2025 H2 Chem P1 Q2 — Atomic structure Solution

The answer is B (Q could be a proton only; R could be an electron only).
• Q: the charge per particle is \(\dfrac{+96500}{F} = \dfrac{+96500}{96500} = +1\) (in units of the electronic charge). Only the proton carries a +1 charge (the electron is \(-1\), the neutron is 0), so Q can only be a proton.
• R: mass of one particle \(< \dfrac{0.500 \times 10^{-3}\ \text{kg}}{6.02 \times 10^{23}} = 8.31 \times 10^{-28}\) kg.
From the Data Booklet, the electron has mass \(9.11 \times 10^{-31}\) kg, below this limit, while the proton and neutron each have mass \(1.67 \times 10^{-27}\) kg, above it. So R can only be an electron.
• A is wrong: R cannot be a proton or a neutron, since one mole of either has a mass of 1.0 g, more than 0.500 g.
• C is wrong: Q cannot be an electron, since one mole of electrons carries a charge of \(-96500\) C, not +96500 C; R also cannot be a proton or a neutron.
• D is wrong: Q cannot be an electron, because its charge is negative.