Answer:

Introduction:
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Hydrogen is the lightest and simplest element in the periodic table, with only one proton and one electron. However, there are three isotopes of hydrogen that differ in their number of neutrons.

Explanation:
The three isotopes of hydrogen are protium, deuterium, and tritium. Let's discuss each isotope in detail:

1. Protium (Symbol: H-1):
Protium is the most common and abundant isotope of hydrogen, making up about 99.985% of naturally occurring hydrogen. It consists of one proton and no neutrons. Protium is the simplest form of hydrogen, and its nucleus contains only a single proton.

2. Deuterium (Symbol: H-2):
Deuterium is a stable isotope of hydrogen that is relatively rare, making up about 0.015% of naturally occurring hydrogen. It consists of one proton and one neutron in its nucleus. Deuterium is often used as a tracer in chemical and biological research.

3. Tritium (Symbol: H-3):
Tritium is a radioactive isotope of hydrogen, with one proton and two neutrons. It is highly unstable and decays over time with a half-life of about 12.3 years. Tritium is used in a variety of applications, including as a tracer in research and in self-luminous exit signs.

Conclusion:
Among the given options, helium (He) is not an isotope of hydrogen. Helium is a completely different element with two protons and two neutrons in its nucleus. It is the second lightest element in the periodic table and is a noble gas.