Increasing Atomic Number Doesn't Always Increase Mass

Protons, Neutrons, and Isotopes

The universe is made up of atoms.
The universe is made up of atoms. Panoramic Images/Getty Images

Since atomic number is the number of protons in an atom and atomic mass is the mass of protons, neutrons, and electrons in an atom, it seems intuitively obvious that increasing the number of protons would increase the atomic mass. However, if you look at the atomic masses on a periodic table, you will see that cobalt (atomic No. 27) is more massive than nickel (atomic No. 28). Uranium (No. 92) is more massive than neptunium (No.93). Different periodic tables even list different numbers for atomic masses. What's up with that, anyway? Read on for a quick explanation.

Neutrons and Protons Not Equal

The reason increasing atomic number doesn't always equate to increasing mass is because many atoms don't have the same number of neutrons and protons. In other words, several isotopes of an element may exist.

Size Matters

If a sizeable portion of an element of lower atomic number exists in the form of heavy isotopes, then the mass of that element may (overall) be heavier than that of the next element. If there were no isotopes and all elements had a number of neutrons equal to the number of protons, then atomic mass would be approximately twice the atomic number. (This is only an approximation because protons and neutrons don't have exactly the same mass, but the mass of electrons is so small that it is negligible.)

Different periodic tables give differing atomic masses because the percentages of isotopes of an element may be considered changed from one publication to another.