Calculate The Oxidation Number of N in Ammonia

What is an Oxidation Number?

The oxidation number (or oxidation state) of an element in a compound is a measure of the degree of oxidation of the element. It indicates the number of electrons an atom has lost, gained, or shared in a chemical bond. Oxidation numbers help predict how atoms will interact in chemical reactions.

Key rules for assigning oxidation numbers:

1. The oxidation number of an atom in its elemental form is 0.
2. The oxidation number of a monatomic ion is equal to its charge.
3. In compounds, the oxidation number of fluorine is always -1.
4. The oxidation number of hydrogen is +1 in most compounds, except when combined with metals where it is -1.
5. The oxidation number of oxygen is usually -2 in compounds, except in peroxides where it is -1.
6. The sum of oxidation numbers in a neutral compound is 0.
7. The sum of oxidation numbers in a polyatomic ion is equal to the charge of the ion.

Calculating the Oxidation Number of N in Ammonia

Ammonia (NH₃) is a neutral compound, meaning the sum of the oxidation numbers of all atoms in the compound must equal zero. To find the oxidation number of nitrogen (N) in ammonia:

1. Assign the oxidation number of hydrogen (H). Since hydrogen is always +1 in compounds, each H in NH₃ has an oxidation number of +1.
2. Let the oxidation number of nitrogen (N) be x.
3. Set up the equation based on the rule that the sum of oxidation numbers in a neutral compound is 0:
   x + (1 × 3) = 0
4. Solve for x:
   x + 3 = 0 x = -3

Therefore, the oxidation number of nitrogen in ammonia is -3.

Example Calculation

Let's work through an example to confirm our calculation.

Consider ammonia (NH₃):

1. Identify the atoms and their counts: 1 N and 3 H.
2. Assign oxidation numbers:
   • Hydrogen (H): +1 each (since it's bonded to a non-metal)
   • Nitrogen (N): Let's call this x
3. Set up the equation:
   x + (1 × 3) = 0
4. Solve for x:
   x + 3 = 0 x = -3

The calculation confirms that the oxidation number of nitrogen in ammonia is -3.

Common Mistakes to Avoid

When calculating oxidation numbers, it's easy to make mistakes. Here are some common errors to watch out for:

1. Assuming hydrogen always has an oxidation number of +1: While this is true in most cases, hydrogen can have an oxidation number of -1 when bonded to metals.
2. Forgetting that the sum of oxidation numbers in a neutral compound must be zero: Always verify that the total oxidation numbers balance to zero for neutral compounds.
3. Incorrectly assigning oxidation numbers to polyatomic ions: Remember that the sum of oxidation numbers in a polyatomic ion must equal the ion's charge.
4. Overlooking the oxidation state of oxygen: Oxygen typically has an oxidation number of -2, but this can vary in certain compounds like peroxides.