What does it mean if atoms have the same atomic number but a different atomic mass?

1 Answer


Assuming you just mean the mass of the atom, it means that they have a different number of neutrons. Otherwise, all atoms with the same atomic number have the same atomic mass.


This is a basic model of the atom:

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Let's begin with some definitions:

  • The atomic number of the atom is the number of protons in the nucleus. These are the orange particles in the model above.
  • The atomic mass of an atom is the weighted average of all the isotopic masses, where each isotope has a different number of neutrons.
  • The mass number is the number of protons plus neutrons.

Since electrons are very small (their size is exaggerated in the model) and take up negligible mass in an atom, the atomic mass is effectively the mass of the proton plus the mass of the neutron... er, minus the energy (in the form of mass) needed to form the atom from its parts.

For light atoms,

#"atomic mass" ~~ "mass of protons" + "mass of neutrons" - "mass defect"#

On the other hand, again:

#"mass number" = "number of protons" + "number of neutrons"#


So, if the atomic number of atoms is the same, the number and therefore mass of the protons would also be the same.

But we know that if the mass number of the atoms is different, the number of neutrons is different.

As mentioned, these atoms with the same number of protons but different number of neutrons are called isotopes.