Difference between Atomic Mass and Atomic Number

Updated on August 25, 2015

The atomic mass and atomic number of an element can be confusing to many people. These two numbers have similarities, but are also different. They are used to reveal different characteristics of the element under study. This article defines each of them and helps differentiate between the two.


Atomic mass
Element showing its atomic number and mass number

Atomic number is used to define the type of element a material or substance is. It is the number of protons present in an element’s nucleus.

The atomic mass, on the other hand, is the number of both the protons and neutrons present in the nucleus of the element. Electrons do not usually weigh much, so the atomic mass is calculated by adding the number of protons to neutrons. It is also referred to as the atomic weight.

Comparison Chart

Atomic massAtomic number
It is the total number of protons and neutrons in an element’s nucleusIt is just the number of protons in an elements nucleus.
Does not define the type of an elementUsed to define an element

Atomic mass vs Atomic number

What is the difference between atomic mass and atomic number? The difference is that one is a count of the protons in an element’s nucleus while the other is a count of both the protons and neutrons.

Atomic mass, also atomic weight, is the total weight of an element. It is found by adding together the number of protons and neutrons in the nucleus of that element. Electrons are disregarded because they do not weigh that much. For elements that exist in several isotopes, the atomic mass of such elements is the weighted average mass of those element’s atoms, taking into consideration the relative natural abundance of the isotopes of those elements.

Atomic number is the total number of the protons that are present in the nucleus of an element’s atom. In a neutral atom, the number of electrons present in an element’s nucleus is equal to the number of protons present, and consequently is equal to the element’s atomic number.

Both the atomic number and atomic mass are closely related. This is because the atomic number of an element is part of its atomic mass. If the atomic number of an element is high, the atomic mass is also expected to be high.


Here is a video with more information about mass number and atomic number of an element:

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