Statement A is incorrect. In group 13, although atomic size increases down the group, the ionic radii of trivalent cations do not decrease regularly. Due to poor shielding by d and f electrons in heavier elements like Ga and In, the ionic radii show irregular trend instead of continuous decrease.
Statement B is incorrect. Electronegativity does not continuously decrease down group 13. After aluminium, electronegativity slightly increases due to ineffective shielding of d and f electrons, which increases effective nuclear charge.
Statement C is correct. Boron has the highest first ionisation enthalpy among group 13 elements because of its small atomic size and strong attraction between nucleus and valence electrons.
Statement D is correct. The trihalides such as $BCl_3$, $AlCl_3$ and corresponding triiodides form covalent bonds due to high polarising power of small and highly charged $M^{3+}$ ions.
Hence, the correct statements are C and D.
Updated for JEE Main 2026: This PYQ is important for JEE Mains, JEE Advanced and other competitive exams. Practice more questions from this chapter.