### Video Transcript

Fill in the blank. The oxidation number of nitrogen is

zero in nitrogen molecules and blank in nitric acid. (A) Plus six, (B) plus five, (C)

plus three, (D) minus five, (E) minus six.

This problem is asking us to find

the oxidation number of nitrogen in nitric acid. In order to solve this problem, let

us first discuss oxidation numbers. Oxidation numbers show the degree

of oxidation of an atom in isolation or in a compound in terms of counting

electrons. Oxidation numbers tell us how many

electrons an element will gain or lose to form a bond or an ion.

Since we need to know the oxidation

number of nitrogen in nitric acid, let us write the formula for nitric acid. Nitric acid is composed of the

hydrogen ion, which has a charge of one plus, and the nitrate ion, NO3, with a

charge of one minus. We will crisscross the charge

numbers and drop the positive and negative signs of the ions. This gives us the formula of HNO3

for nitric acid.

This question reminds us that

nitrogen has an oxidation number of zero in nitrogen molecules. This is because the nitrogen

molecule is solely composed of only one type of element, nitrogen. But we need to find the oxidation

number of nitrogen in HNO3, which does not have an oxidation number of zero since

the nitrogen atom is within a compound composed of more than only one type of

element. When determining the oxidation

number of an atom in a molecule, the molecule will always have a net charge of

zero.

Now, let us discuss some of the

rules for determining the oxidation numbers of elements within a compound that will

help us to answer this question. The first rule we will discuss is

the oxidation number for hydrogen in a compound is plus one. Therefore, the oxidation number of

hydrogen in nitric acid is plus one. The other rule that will assist us

is the oxidation number for oxygen in a compound is minus two. So, since there are three atoms of

oxygen in nitric acid and each oxygen atom has an oxidation number of minus two, the

overall oxidation number is minus six for oxygen.

We now know that the oxidation

numbers of hydrogen, nitrogen, and oxygen must equal zero. We can set up an equation and

substitute the known oxidation numbers, where 𝑁 is the oxidation number for

nitrogen. Therefore, one plus 𝑁 minus six

equals zero. When we solve for nitrogen’s

oxidation number in nitric acid, it is equal to plus five. Therefore, answer choices (A) plus

six, (C) plus three, (D) minus five, and (E) minus six are all incorrect. And answer choice (B) plus five is

the correct answer choice for the oxidation number of nitrogen in nitric acid.