Physical Properties of Boron Family - Practice Questions & MCQ

Electronic configuration
The outer electronic configuration of these elements is ns²np¹. 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-electron cores. Thus, the electronic structures of these elements are more complex than the s-block elements. 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. The 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 an additional 10 d-electrons offers 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).

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

Density
Density increases from boron to thallium. However, boron and aluminium have comparatively low values. This is due to their lower atomic masses as compared to gallium, indium and thallium.

Melting and boiling points
The elements of this group do not show a regular change in their elting points with increase in atomic number. The melting point decreases from B to Ga and then increases. The high melting point of boron is due to the facr that it exists as a giant covalent polymer in both solid and liquid states. The elements Al, In and Tl all have close-packed metal structures. Gallium has an unusal structure. It consists on only Ga₂ molecules. It has thus low melting point. It exists as liquid upto 2000^oC and hence used in high temperature thermometry.

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 an increase in size. The observed discontinuity in the ionisation enthalpy values between Al and Ga, and between In and Tl are due to the 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 ∆_iH₁ < ∆_iH₂ < ∆_iH₃. 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.

Oxidation states
It is in group 13 that we first encounter elements possessing more than one oxidation state. As s²p¹ grouping is present in the outermost energy shell of the elements of the group IIIA, the expected oxidation states are +3 and +1. Boron shows +3 oxidation state in all its compounds. Other members show +3 and +1 oxidation states. The stability of +1 oxidation state increases from aluminium to thallium and the stability of +3 is more important oxidation state for Al, Ga and In whereas +1 oxidation state is more important for Tl.

Electropositive character
The elements of group 13 are less electropositive as compared to the elements of groups 1 and 2. This is due to their size and high ionisation energy. The electropositive character increases from boron to aluminium and then decreases from aluminium to thallium. Boron having very high ionisation energy is considered to be as a semimetal. It is closer to non-metals. Aluminium is a metal and is most electropositive. The increase in electropositive nature from B to Al is due to increases in atomic size. The remaining three elements Ga, In and Tl are less electropositive and less metallic than aluminium and there is a decrease from Ga and Tl.

Complex formation
Group IIIA elements form complexes much more readily than the s-block elements, because of their smaller size, increased charge and availability of vacant orbitals.

Nature of compounds

• The tendency of the formation of ionic compound increases from B to Tl.
• Boron forms only covalent compounds.
• Aluminium forms both ionic as well as covalent compounds.
• Gallium forms mainly ionic compounds except anhydrous GaCl₃ which is covalent in nature.