Short answer
An element is defined by proton count: all carbon atoms have six protons. An isotope is one neutron-count version of that element, such as carbon-12 or carbon-14. Isotopes are not different elements just because their masses differ. 1 2
An isotope is a version of an element; different isotopes of one element share proton number but differ in neutron number.
On this page
At a glance
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| Question | Isotope | Element |
|---|---|---|
| Identity rule | Proton count plus neutron count | Proton count |
| Example | Carbon-12 | Carbon |
| Can one element have several? | Yes, several isotopes | One elemental identity |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Isotope. A variant of an element whose atoms have a particular neutron count and therefore a particular mass number. 1
Element. A class of atoms identified by the same number of protons in their nuclei. 2
Key differences
Atomic number counts protons and defines an element. Mass number counts protons plus neutrons. Two atoms with the same atomic number but different mass number are isotopes of the same element. 1 2
How to tell them apart
Read nuclide notation or a name like uranium-235: the element name gives proton identity, while the number specifies mass number. Compare neutron counts after subtracting the atomic number. 1 2
Where they overlap
Every isotope belongs to an element. Chemical behavior is often similar across an element’s isotopes because electron structure is similar, while masses and nuclear stability can differ. 1 2
Edge cases
Some elements have no stable isotopes. Ion formation changes electrons rather than isotopic identity; radioactive decay may change the element if proton number changes. 1 2
Why the distinction exists
The words allow chemistry to group atoms by chemical identity while nuclear science tracks mass and stability differences within that group. 1 2
Common misconceptions
Carbon-14 is carbon, not a new element. Isotopes differ in neutrons, not in the number of protons that defines the element. 1 2
Examples
Carbon-12 and carbon-14 both have six protons, but the latter has two more neutrons. 1 2
Sources
Sources checked October 3, 2026.
- OpenStax — Atomic Structure and Symbolism. Atomic number and isotopes.
- OpenStax — Phases and Classification of Matter. Elements and compounds.