A hydrogen isotope is a variant of the hydrogen atom that has the same number of protons (1) but a different number of neutrons in its nucleus. Here's a breakdown:

* Protons: All hydrogen atoms have one proton. This is what defines them as hydrogen.

* Neutrons: The number of neutrons is what differentiates the isotopes.

Here are the three main isotopes of hydrogen:

1. Protium (¹H): This is the most common isotope, consisting of one proton and no neutrons. It's what we typically think of as "hydrogen."

2. Deuterium (²H): Also known as "heavy hydrogen," it has one proton and one neutron. It's about twice as heavy as protium.

3. Tritium (³H): This isotope has one proton and two neutrons. It's radioactive and decays over time.

Key Points:

* Isotopes have the same chemical properties: Because they have the same number of protons and electrons, isotopes of an element behave very similarly in chemical reactions.

* Isotopes have different physical properties: Their different masses can lead to slightly different physical properties like density, boiling point, and rate of diffusion.

* Uses: Isotopes of hydrogen have various applications, including:

* Deuterium: Used in nuclear reactors, as a tracer in scientific research, and in some medical imaging techniques.

* Tritium: Used in nuclear fusion research and in luminous paints.

Let me know if you have any other questions!