The Structure Of An Atom

To determine the correct order of atomic radii in group 13 elements, we must understand the general trends of atomic radii across periods and down groups in the periodic table. The elements in group 13 include Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), and Thallium (Tl).

• Atomic radius generally increases down a group due to the addition of electron shells.
• However, there are exceptions to this rule due to the nature of d and f-block contractions and additional electron shielding.

Explanation of the order:

• Boron (B): Being the first element in the group, it has the smallest atomic radius.
• Gallium (Ga): Gallium's atomic radius is unexpectedly smaller than Aluminium (Al) due to the presence of d-orbitals, which cause effective nuclear charge to be higher, pulling the electrons closer.
• Aluminium (Al): Comes next as it doesn't experience the d-orbital contraction seen in Gallium.
• Indium (In): The atomic radius increases significantly due to additional electron shells.
• Thallium (Tl): Has the largest atomic radius in the group, with further electron shells and added f-orbital contraction.

Hence, the correct order of atomic radii in group 13 elements is: B < Ga < Al < In < Tl.

Thus, the correct answer is: