Definition of Isotopes

What's the definition of the isotopes? The video clip shows isotopes of hydrogen.


Sound simple, right? After watching the video clip, I hope you can identify some key words to define isotopes.

Isotopes are atoms that have the same number of protons but different number of neutrons. or

atoms that have the same proton number but different nucleon number.

My students used to give their definition of isotopes as follows:

1. same number of proton but different number of neutrons
2. same proton number but different neutron number.

Sadly, their answers are not considered as a complete definition., The first answer didn't mention the most important keyword: atoms. The second answer used "neutron number" which is incorrect term.

Some examples of isotopes :

1. carbon-12, carbon-13, carbon-14;
2. oxygen-16, oxygen-17, oxygen-18;
3. chlorine-35, chlorine-37.
4. chlorine-35 and chlorine-37.

Isotopes are widely in our daily life. Can you give some examples of isotopes used in medicine, industry, science and archeology?
In fact, the similar question turned out in SPM 2005.