Elements of Group 13 are less metallic than those of groups 1 and 2. Within the group, there is a variation in metallic character. Boron, the smallest element in this group is a non metal. The other elements in this group are fairly reactive metals. This is the first group in which 'change from non-metallic to metallic nature occurs.
The elements of Group 13 have smaller atomic radii and higher electro negativities as compared to s-block elements of the same period. However, these properties do not vary in a regular way down the group, in contrast to the properties of the element of, - Groups 1 and 2. You know that the size and the ionisation energies of atoms depend on effective nuclear charge of the-atom. B and Al follow immediately the s-block elements Be and Mg, respectively. Their size and ionisation energies are as expected. But between the s- and p-block elements of the fourth and successive rows, the d-block elements, i.e., the transition elements get inserted. The insertion of transition elements results in higher effective nuclear charge of the fourth row elements Ga, Ge, etc., than expected by simple extrapolation from the second and third row elements. In other words, the nuclei of these fourth row elements attract electrons more strongly than expected and this affects their properties. Thus, the atomic size of gallium is smaller; its electronegativity and ionisation energies higher than 8 expected. Atoms with dl0 Inner - shell, in general, are smaller and have higher ionisation energies. In a similar way, the inclusion of fourteen electrons in 4f orbital further affects the size and ionisation energy of TI, As a result of this, irregularities in atomic radii, electronegativity and ionisation energies are seen from B to Tl.