Group 13 Elements
The p-block elements display a wide-ranging and diverse chemistry depending on the group to which they belong. The first member of each of the groups 13-17,of the p-block elements differ much in many respects from the other members of their respective groups. Besides, its small size and high electro-negativity, the first member of each group has only four valence orbitals available for bonding and does not form compounds in which the co-ordination number exceeds four. In this unit, we will discuss about group 13 and group 14 elements.
The Group 13 Elements
Group 13 of the periodic table is composed of elements boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl).
- Elements of Group 13 have outer electronic configuration ns2np1, where n varies from 2 to 6. They are expected to form compounds with +3 oxidation state.
- Boron, being the first element of the group, shows anomalous behaviour. It is a nonmetal while others are metals.
- The metallic radii of atoms do not increase regularly on descending the group.10.16).
- Boron is not a metal; the reported radius is half the closest approach in its structure.
- Gallium has an unusual structure; the reported radius is half the closet approach.
- Ga, In and Tl follow immediately after the ten transition elements. Thus, the outer shell is preceded by d10 configurations. Due to the poor shielding of the nuclear charge by d electrons, the outer shell is drawn near the nucleus as compared to their expected normal locations. Thus, their atomic radii are smaller than the expected values. This contraction in size is sometimes called the d-block contraction.
- The atomic radius of Tl is a little larger than In. This is due to the intervention of 4f electrons, which shield the nuclear charge more poorly. The small value of atomic radii is a result of lanthanide contraction.
- The ionic radii of M3+ increase down the group. The reported value of B3+ is an estimated value, as B3+ doesn't exist.
- On descending the group, +1 oxidation state becomes more stable than +3 state due to the inert pair effect. Explanation of Inert Pair Effect In inert pair effect, s electrons do not take part in bonding. The reason behind this is the energy factor. The energy required to unpair them exceeds the energy evolved when they form bonds. Thus, they remain intact.
- The melting points of the Group 13 elements do not show a regular trend (Fig. 10.17).
The very high melting point of boron is due to its unusual crystal structure. The small size and high ionization energy makes boron a nonmetallic element, thus metallic bonds do not exist. The structure of boron is icosahedral (20-faced) with boron atoms at all 12 corners.
Gallium again has an unusual structure. Each metal atom has one close neighbour at a distance of 243 pm and six more distant neighbours at distances between 270 pm and 279 pm.
- The variation of boiling point corresponds to the expected pattern, as no unusual structures exists in liquid phase .
- The densities increase on descending the group.
- The ionization energies do not follow the expected trend of decreasing values on descending the group.
- The standard electrode potential, EĀ°(M3+ | M), becomes more negative in going from B to Al indicating the increase in metallic nature from B to Al. After Al, the values become less and less negative and for Tl, it becomes positive indicating the reduction reaction, M3+ + 3e- ,becomes more easy. This explains why the +3 oxidation state becomes less stable in aqueous solution on descending the group.