The elements Boron (B), Aluminium (Al), Gallium (Ga), Indium (In) and Thallium (Tl) together constitute group 13 of the long form of the periodic table and are known as members of Boron family. Aluminium is the most abundant element in its group and is the 3rd most abundant element found in the earth’s crust after oxygen and silicon.
General Trends in Physical Properties of Boron Family:
(i) Electronic Configuration- The general outer electronic configuration of these elements is ns2 np1 (n = 2 to 6) which shows that these elements have three electrons in the valence shell; one in p-orbital and two in s-orbital.
|Element / Symbol||Atomic Number||Electronic Configuration|
|Boron, B||5||1s2, 2s2 2p1 or [He] 2s2 2p1|
|Aluminium, Al||13||1s2, 2s2 2p6, 3s2 3p1 or [Ne] 3s2 3p1|
|Gallium, Ga||31||1s2, 2s2 2p6, 3s2 3p6 3d10, 4s2 4p1 or [Ar] 3d10, 4s2 4p1|
|Indium, In||49||1s2, 2s2 2p6, 3s2 3p6 3d10, 4s2 4p6 4d10, 5s2 5p1 or [ Kr] 4d10, 5s2 5p1|
|Thallium, Tl||81||1s2, 2s2 2p6, 3s2 3p6 3d10, 4s2 4p6 4d10 4f14, 5s2 5p6 5d10, 6s2 6p1 or [Xe] 4f14, 5d10, 6s2 6p1|
(ii) Density- There is a regular increase in density on moving down the group from B to Tl. Boron and Aluminium have comparatively lower densities due to lower atomic mass as compared to the other elements of the family.
(iii) Atomic radii and Ionic radii- The atomic radii of group 13 elements are smaller than corresponding elements of group 2. However, on moving down the group from B to Tl, atomic radii shows an increasing trend due to increase in screening effect which overcomes the effect of increased nuclear charge. Like atomic radii, ionic radii also shows an increasing trend on moving down the group.
(iv) Melting points and Boiling points- The elements of group 13 do not show a regular trend in their melting points because of structural difference. It first decreases from B to Ga and then increases from Ga to Tl. The melting point of B is very high.
However, boiling points of these elements regularly decreases on moving down the group from B to Tl indicating that the strength of bonds holding atoms in their liquid state decreases from B to Tl.
(v) Electronegativity- It decreases from B to Tl on moving down the group due to increase in atomic size leading to a decrease in the attraction of nucleus for electrons.
(vi) Ionisation energy- The valence electrons in these elements occupy s and p-orbitals. The p-electrons are held less firmly by the nucleus in comparison to s-electrons. Thus, first ionisation energy (IE1) of these elements are low while 2nd and 3rd ionisation energies are comparatively high because the successive electrons have to be removed from s-orbital. It goes on decreasing on moving down the group due to increase in atomic size and screening effect.
(vii) Oxidation States- These elements contain 3 electrons in the valence shell (ns2 np1) and so they show an oxidation state +3 most commonly (by using two s-electrons and one p-electron) and also +1 oxidation state (by using only p-electron). The stability of +1 oxidation state increases and that of +3 decreases on moving down the group from B to Tl. Thus, Tl is more stable in +1 oxidation state and B in +3 oxidation state. The tendency of these elements to show +1 oxidation state is due to the reason that very high energy is required to unpair the two s-electrons in the valence shell and so they remain paired and not participate in bonding. This effect is known as “Inert pair effect” and is more predominant in case of heavier p-block elements.
(ix) Electropositive Character ( or Metallic Character)- Due to high ionisation energies, the elements of group 13 are less electropositive than 1 and 2 group elements. On moving down the group 13, the electropositive character increases from B top Al and then it slightly decreases from Al to Tl because of the slight difference in their first ionisation energies. Boron, because of its small atomic size and high ionisation energy is regarded as a semi-metal, it is closer to non-metals than to metals in many of its properties.
(x) Nature of Compounds- According to Fajan’s rule, the smaller the size of cation, the greater is the tendency for forming a covalent bond. Boron due to its small size and a high sum of first three ionisation energies does not form B3+ ion. Thus, it does not form ionic compounds but forms covalent compounds. The same is true for some of the compounds of Al. Example- Covalent compounds of B are BF3 and BCl3 and that of Al is AlCl3. However, on moving down the group, atomic size increases and ionisation energy decreases due to which the tendency to form covalent compounds decreases while that of ionic compounds increases.