Atomic Masses of All 118 Elements and a Molar Mass Calculator

Molar mass calculator

Write your formula with the correct capital and lowercase letters: CO is carbon monoxide, and Co is cobalt. Write water of crystallization with a dot: CuSO₄·5H₂O.

How do you find molar mass by hand?

1. Count the atoms of each element in the formula. H₂SO₄ has 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms.
2. Look up the atomic mass of each element in the table: H 1.008, S 32.06, O 15.999.
3. Multiply each atom count by its atomic mass and add the results: 2 × 1.008 + 32.06 + 4 × 15.999 = 98.07 g/mol.

Write the answer in g/mol. In class, problems often use rounded values: H 1, O 16, S 32. With these values, the molar mass of H₂SO₄ is 98 g/mol.

Why is atomic mass not a whole number?

Atoms of the same element can have different numbers of neutrons. These atoms are called isotopes. Atomic mass is an average of the isotopes, weighted by how common each one is in nature. For example, about 76% of natural chlorine has a mass number of 35, and 24% has a mass number of 37, so the average is 35.45.

What does a number in brackets mean?

A number in square brackets, such as [98], is the mass number of the longest-lived isotope of an element that has no stable isotopes. Elements like this have no natural average mass.

Atomic mass table for all 118 elements

Values below 100 are rounded to three decimal places, and values above 100 are rounded to two. You can search by element name or symbol.

Sources

• CIAAW: Standard Atomic Weights
• NIST: Atomic Weights and Isotopic Compositions