Atomic Mass


Isotopes are identified by their mass number, which is the total number of protons and neutrons. Remember, the electrons have so small of mass in comparison to the protons and neutrons that it's practically zero and doesn't need to be considered. Protium is referred to as hydrogen-1, with its mass number of one. Protium is distinguished from deuterium, hydrogen-2, by its increase in mass number. Similarly, carbon-14, used for dating rocks, is differenciated from carbon-12 by two neutrons, which increases the mass number by two.


An alternate way of writing mass numbers, commonly used by nuclear physicists, is to write the mass number as a superscript to the left of the atomic symbol and the atomic number as a subscript below the mass number. This allows for easy calculation of the number of protons, by subtracting the atomic number from the mass number. Continuing with our example of hydrogen, using this notation, deuterium would be written as



learning activity Scroll your mouse over the isotopes shown to learn about the placement of numbers in this notation.
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The number of neutrons for deuterium can be determined by subtracting the number of protons (1) from the mass number (2).

2 - 1 = 1 neutron.


Uranium is an element in which its isotopes play a huge role in many common functions. Uranium-238 has an atomic number of 92.

Uranium can be written, using the symbolic notation as:


If we take the mass number minus the atomic number (238 - 92) can determine that this isotope of uranium has 146 neutrons!

Definitely not equal to the number of protons!

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