Subatomic masses: a free neutron has a rest mass approximately equal to the mass of which neutral atom?

Introduction / Context:
Comparing subatomic particle masses with atomic masses offers useful intuition in nuclear physics. The neutron is electrically neutral and slightly heavier than the proton. Matching its mass to a familiar neutral atom highlights scale and aids in back-of-the-envelope estimates.

Given Data / Assumptions:

• Neutron rest mass ≈ 1.008665 atomic mass units.
• Hydrogen atom (protium) mass ≈ 1.007825 atomic mass units.
• Deuterium atom mass ≈ 2.0141 atomic mass units; helium atom ≈ 4.0026 atomic mass units.

Concept / Approach:
The hydrogen atom consists of one proton and one electron. Despite the electron’s small mass, the hydrogen atom’s total mass is very close to 1 u and is near the neutron’s mass. Deuterium and helium atoms are about two and four atomic mass units, respectively, and therefore not “approximately equal” to the neutron mass.

Step-by-Step Solution:

Verification / Alternative check:
Mass-energy equivalence (E = m*c^2) tracks the small mass differences; binding energies account for fine differences between free nucleons and bound atomic systems.

Why Other Options Are Wrong:

• Deuterium: roughly twice the neutron mass.
• Helium: roughly four times the neutron mass.
• None of these/Lithium: much larger deviations from neutron mass.

Common Pitfalls:
Confusing “hydrogen nucleus” (a proton) with the “hydrogen atom” (proton + electron) when comparing masses; overlooking orders of magnitude.

Final Answer:
Hydrogen