Atomic Mass Formula

Atomic Mass Formula

Atomic mass is the total of the masses of the electrons, neutrons, and protons in an atom, or in a group of atoms, the average mass. When compared to protons or neutrons electrons have so much less mass that they don’t influence the calculation. Thus, the total of the masses of neutrons and protons is the atomic mass.


Atomic mass = Number of protons + number of neutrons + number of electrons


There are three methods to know the atomic mass, depending on one’s circumstances.

1) Use the Periodic Table!

If it’s one’s first encounter with chemistry, their tutor will want them to study how to make use of the periodic table to find the atomic weight of an element. This digit is typically written underneath an element’s representation. Search for a decimal number, which is the weighted average of the atomic masses of every natural isotope of an element.

2) Sum of Protons and Neutrons

Add up the mass of neutrons and protons to compute the atomic mass of a single atom of an element.


3) For All Atoms of an Element – Weighted Average

The weighted average of every element’s isotopes grounded on their abundance in nature is the atomic weight of an element. It is simple to compute the atomic mass of any element with these tips.

Usually, in these questions, one is provided with a list of isotopes with their natural abundance and their mass either as percent value or a decimal. Multiply each isotope’s abundance by its mass. If someone’s abundance is a percent, they should divide their solution by 100. Add these values up. The solution is the whole atomic weight or atomic mass of the selected element.

The atomic mass unit can be associated to other units of mass using the conversion factor

                                          1u = 1.66054 X 10-24g












Problem 1: Find the mass number of an element whose atomic number is 16 and the number of neutrons in its atom is 19.


Atomic number or number of protons = 16

Number of Neutrons = 19

Mass number (A) = Number of neutrons + Number of protons

Mass number (A) = 19 + 16

Mass number (A) = 35


Problem 2: What is the atomic weight of an element that consists of two isotopes? Isotope A has an abundance of 75.00 percent, and its mass is 14.000 units. The Isotope B has a mass of 15.000 atomic mass units and an abundance of 25.00 percent.


The word percentage abundance points out how many atoms of an isotope are made up of 100 atoms of the element. In this instance, 100 atoms of the element would comprise 75 atoms of isotope A and 25 atoms of isotope B.

For computing the atomic weight of the element, we multiply the mass of individual isotopes by the decimal equivalent of their percentage abundance and then add the results.

Atomic mass = (14.000u x 0.7500) + (15.000u x 0.2500)

Atomic mass = 14.25u


Practise This Question

Identify a cleaner vehicle fuel from the given options.