while electrons have practically no mass.
Protons and neutrons each have mass of roughly 1 atomic mass unit (amu), and
an atom’s mass number is equal to the sum of its protons and its neutrons.
Now, we’ve said that in an electrically neutral atom, the number of protons is
equal to the number of electrons. But this says nothing about neutrons. Most
carbon atoms, for instance, have 6 neutrons in their nuclei. A few have 8. All
have 6 protons in their nuclei, so they all have the atomic number 6, and they’re
all carbon. But they can differ in the number of neutrons.
If two atoms of the same element differ in the number of neutrons in their nuclei,
they are said to be isotopes; a carbon atom that has 6 neutrons and a carbon atom
that has 8 neutrons are isotopes.
Since an atom’s mass number is equal to the sum of its protons and its neutrons,
two different isotopes of the same element will have different mass numbers. For
instance, the carbon atom that has 6 neutrons in its nucleus has a mass number of
6 protons + 6 neutrons = 12 amu. And, because its mass number is 12, we call it
carbon-12. The carbon atom that has 8 neutrons in its nucleus has a mass number
of 6 protons + 8 neutrons = 14 amu. And, because its mass number is 14, we call
it carbon-14.