Answer:
the number of neutrons in the nucleus
Step-by-step explanation:
The mass number 205 is the total number of protons and neutrons in the nucleus. Lead has 82 protons, so a mass number of 205 indicates a nucleus with 123 neutrons.
Similarly, the mass number 208 indicates a lead nucleus with 126 neutrons.
The difference in atomic structure is the difference in the number of neutrons in the nucleus.