Al but the next element Ga has slightly higher IE, than Al, it again decreases in In and increases in the last element Tl as follows :

The irregularity observed in case of Gallium is due to the ineffective shielding of nuclear charge because of intervening d electrons, which cause the increase in nuclear charge leading to high I.E.

Electronegativity of boron family

Down the group, electronegativity first decreases from B to Al and then increases marginally. This is because of the discrepancies in atomic size of the elements.

Physical Properties of boron family

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.

DENSITY

It increases regularly on moving down the group from B to Tl

MELTING AND BOILING POINTS

M.P. and b. p. of group 13 elements are much higher than those of group 2 elements

The m.p. decreases from B to Ga and then increases, due to structural changes in the elements

Boron has a very high m. p. because of its three dimensional (B₁₂-icosahedral) structure in which B atoms are held together by strong covalent bonds.

Low m. p. of Ga is due to the fact that it consists of only Ga₂ molecules, and Ga remains liquid upto 2273K therefore it is used in high temperature thermometry.

Chemical Properties of boron family

Oxidation state and trends in chemical reactivity

Inert pair effect

It is the reluctance of the s-electrons of the valence shell to take part in bonding and occurs due to ineffective shielding of the ns² electrons by the intervening d and f electrons.

It increases down a group and thus the lower elements of group show lower oxidation states.

OXIDATION STATES

· B and Al show an oxidation state of +3 only while Ga, In and Tl show oxidation states of both +1 and +3.

· As we move down in the group 13, due to inert pair effect, the tendency to achieve +3 oxidation state goes on decreasing and the tendency to acquire +1 oxidation state goes on increasing.

· Stability of +1 oxidation state follows the order Ga < In < Tl

· Tl⁺ compounds are more stable than Tl³⁺ compounds.

Reactivity group 13 elements towards air

Crystalline boron is unreactive whereas amorphous boron is reactive. It reacts with air at 700°C as follows

4B + 3O₂→ 2B₂O₃

2B + N₂→ 2BN

AI is stable in air due to the formation of protective oxide film.

4Al + 3O₂→ 2Al₂O₃

Thallium is more reactive than Ga and In due to the formation of a unipositive ion, TI⁺.

4Tl + O₂→ 2Tl₂0

Reactivity group 13 elements towards acids and alkalies

Reaction with alkaliesBoron dissolves in alkalies and gives sodium borates.

Aluminium also reacts with alkali and liberates hydrogen.

Reactivity group 13 elements towards halogens

PREPARATION OF HALIDES

All the elements of boron family (except thallium which forms thallous monohalides ) form trihalides of type MX₃ where X= F, Cl, Br and I.

B₂O₃ + 3C + 3Cl₂ 2BCl₃ + 3CO

Al₂O₃ + 3C + 3Cl₂ 2AlCl₃ + 3CO

All the boron trihalides, BX₃ and aluminium trihalides AlX₃ (except AlF₃ which is ionic) are covalent compounds whereas former exist as only monomers and latter as dimers, because boron atom is too small to coordinate with four large halide ions and in case of much smaller F^– ion , the energy released during the formation of the bridge structure is not sufficient for the cleavage of the typical p-p bond in BF₃.

BX₃

Al₂X₆

BF₃ is a colourless gas, BCl₃ and BBr₃ are colourless fuming liquids whereas BI₃ is a white fusible solid at room temperature.

The covalent character of trihalides decreases on moving from Ga to Tl.

Hybridisation of Boron in BCl₃ is sp²

NATURE OF TRIHALIES

Trihalides of group 13 elements behave as lewis acids due to having a strong tendency to accept a pair of electrons.

The relative strength of lewis acids of boron trihalides increases in the order :

BF₃ < BCl₃ < BBr₃ < BI₃.

The halides of group 13 elements behave as lewis acids and the acidic character decreases as follows:

BX₃ > AlX₃ > GaX₃ > InX₃ (where X=Cl, Br or I)

BF₃ and anhydrous AlCl₃ are used as a catalyst in Friedel Crafts reactions.

TlCl₃ decomposes to TlCl and Cl₂ above 40ºC and hence acts as an oxidising agent, whereas TlBr₃ converts into Tl[TlBr₄] at room temperature.

TlCl₃ TlCl + Cl₂ ;2TlBr₃Tl[TlBr₄] +Br₂

While TlI₃ is an ionic compound containing Tl(I) and ions.

Anomalous properties of boron

Boron shows anomalous behaviour with the other members of the group, due to the following reasons:

(i) Smallest size in the group.
(ii) High ionisation energy.
(iii) Highest electronegativity in the group.
(iv) Absence of vacant d-orbital.

A few points of difference are

1. It is a non-metal while other members of the group are metallic.
2. It shows allotropy while other members do not.
3. It has the highest melting point and boiling point in group 13.
4. It forms only covalent compounds while other members form both ionic and covalent compounds.
5. The halides of boron exist as monomers while AlCI:! exists as a dimer.
6, The oxides and hydroxides of boron are weakly acidic while those of aluminium arc amphoteric and those of other elements are basic.
7. It can be oxidised by concentrated HNO₃while aluminium becomes passive due to the formation of oxide layer on the surface.