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1.
Each element has many versions that contain a different number of neutrons in the nucleas. Each different version of the element is called an isotope.
Carbon-13 is and isotope of Carbon
by PrivateDonut February 10, 2005
42 7
 
2.
Isotopes are atoms of the same element which have differing number of neutrons.

This means they have the same number of protons.
Deuterium is an isotope of hydrogen.
Deuterium is hydrogen-2, i.e. hydrogen with a mass number of 2 (1 proton + 1 neutron).

Tritium is a rarer isotope of hydrogen, and is also known as hydrogen-3 (it has 1 proton + 2 neutrons).
by ChemMan May 12, 2010
8 2
 
3.
Lighter and heavier versions of the same thing, (but usually in reference to atoms)
Bud and Bud Lite are "isotopes" of Bud. When you gain weight, you just become an "isotope" of your former self.
by 387mqr August 15, 2008
11 6