Mass Number Versus Atomic Number and Atomic Mass

Mass number, atomic mass, and atomic number are three related concepts in chemistry.

• Atomic number is the number of protons in an atom of an element. It is a whole number or the symbol Z in general notation. It isn’t always listed because you can figure out the number of protons if you know the element symbol.
• Mass number is the sum of the number of protons and neutrons in an atom. In other words, it is the number of nucleons in an atom. Mass number is also a whole number, with the symbol A in general notation. It is given on the upper right or upper left side of an element symbol.
• Atomic mass (atomic weight) is the average number of protons and neutrons in a sample of an element. It is a number calculated based on the natural abundance of isotopes of an element, so it doesn’t have to be a whole number. For example, the atomic mass of helium is 4.003 rather than 4.

Atomic Mass and Mass Number

Both atomic mass and mass number reflect the number of protons and neutrons in a sample. The difference is that atomic mass on the periodic table is the average mass of all the isotopes of a naturally-occurring sample of an element. In contrast, the mass number is the number of protons and neutrons of a single atom of an element. For example, the mass number of ⁷₃Li is 7, while the atomic mass of lithium is 6.941. What this tells you is some atoms of natural lithium have a mass number lower than 7.

Atomic Number and Mass Number Examples

Atomic number is an element’s identity. The periodic table of elements lists elements according to increasing atomic number, with hydrogen having an atomic number of 1, helium with an atomic number of 2, and so on until you reach oganesson, with atomic number 118. If you look up an element on the table, the atomic number is the whole number given on an element tile.

Sometimes you’ll see atomic number in isotope notation, but other times you’ll only see the mass number. For example, in ⁴₂H is the mass number is 4 and the atomic number is 2. The mass number is 4 in ⁴He. Another way to write mass number is following the element symbol. For example, He-4 and He-3 designate two isotopes of helium. The first has a mass number of 4, while the second has a mass number of 3.

Hint: If there are two numbers, the smaller number is the mass number, while the larger number is the mass number.

Atomic number does not change for any element isotope. Only mass number changes, because the number of neutrons determines the isotope.

There is one instance where atomic number, atomic mass, and mass number are the same. This is for a pure sample of the isotope of hydrogen called protium, which has one proton and no neutrons. All of the numbers are “1.”

Find the Number of Protons and Neutrons

You can use the mass number and either the atomic number or element symbol to determine the number of protons and neutrons in an atom.

For example: Find the number of protons and neutrons in ¹⁴₆C (also written as carbon-14).

The larger number is the sum of the protons and neutrons. The number of protons or atomic number is 6, which you get either from the notation or by looking up the atomic number of carbon on a periodic table. To get the number of neutrons, subtract the number of protons from the mass number:

Number of Neutrons = Mass Number – Atomic Number
Number of Neutrons = 14 – 6
Number of Neutrons = 8

The number of protons is the atomic number, 6.

References

• Bishop, Mark. “The Structure of Matter and Chemical Elements”. An Introduction to Chemistry. Chiral Publishing. ISBN 978-0-9778105-4-3.
• Jensen, William B. (2005). “The Origins of the Symbols A and Z for Atomic Weight and Number”. J. Chem. Educ. 82: 1764.