`color{green}(★)` The outer electronic configuration of these elements is `color{red}(ns^2 np^1)`.

`color{green}(★)` A close look at the electronic configuration suggests that while boron and aluminium have noble gas core, gallium and indium have noble gas plus `color{red}(10 d)`-electrons, and thallium has noble gas plus `color{red}(14 f)`- electrons plus `color{red}(10 d)`-electron cores.

`color{green}(★)` This difference in electronic structures affects the other properties and consequently the chemistry of all the elements of this group.

`color{green}(★)` On moving down the group, for each successive member one extra shell of electrons is added and, therefore, atomic radius is expected to increase.


`color{green}(★)` However, a deviation can be seen. Atomic radius of `color{red}(Ga)` is less than that of `color{red}(Al)`. This can be understood from the variation in the inner core of the electronic configuration. The presence of additional `color{red}(10 d)`-electrons offer 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).

`color{green}(★)` The ionisation enthalpy values as expected from the general trends do not decrease smoothly down the group.

`color{green}(★)` The decrease from `color{red}(B)` to `color{red}(Al)` is associated with increase in size.

`color{green}(★)` The observed discontinuity in the ionisation enthalpy values between `color{red}(Al)` and `color{red}(Ga)`, and between In and `color{red}(Tl)` are due to inability of `color{red}(d-)` and `color{red}(f)`-electrons ,which have low screening effect, to compensate the increase in nuclear charge.

`color{green}(★)` The order of ionisation enthalpies, as expected, is `color{red}(D_iH_1 < D_iH_2 < D_iH_3)`.

`color{green}(★)` The sum of the first three ionisation enthalpies for each of the elements is very high.

`color{green}(★)` Down the group, electronegativity first decreases from `color{red}(B)` to `color{red}(Al)` and then increases marginally (Table 11.2). This is because of the discrepancies in atomic size of the elements.

`color{green}(★)` Boron is non-metallic in nature.

`color{green}(★)` It is extremely hard and black coloured solid. It exists in many allotropic forms.

`color{green}(★)` Due to very strong crystalline lattice, boron has unusually high melting point.

`color{green}(★)` Rest of the members are soft metals with low melting point and high electrical conductivity.

`color{green}(★)` It is worthwhile to note that gallium with unusually low melting point (303K), could exist in liquid state during summer. Its high boiling point (2676K) makes it a useful material for measuring high temperatures.

`color{green}(★)` Density of the elements increases down the group from boron to thallium.