How to read an element entry
An atomic number identifies an element by its number of protons. The chemical symbol is the standardized one- or two-letter abbreviation used in formulas and equations.
Standard atomic weights reflect the relative atomic masses and isotopic abundances of normal terrestrial materials. Some values are intervals; compact tables often display an abridged or conventional value.
How to use the periodic table element lookup
- Search: Enter an exact atomic number or any part of an element symbol.
- Select an element: Choose a matching element tile.
- Read the entry: Review its atomic number, symbol, and representative mass value.
- Check precision needs: Use current IUPAC or CIAAW data for interval, isotope-specific, or high-precision work.
Formula and variables
Each atomic number maps to exactly one chemical element; changing the proton count changes the element.
Atomic number Z = number of protons- Z — Atomic number
- Number of protons in the nucleus (whole number)
- Ar — Relative atomic mass
- Isotopically weighted relative mass where a standard value exists (dimensionless)
Find iron by symbol
Search for the chemical symbol Fe.
- Search
- Fe
- Select Fe from the filtered element list.
Result: Iron has atomic number 26 and a representative atomic mass of 55.845 u.
Every iron atom has 26 protons; isotope neutron counts can differ.
Understanding your results
Atomic mass is not mass number
A standard atomic weight is generally an abundance-weighted value for terrestrial material. A mass number is the whole-number total of protons and neutrons for one nuclide.
- Element symbols are case-sensitive in chemical formulas.
- Bracketed or conventional mass numbers are used for elements without a standard atomic weight.
Assumptions
- The element order follows atomic numbers 1 through 118.
- A single abridged representative value is sufficient for the lookup.
Limitations
- Does not show standard-atomic-weight intervals or isotope abundance tables.
- Does not calculate isotope-specific exact mass.
- Representative values may be revised by IUPAC and CIAAW.
Common mistakes
- Confusing atomic number with atomic mass.
- Changing the capitalization of a chemical symbol.
- Treating a conventional mass number as a standard atomic weight.
Practical use cases
Formula work
Confirm a symbol before entering a chemical formula.
Periodic-table review
Find the atomic number and representative mass associated with an element.
Frequently asked questions
Why are some atomic masses not whole numbers?
Many standard atomic weights reflect a weighted mixture of naturally occurring isotopes.
Why do some sources show brackets around a value?
For elements without a standard atomic weight, periodic tables may show the mass number of a selected nuclide in brackets.
How many confirmed elements are in the table?
The lookup contains the 118 currently named elements, from hydrogen through oganesson.
Sources and review
- Periodic Table of Elements — International Union of Pure and Applied Chemistry. Accessed 2026-07-13.
- Abridged Standard Atomic Weights — Commission on Isotopic Abundances and Atomic Weights. Accessed 2026-07-13.
Reviewed 2026-07-13.