Final answer:
Tritium differs from hydrogen-1 by having two neutrons in its nucleus, compared to none in hydrogen-1, making it heavier and radioactive.
Step-by-step explanation:
An atom of tritium differs from an atom of hydrogen-1 in terms of the number of neutrons in its nucleus. Hydrogen-1, also known as protium, has one proton and no neutrons, giving it a mass number of 1. In contrast, tritium has one proton and two neutrons, which results in a mass number of 3.
The presence of two neutrons in tritium compared to none in hydrogen-1 makes tritium much heavier and also radioactive. Tritium is used in various applications, including research and luminescent devices, due to its radioactive properties.