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Atomic Mass Formula

Chemists have agreed that a neutral atom of the isotope carbon-12 is the appropriate standard. Its mass has been set at exactly 12 atomic mass units. It follows that 1 atomic mass unit is 1/12 of the mass of a neutral carbon-12 atom. All other atomic masses are measured relative to this value. The atomic mass is the mass of an atomic particle or molecule.

With this system, the masses of a proton and a neutron are nearly equal to 1 atomic mass unit, while the electron has a mass close to 0.0005 atomic mass unit. Atomic mass is also called atomic weight. For atoms, the protons and neutrons of the nucleus account for almost all of the mass, and the atomic mass measured in u has nearly the same value as the mass number.

Masses of fundamental atomic particles are often expressed in atomic mass units(u).

"One atomic mass unit (1u) is one twelfth of the mass of an atom of carbon with six protons and six neutrons."

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

1u = 1.66054 X 10-24g

Atomic mass = Number of protons + number of neutrons + number of electrons
The approximate mass of an atom can be estimated if the number of neutrons and protons is known. The sum of the number of protons and neutrons for an atom is called its mass number and is given by the symbol A.

A = mass number = number of protons + number of neutrons

The symbolization of an atom is represented below.

The subscript Z is optional because the element's symbol tells us what the atomic number must be.

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Atomic Mass Problems

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Solved problems based on atomic mass and mass number are given below.

Solved Examples

Question 1: The atomic number of an element is 17 and the number of neutrons in its atom is 18. Find the mass number of the element?
Atomic number or number of protons = 17
Number of neutrons = 18

Mass number(A) = Number of protons + Number of neutrons
Mass number(A) = 17 + 18
Mass number(A) = 35


Question 2: An element consists of two isotopes. Isotope A has an abundance of 75.00 percent and its mass is 14.000 atomic mass units. Isotope B has an abundance of 25.00 percent and its mass is 15.000 atomic mass units. What is the atomic mass of the element?
The term percentage abundance indicates how many atoms of an isotope are contained in 100 atoms of the element. In this example, 100 atoms of the element would contain 75 atoms of isotope A and 25 atoms of isotope B.

To calculate the atomic mass of the element, we multiply the mass of each isotope by the decimal equivalent of its percentage abundance and then add the results.

Atomic mass = (14.000u x 0.7500) + (15.000u x 0.2500)
Atomic mass = 14.25u


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