Question

Which of the following properties does not support anomalous behavior of boron?
A.Small size
B.High electronegativity
C.High ionization enthalpy
D.Formation of trihalides

Answer 1

Boron is an element which belongs to the group 13 of the periodic table. Boron is the first member of the group 13.
-The first member of each group of p-block elements differ from the other heavier elements in many aspects. Only boron is a non-metal. All the other elements of group 13 show metallic properties.

Complete step by step answer:
Atomic radius generally increases down a group from top to bottom. So, the element at the top of the group has the smallest size. Thus, boron has the smallest size among group 13 elements.
The electronegativity decreases from boron to aluminium and then shows a regular increase down the group. But overall, the electronegativity of boron is the highest in group 13.
The ionization enthalpy or ionization energy of group 13 elements is lower than that of the corresponding alkaline earth metals. Aluminium has larger size and greater screening effect than boron. So, the ionization energy of boron is higher than that of aluminium. In fact, it is higher than that of all group 13 elements.
Also, boron does not have d orbitals available for bonding and so cannot expand its valence shell to accommodate more than 4 pairs of electrons.
Thus, the anomalous behavior of boron is due to its small size, high electronegativity, high ionization energy and non-availability of d orbitals.
Formation of trihalides is not an anomaly because other elements of group 13 also form trihalides. For example, aluminium forms aluminium trichloride. So, option D is correct.

Note:
Some differences between boron and aluminium are as follows.
-Boron is a poor conductor of electricity but aluminium is a good conductor of electricity.
-Boron shows allotropy but aluminium does not.
-Boron forms covalent compounds while aluminium forms covalent as well as ionic compounds.