Video Transcript
The electronegativities of nitrogen, 3.04, and bromine, 2.96, are very similar. However, nitrogen has a much greater ability to form hydrogen bonds than bromine. How can this difference in hydrogen-bonding ability be rationalized? (A) An atom of nitrogen contains more lone pairs than an atom of bromine, giving it a
greater ability to form hydrogen bonds. (B) An atom of bromine contains more protons in its nucleus and so attracts an atom
of hydrogen strongly enough to form covalent bonds instead of hydrogen bonds. (C) An atom of bromine is more reactive than an atom of nitrogen and so easily forms
covalent bonds, leaving it with no lone pairs with which to hydrogen bond. (D) The larger size of a bromine atom means it has weaker forces of attraction to
hydrogen atoms on other adjacent molecules. Or (E) bromine can only form one bond with an atom of hydrogen, whereas nitrogen can
form three, giving it a greater ability to hydrogen bond.
Let’s clear some space on screen to discuss hydrogen bonds.
Hydrogen bonds are dipole–dipole interactions that exist between covalently bonded
hydrogen atoms and electron lone pairs on strongly electronegative elements. These strong interactions occur due to the uneven distribution of charge. For example, in water, oxygen is the strongly electronegative element. This means that the electrons shared in the covalent bonds with hydrogen atoms are
pulled more closely to the oxygen atom. This results in a partial negative charge on oxygen, which we can represent with a 𝛿
minus symbol. This gives the hydrogen atoms a partial positive charge, which we can represent with
a 𝛿 plus symbol.
For an electronegative element covalently bonded to hydrogen to form hydrogen bonds,
there must be an uneven distribution of charge. There must also be at least one lone pair of electrons present on the electronegative
element. A substance like hydrogen bromide has hydrogen atoms covalently bonded to a strongly
electronegative atom with lone pairs and uneven distribution of charge. However, strong hydrogen bonds do not form. Why not? The reason for this is that strong hydrogen bonds only form when the strong partial
negative charge is densely concentrated into a small area.
Let’s take a look at the atomic radii of nitrogen and bromine to better understand
this concept.
On the periodic table, atomic radius decreases from left to right. It also increases from top to bottom. On the periodic table, we will find nitrogen in period two in group 15, while we find
bromine in period four, group 17. For atomic radius, the vertical trend down a group is much stronger than the
horizontal trend across a period. Therefore, a bromine atom has a much larger radius than a nitrogen atom.
While these atoms both have very similar electronegativities, they have vastly
different atomic radii. Both nitrogen and bromine form covalent bonds with hydrogen atoms. Both nitrogen and bromine will likely have lone pairs of electrons and uneven
distributions of charge when forming molecules with hydrogen. However, because of their smaller size, the nitrogen atoms have a higher charge
density and the negative partial charge is more concentrated. A greater charge density in the electronegative atom means a stronger attraction
between itself and the partial positive charge of the hydrogen on neighboring
molecules.
So for the larger bromine atom, this means the forces of attraction to neighboring
molecules are weaker due to the partial negative charge being spread out over a
larger radius. With this in mind, let’s have another look at our answer choices.
We can see that answer choice (D) describes the reasoning we discussed. Therefore, the difference in hydrogen-bonding ability between nitrogen and bromine
can be rationalized by answer choice (D). The larger size of a bromine atom means it has weaker forces of attraction to
hydrogen atoms on other adjacent molecules.
