Boron is the first element of group 13 and only has 2 shells. Because of its small size, the first three ionization enthalpies are very high. This prevents boron from forming +3 ions, and hence it forms only covalent compounds. As we move from boron to aluminum, there is a considerable decrease in the ionization enthalpies which makes it possible for aluminum to form Al3+ ions. Aluminium is thus, highly electropositive in nature. As we move down the group, the shielding effect of d and f orbitals becomes very poor and the ns electrons get held tightly due to effective nuclear charge. This, therefore, restricts its participation in forming bonds. As a result, only p orbital electrons participate in the bond formation. In Ga, Tl and In both +1 and +3 oxidation states are observed. The compounds with +1 oxidation state are more ionic than +3 oxidation state in terms of energy considerations.
In trivalent oxidation state, most of the compounds are covalent and hence hydrolyzed in water. For example, the trichlorides form tetrahedral species on hydrolysis in water. Now, let us have a look at the chemical reactivity of these elements:
1. Reactivity with air – In the crystalline form, boron is unreactive. Whereas aluminum on reaction with air forms a thin layer of oxide which prevents the metal from any further attack. When boron is taken in amorphous form and heated, it forms B2O3, and aluminum on heating forms Al2O3. They also form nitrides when reacted with nitrogen molecule at high temperature.
2. Reactivity with acids and alkalis – Aluminium being amphoteric in nature dissolves in both acids and alkalis. In both, these reactions hydrogen molecule is liberated. Boron does not react with acids or alkalis.
3. Reactivity with halogens – These elements on reacting with halogens form trihalides (Tl and I3 are exceptions in this case).
We are now aware of the oxidation state and chemical reactivity of group 13 elements. Now, suppose we have a bottle of anhydrous aluminum chloride. The bottle has white fumes around it. What can be the possible reason for these fumes? Give it a thought before reading the answer ahead. This happens because anhydrous aluminum chloride is hydrolyzed with atmospheric moisture and this reaction librates HCl gas. This moist HCl is white in color and appears as fumes.
We have seen the properties of oxidation state and chemical reactivity of aluminum and boron. For any further query on this topic, call the mentor support team at Byju’s.