In the compound ammonia (NH3), what is the oxidation number of the nitrogen atom?

Introduction / Context:
Assigning oxidation numbers is a basic skill in inorganic chemistry and redox reactions. Oxidation numbers help chemists keep track of electrons gained or lost in reactions and identify oxidising and reducing agents. Ammonia, NH3, is a familiar compound, and determining the oxidation number of nitrogen in NH3 is a standard example used in teaching the rules for oxidation states.

Given Data / Assumptions:

• The compound is ammonia, NH3.
• We assume the usual convention that hydrogen has oxidation number +1 when bonded to non metals.
• Ammonia is a neutral molecule, so the sum of oxidation numbers must be zero.
• We denote the oxidation number of nitrogen by x.

Concept / Approach:
The general rule is that for a neutral molecule, the sum of oxidation numbers of all atoms equals zero. Hydrogen usually has oxidation number +1 when bonded to non metals and minus 1 when bonded to metals. In NH3, nitrogen is the central non metal and each hydrogen is attached to it. Thus, each hydrogen is assigned +1. If the oxidation number of nitrogen is x, then x plus three times 1 must equal zero because the molecule is neutral. Solving this simple equation gives the oxidation number of nitrogen.

Step-by-Step Solution:
Step 1: Write the formula for ammonia: NH3.Step 2: Assign the usual oxidation number for hydrogen in a compound with a non metal, which is +1.Step 3: Let the oxidation number of nitrogen be x.Step 4: Use the rule that the sum of oxidation numbers in a neutral molecule is zero.Step 5: Set up the equation: x + 3*(+1) = 0.Step 6: Simplify the equation to x + 3 = 0.Step 7: Solve for x: x = -3.Step 8: Conclude that the oxidation number of nitrogen in NH3 is -3.

Verification / Alternative check:
As an additional check, consider related nitrogen compounds. In ammonium ion, NH4+, nitrogen still has oxidation number -3 because 4*(+1) + x = +1 gives x = -3. In nitrate ion, NO3−, nitrogen has a positive oxidation state, showing that nitrogen can have a range of oxidation numbers from -3 to +5. This variation is consistent with its position in group 15 and supports the idea that -3 is the lowest common oxidation state for nitrogen in hydrides like ammonia.

Why Other Options Are Wrong:
Option a, -1, would give a total of -1 + 3 = +2, which does not sum to zero for a neutral molecule. Option b, +1, would give +1 + 3 = +4, again not zero. Option c, 0, would give 0 + 3 = +3, which is not correct. Option e, +3, would make the sum +3 + 3 = +6, far from zero. Only -3 makes the sum of oxidation numbers in NH3 equal to zero.

Common Pitfalls:
Students sometimes confuse formal charge with oxidation number or forget that hydrogen usually has +1 oxidation state when bonded to non metals. Another mistake is to think that nitrogen should always have a positive oxidation number because it is a non metal. In fact, nitrogen is more electronegative than hydrogen, so in NH3 it takes electrons and has a negative oxidation state. Carefully applying the sum of oxidation numbers rule avoids these errors.

Final Answer:
In ammonia, NH3, the oxidation number of nitrogen is -3.