Isotopes
Isotopes show that an element can keep the same chemical identity while its nucleus changes. The key is to separate proton number from neutron number.
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Isotopes in one minute
Isotopes are nuclides with the same atomic number but different mass numbers. In practical terms: same number of protons, different number of neutrons.
Same protons means the same element. Different neutrons means a different isotope. Isotopes can differ in mass and nuclear stability even though they belong to the same element.
See how the idea connects
These are explanatory steps, not buttons. Read from left to right to follow the cause-and-effect chain.
The element identity stays the same.
The nucleus becomes a different isotope.
Different neutron count changes mass number.
Isotopes can have very different nuclear stability.
How can two atoms be the same element but different isotopes?
An element is defined by its proton number. Neutrons contribute to nuclear mass and stability but do not set the element identity. That is why carbon can occur as carbon-12, carbon-13 and carbon-14 without any of them becoming a different element.
The IUPAC definition captures this precisely: isotopes share the same atomic number and differ in mass number.
How do you read isotope notation?
The mass number is commonly written with the element name, such as carbon-14, or as a superscript before the symbol, such as 14C. If the atomic number is also shown, it may appear as a lower-left subscript.
Z = atomic number = protons
X = element symbol
Once A and Z are known, neutron number follows from N = A − Z.
Worked example: carbon-14
Carbon has atomic number 6, so every carbon nucleus has 6 protons. Carbon-14 has mass number 14.
A neutral carbon-14 atom therefore has 6 protons, 8 neutrons and 6 electrons. The isotope is radioactive because its nucleus is unstable, but it is still carbon because the proton number remains 6.
Do isotopes have exactly the same chemistry?
Isotopes of one element have the same proton number and, for neutral atoms, the same electron count and broadly the same electron configuration. That is why they generally take part in the same kinds of chemical reactions.
They are not physically identical, however. Their masses differ, and that can produce measurable isotope effects. Nuclear stability can differ dramatically: carbon-12 is stable while carbon-14 is radioactive; uranium-235 and uranium-238 are both radioactive but have different nuclear behavior.
Why isotopes matter across the five reviewed elements
Natural magnesium is a mixture of stable isotopes, so its periodic-table atomic-weight value is an average rather than one isotope mass.
Open Magnesium →Carbon-14 connects isotope identity to radioactive dating, while carbon-12 anchors the atomic-mass scale.
Open Carbon →Natural gold is dominated by one stable isotope, Au-197, which is why its isotope story is simpler than magnesium’s.
Open Gold →U-235 and U-238 share 92 protons but differ in neutron count and nuclear behavior.
Open Uranium →Oganesson is known from short-lived synthetic isotopes; there is no stable natural isotope mixture to average.
Open Oganesson →What students often mix up
Isotope does not mean ion. Isotopes differ in neutrons; ions differ in electrons.
Mass number is not the same as relative atomic mass or standard atomic weight.
Radioactive isotope does not mean “different element”; element identity still comes from proton number.
Check your understanding
Answer before opening the explanation. The aim is understanding, not speed.
1An isotope has Z = 92 and A = 238. How many neutrons does it have?
146 neutrons, because 238 − 92 = 146. This is uranium-238.
2Two nuclei have 6 protons but 6 and 8 neutrons. Same element or different elements?
Same element: both are carbon. They are different isotopes because their neutron counts differ.
3What must change for an atom to become a different element?
The proton number must change.
Sources and terminology
Definitions and reference claims are anchored to authoritative scientific organizations and peer-reviewed literature where needed. Element Lookup adds teaching explanation, examples and visual structure; it does not treat AI as the source of scientific definitions or numbers.
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