Is mass number the same as the atomic number?

1 Answer
Mar 27, 2016


Nope, they're both different numbers representing different things!


The atomic number is the number of protons in an atom's nucleus. It's usually that big number that increases by 1 for every element on the Periodic Table. For example, the atomic number for beryllium is 4, because it has 4 protons; the atomic number for Einsteinium is 99. because it has 99 protons.

The mass number is the total mass of the protons AND neutrons in the nucleus. It is measured in atomic mass units (amu). So carbon-12 has a mass number of 12 amu, for it has 6 protons and 6 neutrons. Selenium-80 has a mass number around 80 amu, for it has 34 protons and 46 neutrons. Notice that the isotope number is the same as the mass number.