Chemistry 9701/13 — October/November 2023

Background Concept

The proton number, also called atomic number, is given the symbol Z. It tells you the number of protons in the nucleus of an atom. In a neutral atom the number of electrons equals Z, because the positive charge of the nucleus is exactly balanced by the negative charge of the electrons.

When an atom forms an ion it gains or loses electrons, never protons. A positive ion has fewer electrons than protons; a negative ion has more electrons than protons. This can be summarised as:

$\text{number of electrons} = \text{proton number} - \text{ionic charge}$

where the ionic charge is used with its sign. For example, for $\text{Na}^+$ the charge is $+1$, so you subtract one electron; for $\text{N}_3^-$ the charge is $-1$, so you add one electron. For a polyatomic ion, first add the proton numbers of all the atoms shown in the formula, then apply the charge.

Understanding the Question

The stem states that sodium azide, $\text{NaN}_3$, is made of positive sodium ions and negative azide ions. It asks for the number of electrons in each ion. This is a direct application of the relationship between proton number, ionic charge, and electron count. No chemistry of the azide ion beyond its formula and charge is needed.

Approach

Find the proton number of sodium from the periodic table: sodium has Z = 11. A neutral sodium atom therefore has 11 electrons. Since $\text{Na}^+$ has lost one electron, it has 10 electrons.

For the azide ion, count all the protons in the whole ion. The formula $\text{N}_3^-$ means three nitrogen atoms. Each nitrogen atom has Z = 7, so the total proton number is $3 \times 7 = 21$. The $-1$ charge means there is one more electron than protons, giving 22 electrons.

Step-by-Step Reasoning

1. Sodium ion: A neutral sodium atom has 11 protons and therefore 11 electrons. The $+1$ charge on $\text{Na}^+$ means the atom has lost one electron:
   $11 - 1 = 10$
   So the sodium ion has 10 electrons.

2. Azide ion: The ion is $\text{N}_3^-$, so it contains three nitrogen atoms. Each nitrogen atom has $Z = 7$, giving total protons:
   $3 \times 7 = 21$
   The superscript minus means the ion has one more electron than it has protons:n $21 + 1 = 22$
   So the azide ion has 22 electrons.

The correct pair is therefore 10 electrons for the sodium ion and 22 electrons for the azide ion, which is option B.

Key Takeaways

• For any ion: electrons = proton number - ionic charge, with the charge written with its sign.
• Positive ions have fewer electrons than protons; negative ions have more electrons than protons.
• For a polyatomic ion, add the proton numbers of all atoms in the formula before adjusting for the overall charge.

Common Mistakes

• Using the mass number instead of the proton number: sodium has a mass number close to 23, but Z = 11; the mass number is irrelevant here.
• Treating the azide ion as a single nitrogen atom: $\text{N}_3^-$ contains three nitrogen atoms, so total protons are 21, not 7.
• Getting the sign backwards: a negative ion has more electrons than protons, so for $\text{N}_3^-$ you add one electron, not subtract one.
• Options A and C give the azide ion as 20 electrons, which would correspond to 21 protons minus one electron. Only B and D give 22 for azide, but D uses 12 for sodium, misunderstanding that $\text{Na}^+$ has lost an electron.

Things to Be Careful About

• Use $Z = 11$ for sodium, not its relative atomic mass of approximately 23.
• In $\text{N}_3^-$, the subscript 3 multiplies the nitrogen atoms; the superscript $-1$ is applied only after summing the protons.n- Give the answer in the same order as the table: sodium ion first, azide ion second.