Video Transcript
Nitric acid can exist as two major
resonance structures. One of these structures is shown
here. Which of the following structures
is the other major resonance structure adopted by nitric acid?
In the five answer choices, we see
five possibilities for the way that electrons can be distributed in the second
resonance structure of nitric acid. Resonance structures are a set of
two or more Lewis structures that demonstrate the delocalization of electrons in a
chemical species. Resonance structures have the same
connectivity of atoms, the same number of electrons, and the same chemical
formula.
The chemical formula of nitric acid
is HNO3. When looking at the structures in
the answer choices, we can see that they all have the chemical formula HNO3 and that
the atoms are connected in the same way. It’s difficult to tell if the
structures have the same number of electrons because the lone pairs of electrons are
not shown.
In the initial structure, we notice
that there is a double bond between the nitrogen atom and an oxygen atom that has no
other covalent bonds. In addition, there is a single bond
between the nitrogen atom and another oxygen atom that also has no other covalent
bonds. It’s important to remember that
oxygen tends to have no more than two covalent bonds in a molecule, and a hydrogen
atom will only have one single bond.
Resonance structures are often
possible for a chemical species, in which a double bond is located next to a single
bond. The oxygen atom involved in the
double bond has two lone pairs of electrons. To have an octet of electrons, the
oxygen atom on the left must have three lone pairs. We won’t discuss how to calculate
formal charge in this video. However, it’s important to note
that the leftmost oxygen atom has a formal charge of negative one and the nitrogen
atom has a formal charge of positive one.
Now, resonance structures differ by
the locations of lone pairs and bonding pairs. To draw a second resonance
structure for nitric acid, we can convert a lone pair on the leftmost oxygen atom to
a bonding pair, and also convert a bonding pair from the double bond into a lone
pair on the top oxygen atom. So, the second resonance structure
looks as if the double bond changed locations.
In reality, the electron pairs that
we moved are delocalized, which means the electron density is spread out over the
two oxygen atoms we’ve been discussing. However, in the resonance
structure, the leftmost oxygen atom has two lone pairs and is shown as having a
double bond, and the top oxygen atom has three lone pairs, a single bond, and is
assigned a formal charge of negative one. The number of bonds around the
nitrogen atom remained the same, so its formal charge did not change.
When looking at the answer choices,
we can see that (A) cannot be the correct answer because the structure does not have
a double bond. Structures (B) and (D) are also
unlikely because one of the oxygen atoms has more than two bonds. In addition, a hydrogen atom cannot
have more than one single covalent bond. Structure (E) is also incorrect
because it contains two double bonds. This leaves answer choice (C),
which matches the structure we drew.
In conclusion, the structure that
is the second major resonance structure adopted by nitric acid is the one shown in
answer choice (C).
