Hydrogen has three isotopes : protium (¹H), deuterium (²H or D) and tritium (³H or T). They have nearly same chemical properties but different physical properties. They differ in :

Solution

Chemistry ma’am · 3 steps

Isotopes share protons and electrons; only the neutrons change, so only the mass changes.

1. What isotopes share. Isotopes of an element have the same number of protons, so the same atomic number. In a neutral atom the electrons equal the protons, so the electronic configuration is the same too.
2. What changes. Only the number of neutrons is different. Each extra neutron adds about 1 u, so the atomic mass changes.

   Isotope	Protons	Neutrons	Electrons	Mass number
   Protium, ¹H	1	0	1, 1s¹	1
   Deuterium, ²H	1	1	1, 1s¹	2
   Tritium, ³H	1	2	1, 1s¹	3

3. So. The three isotopes differ only in atomic mass, option (d). The same 1s¹ configuration is why their chemical properties are nearly the same, and the different masses are why their physical properties differ.

The idea to keep. Chemistry follows the electrons and physics follows the mass. Isotopes keep the protons and electrons and change only the neutrons, so they differ in mass and not in chemical behaviour.

Answer from the NTA final answer key. Solution by PrepWiser.