All three hydrogen isotopes, protium (H), deuterium (D), and tritium (T), have one proton and one electron, so their chemical behaviour is almost identical. The differences between them come only from the number of neutrons in the nucleus, which changes the mass but barely touches the electron cloud that decides bond length and bonding.
- Internuclear distance in hydrogen and deuterium is same: bond length is set by how the electron clouds of the two atoms overlap, and since H and D have identical electronic structure, the H-H and D-D internuclear distances come out essentially equal. This statement is correct.
- Enthalpy of bond dissociation of deuterium is less than hydrogen: the heavier D nucleus vibrates with a smaller zero-point energy than H, which actually makes the D-D bond slightly stronger and harder to break, not weaker. This statement has it backwards.
- Order of relative abundance of isotopes is H > T > D: natural hydrogen is almost entirely protium, with a small trace of deuterium and only a radioactive trace of tritium, so the real order is H greater than D greater than T. This statement has D and T swapped.
- Ratio of neutrons to protons is greater for D than T: D has 1 neutron and 1 proton, giving a ratio of 1, while T has 2 neutrons and 1 proton, giving a ratio of 2. The ratio is actually larger for T, so this statement is backwards too.
Only the first statement survives this check, since the other three each get some detail of mass, abundance, or ratio reversed.
Let's summarize:
- Isotopes share electronic structure, so bond length is nearly identical across H and D.
- Heavier isotopes tend to have slightly stronger bonds and lower natural abundance, not higher.
So the correct statement is that the internuclear distance in hydrogen and deuterium is the same.