Chemistry 9701/12 — May/June 2011

Background Concept

An atom is described by two numbers written as a nuclide symbol $^{A}_{Z}\text{X}$:

• Proton number (atomic number) $Z$ — the number of protons; this defines the element.
• Nucleon number (mass number) $A$ — the total number of protons + neutrons.

The number of neutrons is found by subtraction: neutrons = $A - Z$.

In a neutral atom, the number of electrons equals the number of protons.

Understanding the Question

We are asked to compare an atom of helium-4 with an atom of tritium, hydrogen-3. The phrase "the same in an atom" means we must compare the actual counts of sub-atomic particles (and the mass) in these two specific nuclides — not general properties of the elements helium and hydrogen.

Approach

1. Write each nuclide in $^{A}_{Z}\text{X}$ form.
2. Read off the proton number $Z$ for each.
3. Calculate neutrons = $A - Z$ for each.
4. Note electrons = protons (neutral atoms).
5. Compare each quantity across the two atoms.

Step-by-Step Reasoning

Step 1 — Helium-4 ($^{4}\text{He}$)

• Helium has $Z = 2$ (second element in the Periodic Table).
• $A = 4$, so neutrons = $4 - 2 = 2$.
• Electrons = 2.

Step 2 — Tritium ($^{3}\text{H}$)

• Hydrogen has $Z = 1$.
• $A = 3$, so neutrons = $3 - 1 = 2$.
• Electrons = 1.

Step 3 — Compare

• Electrons: 2 vs 1 → different.
• Neutrons: 2 vs 2 → same.
• Protons: 2 vs 1 → different.
• Relative atomic mass: 4 vs 3 → different.

Only the number of neutrons matches, so the correct option is B.

Key Takeaways

• The nucleon number $A$ is the sum of protons and neutrons; neutrons = $A - Z$.
• Two different nuclides can share the same neutron number even though they are different elements.
• The proton number uniquely identifies an element; changing protons changes the element entirely.

Common Mistakes

• Assuming that because both nuclides have mass number close to 3 or 4, they must share the same particles — each must be worked out separately.
• Confusing "relative atomic mass" (the nucleon number for a specific isotope) with a general property of the element.
• Forgetting that in a neutral atom, electrons = protons.

Things to Be Careful About

• Read the mass number carefully: $^{4}\text{He}$ has 4 nucleons, $^{3}\text{H}$ has 3 nucleons.
• Do not mix up proton number and nucleon number; the question asks about specific quantities, so each must be computed explicitly.
• Remember that tritium is hydrogen-3, so its proton number is 1, not 3.