The HF bond length is \(92 \mathrm{pm}, 16 \%\) shorter than the sum of the covalent radii of \(\mathrm{H}(37 \mathrm{pm})\) and \(\mathrm{F}(72 \mathrm{pm})\). Suggest a reason for this difference. Similar data show that the difference becomes smaller down the group, from HF to HI. Explain.

The covalent radius is a measure of the size of an atom within a molecule. Specifically, it is half the distance between two atoms that are bonded together.
Atoms bond to form molecules by sharing electrons, and the covalent radius gives us a way to understand how large an atom is when it is part of such a molecule.
For instance, in the case of hydrogen fluoride (HF), the covalent radius of hydrogen is 37 picometers (pm) and that of fluorine is 72 pm. By adding these values, we can estimate the bond length to be 109 pm. However, the actual HF bond length is shorter, at 92 pm.
The difference suggests that other factors, such as electronegativity, play a role.

Electronegativity is a measure of how strongly an atom attracts the electrons in a bond.
Fluorine is the most electronegative element, which means it has a very strong pull on the electrons in a bond.
Because fluorine strongly attracts electrons, the shared electron pair in the HF molecule is pulled closer to the fluorine atom. This causes the bond to be shorter than the sum of the covalent radii of hydrogen and fluorine.
High electronegativity leads to stronger bonds and shorter bond lengths.
This principle explains why the HF bond length is 92 pm, rather than the 109 pm expected from adding the covalent radii.

In the periodic table, elements are arranged in groups (columns) and periods (rows), with each group sharing similar chemical properties.
The halogens are found in Group 17, and they include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).
One key trend down the group is a decrease in electronegativity. Fluorine, at the top of the group, is the most electronegative, while iodine, further down, has a lower electronegativity.
As we move from HF to HI, the bond length increases. This increase is because the atoms get larger (which can be seen from their increased covalent radii).
Additionally, the decrease in electronegativity means that the bond attraction is weaker, causing less shortening of the bond length.
Hence, HF has a bond length that is much shorter compared to HI, as the electronegativity of fluorine results in a stronger pull on the bonding electrons.