Isotopes share a proton count but have different neutron counts.
Key knowledge
Isotopes are atoms of the same element with different numbers of neutrons. Their atomic number is the same, while their mass number A, the total number of protons and neutrons, differs.
Values and equations
A = Z + N
A
Mass number: total nucleon count
Z
Number of protons
N
Number of neutrons
Conditions and scope
Mass number counts nucleons; it is not an atomic mass measured in kilograms.
A change in electron count changes ionic charge, not isotope identity.
Counts for one specified atomic nucleus; all three quantities are dimensionless integers.
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References
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Source definitions and magnesium isotope examples; isotope abundances and nuclear lifetimes are outside this record.
2026-10-10 · English isotope identity and mass-number distinction checked against Atomic Structure and Symbolism.
Foundational review
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Sources from other places
Scope or convention differs
Same proton number, different neutron counts and mass numbers.
A mass number counts nucleons; it is not an exact measured atomic mass.
Isotopes are atoms of the same element with different numbers of neutrons. Their atomic number is the same, while their mass number A, the total number of protons and neutrons, differs.
An atom has a nucleus surrounded by electrons. Its atomic number Z is the number of protons in the nucleus and identifies the element. A neutral atom has the same number of electrons as protons; gaining or losing electrons forms an ion of that element.
Isotope identity uses the shared atomic number of an element.
Radiometric dating combines isotope measurements with a decay model to estimate elapsed time. Interpretation requires accounting for initial daughter material and subsequent isotope gain or loss. A mineral's isotopic age can record formation or later resetting, rather than every event in the rock's history.
A stated isotope system and sample-retention history underlie an age interpretation.
An isotope delta compares a specified sample isotope ratio with the same ratio in a stated reference. Equilibrium and kinetic fractionation can change isotope partitioning between substances; mixing also changes measured ratios. A delta is a relative ratio difference, not an absolute isotope abundance or a unique process diagnosis.
The proton–proton chains and CNO cycles convert hydrogen into helium through different reaction sequences; CNO nuclei participate in a catalytic cycle. Helium burning can form carbon through the triple-alpha sequence. Rates and pathway contributions depend on temperature, density and composition, and neutrinos carry away part of the released energy.
Isotope identities and nuclear counts specify each pathway.
A mass spectrometer ionizes material, separates ions by mass-to-charge ratio and detects their abundance. Ionization and fragmentation produce method-dependent spectral patterns. A molecular ion, isotope pattern and fragments can constrain composition and connectivity, but the measured m/z is not automatically a neutral molecular mass.
Isotope composition contributes to spectral patterns.
The atomic mass constant mu is one twelfth of the mass of a free carbon-12 atom at rest in its ground state. A sample's relative atomic mass is its mean atomic mass divided by mu. Isotope fractions determine the mean; this is distinct from an isotope's integer mass number. Standard atomic weights describe specified normal terrestrial materials and may have intervals because isotope abundances vary.
The average is over isotopes of one element; isotope mass number and dimensionless relative atomic mass remain distinct.