Isotopes and Nuclear Charge-to-Mass Ratio

Steps:

• ^1H nucleus: 1 proton (charge +1e), mass ≈1 u; ^2H nucleus: 1 proton + 1 neutron (charge +1e), mass ≈2 u.
• Both isotopes have atomic number Z=1, so same nuclear charge of +1e.
• Charge per unit mass for ^1H: +1e / 1 u = +1e/u.
• For ^2H: +1e / 2 u = +0.5e/u, half that of ^1H.

Why D is correct:

• Charge-to-mass ratio is nuclear charge divided by mass number; deuterium's doubled mass halves this ratio relative to hydrogen, per the definition of specific charge in nuclear physics.

Why the others are wrong:

• A: Both neutral atoms have 1 electron each, as electron number equals Z.
• B: Deuterium nucleus mass (2 u) exceeds hydrogen's (1 u) due to extra neutron.
• C: Both nuclei have +1e charge from single proton.

Final answer: D