Atomic Mass Calculator: Element Atomic Weight Lookup

What is atomic mass?

Atomic mass is the average mass of one atom of a chemical element, measured in atomic mass units (u). This calculator looks up the atomic mass of any element by its name or chemical symbol, such as "Oxygen" or "O".

An atomic mass unit is defined as exactly 1/12 the mass of a carbon-12 atom. Most elements exist in nature as a mix of isotopes, atoms with the same number of protons but different numbers of neutrons. The atomic mass shown on a periodic table is the weighted average of those isotopes, based on how common each one is.

Because it is an average, atomic mass is rarely a whole number. Chlorine, for example, has an atomic mass of 35.45 u, even though every chlorine atom has either 35 or 37 nucleons.

Atomic mass formula

The average atomic mass of an element is calculated by multiplying the mass of each isotope by its natural abundance, then adding the results together:

$\text{Atomic Mass} = \sum_{i} (f_i \times m_i)$

Here, $f_i$ is the fraction of atoms that are isotope $i$ (written as a decimal), and $m_i$ is the mass of that isotope in atomic mass units. The sum covers every isotope of the element found in nature.

Worked example: chlorine

Natural chlorine is about 75.77% chlorine-35 (mass 34.97 u) and 24.23% chlorine-37 (mass 36.97 u). Applying the formula:

$(0.7577 \times 34.97) + (0.2423 \times 36.97) \approx 35.45 \text{ u}$

This matches the value shown on the periodic table and returned by the calculator when a user searches "Chlorine" or "Cl".

The values in this calculator come from the standard atomic weights published by the International Union of Pure and Applied Chemistry (IUPAC).

How to use the elemental mass calculator

1. Type an element's name (for example, "Hydrogen") or its symbol (for example, "H") into the input field.
2. The calculator shows the element's name, symbol, atomic number, and atomic mass.
3. Use the copy button to copy the atomic mass value.

The search is not case sensitive. It matches names and symbols regardless of capitalization, so "iron", "Iron", "Fe", "fe", and "FE" all return the same result: Iron, atomic number 26, atomic mass 55.845 u.

Quick examples

Why atomic mass matters

Calculating molecular weight

Atomic mass is the building block for molecular weight, the mass of a whole molecule. To find it, multiply each element's atomic mass by how many atoms of it appear in the molecule, then add the totals.

Water (H₂O) has two hydrogen atoms and one oxygen atom:

$(2 \times 1.008) + (1 \times 15.999) = 18.015 \text{ u}$

Carbon dioxide (CO₂) has one carbon atom and two oxygen atoms:

$(1 \times 12.011) + (2 \times 15.999) = 44.009 \text{ u}$

Preparing solutions

Chemists use molecular weight to weigh out compounds for a solution of a known concentration. For sodium chloride (NaCl), sodium contributes 22.990 u and chlorine contributes 35.45 u, giving a molecular weight of 58.44 g/mol. Making one liter of a 1 molar (1 M) NaCl solution requires weighing out 58.44 grams of salt.

Connecting atoms to grams

One mole of any element weighs its atomic mass in grams and contains about 6.022 × 10²³ atoms, a quantity known as Avogadro's number. This relationship links the microscopic scale of atoms to masses that can be measured on a lab balance, and it underlies most calculations in stoichiometry, the branch of chemistry that deals with the quantities of substances in reactions.

A short history of atomic mass

In 1803, John Dalton published the first table of atomic weights, setting hydrogen equal to 1 and measuring other elements relative to it. When Dmitri Mendeleev built the periodic table in 1869 by ordering elements by atomic weight, a few elements appeared to be out of place. That puzzle was not solved until isotopes were discovered decades later.

In 1961, scientists agreed to define carbon-12 as exactly 12 u, replacing earlier standards based on oxygen. This is still the reference used today. IUPAC reviews atomic weight measurements periodically and updates the standard values when new evidence justifies it, though changes are usually too small to affect ordinary calculations.

Frequently asked questions

What is the difference between atomic mass and atomic weight?

Strictly, atomic mass refers to the mass of a single atom or isotope, while atomic weight is the weighted average across all of an element's naturally occurring isotopes. In everyday use, including on periodic tables, the two terms are treated as synonyms.

Why are atomic masses not whole numbers?

Because they are averages of two or more isotopes with different masses. Chlorine's atomic mass of 35.45 u falls between its two common isotopes, chlorine-35 and chlorine-37, because both appear in nature in different proportions.

What does it mean when an atomic mass is shown without decimals, like technetium at 98?

Elements with no stable isotopes, such as technetium and plutonium, do not have a natural isotope mixture to average. Their listed mass is the mass of their most stable or most studied isotope instead.

Is the element search case sensitive?

No. Typing "fe", "Fe", or "FE" all return iron, and the same applies to element names.

How do I find the molecular weight of a compound?

Multiply each element's atomic mass by the number of times it appears in the formula, then add the results. For water, that is (2 × 1.008) + 15.999 = 18.015 u.

Where do these atomic mass values come from?

They follow the standard atomic weights published by IUPAC, the international body responsible for chemical nomenclature and measurement standards.