Atomic Weight Calculator - Element Atomic Mass Finder
Enter an atomic number from 1 to 118 to find the element's symbol, name, and atomic weight in atomic mass units. Values follow standard IUPAC atomic weights.
Elemental Calculator - Atomic Weight Finder
Documentation
The atomic weight finder is a lookup tool that returns an element's atomic weight from its atomic number. Enter a number from 1 to 118 and the tool displays the matching element's symbol, name, and atomic weight in atomic mass units (amu).
What atomic weight means
Atomic weight, formally called relative atomic mass, is the average mass of an element's atoms as they occur in nature. It is not the mass of a single atom.
Most elements occur naturally as a mix of isotopes: atoms with the same number of protons but different numbers of neutrons. Atomic weight is the weighted average of the masses of these isotopes, weighted by how common each one is.
Chlorine is a common example. About 76% of natural chlorine atoms are chlorine-35 and about 24% are chlorine-37. Averaging these masses by abundance gives chlorine's atomic weight of 35.45 amu, a value that sits between the two whole mass numbers.
The atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom. Chemists have used this standard since 1961.
Atomic weight formula
For an element with more than one isotope, atomic weight is calculated as:
Atomic weight = Σ (fᵢ × mᵢ)
Here fᵢ is the fraction of atoms that are isotope i, and mᵢ is the mass of isotope i. The fractions of all isotopes of an element add up to 1.
An element with only one stable isotope has an atomic weight equal to that isotope's mass. Elements with no stable isotopes at all, such as technetium and every element heavier than uranium, are instead assigned the mass number of their most stable known isotope.
How to find an element's atomic weight
- Enter an atomic number between 1 and 118 in the input field. The atomic number is the number of protons in an atom's nucleus, and it is unique to each element — 6 is always carbon, 79 is always gold.
- The calculator shows the element's symbol, full name, and atomic weight.
- A copy button lets the result be pasted elsewhere, such as into a lab report or a homework answer.
A number outside the 1–118 range produces an error message, since no confirmed element exists there.
Example: finding oxygen's atomic weight
Oxygen has atomic number 8. Entering 8 returns:
- Symbol: O
- Name: Oxygen
- Atomic weight: 15.999 amu
That value can be used to find the molecular weight of a compound. Water (H₂O) contains two hydrogen atoms and one oxygen atom:
2 × 1.008 (hydrogen) + 1 × 15.999 (oxygen) = 18.015 amu
The atomic number (8) should not be confused with the atomic weight (15.999). The atomic number counts protons only. Atomic weight reflects the combined mass of protons and neutrons across all of an element's naturally occurring isotopes.
Atomic number vs. atomic weight vs. mass number
| Property | Definition | Carbon example |
|---|---|---|
| Atomic number | Protons in the nucleus | 6 |
| Atomic weight | Average mass of naturally occurring atoms | 12.011 amu |
| Mass number | Protons plus neutrons in one isotope | 12 (for carbon-12) |
Atomic number sets an element's identity and its place on the periodic table. Atomic weight and mass number both describe mass, but atomic weight is an average across isotopes, while mass number belongs to a single, specific isotope.
History of atomic weight measurement
John Dalton proposed the first table of atomic weights in the early 1800s, using hydrogen as the reference point. His figures were rough estimates, some off by 10% or more.
Dmitri Mendeleev built the periodic table in 1869 by arranging elements in order of atomic weight. A few elements, including argon and potassium, seemed out of order compared with their chemical properties. The puzzle was not solved until isotopes were discovered.
Frederick Soddy identified isotopes in 1913, showing that most elements are natural mixtures of atoms with different masses. This explained why so many atomic weights fall between whole numbers. Mass spectrometry, developed over the following years, let chemists measure isotope masses and abundances directly instead of estimating them.
In 1961 the carbon-12 standard replaced hydrogen as the reference point for the atomic mass unit. The International Union of Pure and Applied Chemistry (IUPAC) has reviewed and updated standard atomic weights since, most recently in 2021, as measurement techniques have improved. Elements 113, 115, 117, and 118 were formally named in 2016, completing the seventh row of the periodic table.
Frequently asked questions
What is the difference between atomic weight and atomic mass? Atomic mass is the mass of a single atom or isotope. Atomic weight, formally relative atomic mass, is the average mass of all of an element's naturally occurring isotopes, weighted by their abundance. The two terms are often used interchangeably in everyday chemistry, but atomic weight is the more precise term for the value this calculator returns.
Why are atomic weights not whole numbers? Two reasons. Most elements are natural mixtures of isotopes with different masses, so the average falls between whole numbers. Also, the mass of a nucleus is slightly less than the sum of its protons and neutrons, because some mass converts into the binding energy that holds the nucleus together.
How accurate are the values in this calculator? The values follow IUPAC's standard atomic weights, accurate to four or five significant figures. That level of precision covers ordinary classroom and laboratory calculations. Work that requires full uncertainty ranges should consult IUPAC's published tables directly.
How is atomic weight related to molar mass? The two values are numerically equal but use different units. Carbon's atomic weight is 12.011 amu per atom; its molar mass is 12.011 grams per mole, where a mole is 6.022 × 10²³ atoms.
Does atomic weight affect how an element reacts? Not directly. Chemical reactivity depends mainly on electron configuration, not mass. Isotope mass can slightly change reaction speed, an effect called the kinetic isotope effect, but it does not change which reactions occur.
Why do some periodic tables show atomic weight ranges instead of single numbers? For a handful of elements, natural isotope ratios vary depending on where a sample was taken. IUPAC lists these elements with an interval rather than one fixed number. This calculator uses IUPAC's conventional single value for each element, which fits typical samples.