Chemistry 9701/11 — October/November 2023

Background Concept

A neutral atom contains equal numbers of protons and electrons. The number of protons is the proton number (atomic number), which is unique to each element. When an atom forms an ion, it either loses electrons (forming a positive ion) or gains electrons (forming a negative ion). For a polyatomic ion, the total number of electrons is found by adding the electrons of all atoms in the ion and then adjusting for the overall charge: a negative charge means extra electrons, a positive charge means fewer electrons.

Understanding the Question

This question asks for the number of electrons in two ions present in sodium azide: the sodium ion and the azide ion. The formula $\text{NaN}_3$ tells us the compound contains $\text{Na}^+$ and $\text{N}_3^-$. We are given four pairs of numbers and must select the correct pair. The key is to recall the proton numbers of sodium and nitrogen and then apply the ionic charges correctly.

Approach

First, identify the proton numbers of the elements involved: sodium is 11 and nitrogen is 7. For a monatomic ion, subtract the positive charge from the proton number. For a polyatomic ion, multiply the proton number of each atom by the number of atoms, then add electrons for each negative charge (or subtract for each positive charge).

Step-by-Step Reasoning

1. Sodium atom: proton number = 11, so a neutral sodium atom has 11 electrons.
2. The sodium ion is $\text{Na}^+$, meaning it has lost one electron: $11 - 1 = 10$ electrons.
3. Azide ion: formula $\text{N}_3^-$, containing three nitrogen atoms.
4. Each nitrogen atom has proton number 7, so three nitrogen atoms contribute $3 \times 7 = 21$ protons, and a neutral $\text{N}_3$ unit would have 21 electrons.
5. The $^-$ charge means one extra electron has been gained: $21 + 1 = 22$ electrons.
6. Therefore the correct pair is sodium ion = 10, azide ion = 22, which is option B.

Distractors: Option A gives azide as 20, which might come from forgetting the extra electron or miscounting the nitrogen atoms. Option C gives sodium as 12, which would mean adding an electron to a neutral sodium atom instead of removing one. Option D combines both errors.

Key Takeaways

• The number of electrons in an ion is the total proton number adjusted by the ionic charge.
• For a polyatomic ion, sum the electrons of all atoms, then adjust for the overall charge.
• A positive charge means electrons lost; a negative charge means electrons gained.

Common Mistakes

• Using the mass number instead of the proton number when counting electrons.
• Adding an electron for a positive ion or removing one for a negative ion.
• Forgetting that the azide ion contains three nitrogen atoms and carries a single negative charge.

Things to Be Careful About

• Always use the proton number (atomic number), not the mass number, when counting electrons.
• Check the sign of the ionic charge carefully: $\text{Na}^+$ has fewer electrons than $\text{Na}$, while $\text{N}_3^-$ has more electrons than three neutral nitrogen atoms.
• In a polyatomic ion, the charge applies to the whole ion, so adjust the total electron count by the magnitude of the charge.