Question 53

The correct statements from the following are:
A. Ionic radii of trivalent cations of group 13 elements decreases down the group.
B. Electronegativity of group 13 elements decreases down the group.
C. Among the group 13 elements, Boron has highest first ionisation enthalpy.
D. The trichloride and triiodide of group 13 elements are covalent in nature.
Choose the correct answer from the options given below :

Solution

. Ionic Radii: The distance between the nucleus of a trivalent cation ($$M^{3+}$$) and its outermost electron shell. General Trend: It increases down the group ($$\text{B}^{3+} < \text{Al}^{3+} < \text{Ga}^{3+} < \text{In}^{3+} < \text{Tl}^{3+}$$) because new principal energy levels (shells) are added. Statement A is False.

B. Electronegativity: The tendency of an atom to attract a shared pair of electrons. General Trend: It decreases from Boron to Aluminium, but then increases slightly from Aluminium to Thallium. Exception: This irregular trend occurs because of the poor shielding effect of d and f electrons in heavier elements, which increases the effective nuclear charge. Statement B is False.

C. Ionisation Enthalpy: The energy required to remove the most loosely bound electron from an isolated gaseous atom. General Trend: It generally decreases down the group. Exception: Boron has the highest value because it has the smallest atomic size and a very high nuclear charge density, making its electrons hardest to remove. Statement C is True.

D. Covalent Nature: The sharing of electron pairs between atoms rather than total transfer. General Trend: Group 13 halides (trichlorides and triiodides) are predominantly covalent. Exception: According to Fajan's Rule, the small size and high $$+3$$ charge of these cations result in high polarizing power, which distorts the electron cloud of the large halide ions to create covalent character. Statement D is True.

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