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Element by Number Lookup

Enter an atomic number to look up the matching chemical element and its symbol.

Result

Gold
Au
79
196.967 u

How it is calculated

You enter an atomic number Z, the number of protons in the nucleus. The tool rounds Z to the nearest whole number and looks it up in a table of all 118 elements, returning the symbol, the name, the number and the standard atomic mass in unified atomic mass units (u), shown with three decimals. For an element with no stable isotope the mass is the mass number of its longest-lived or best-known isotope, in square brackets, as periodic tables print it: technetium is [98] u. Numbers below 1 or above 118 give no result.

Worked example

Enter Z = 79. It is a whole number, so no rounding is needed, The tool returns the symbol Au, the name Gold, the number 79 and the mass 196.967 u. That mass is the weighted average over the element's natural isotopes, so a single atom of gold-197 is close to it but not identical.

Try this example →

Common mistakes

  • Reading a bracketed mass such as [294] u as a measured average. It is the mass number of one isotope, because the element has no natural mix to average.
  • Entering a decimal like 26.6. It is rounded to 27 and shows cobalt, not iron, with no warning.
  • Confusing atomic number with mass number. Iron is Z = 26, not 56 (its mass number), and 56 would give barium.
  • Reading the mass in u as grams. It is the mass of one atom in atomic mass units; the same figure is the molar mass in g/mol.
  • Treating the mass as that of one particular isotope. Without brackets it is the average for natural abundance.

Sources

About this calculator

Every chemical element has a unique atomic number — the count of protons in its nucleus — which is what actually defines the element on the periodic table, as codified by IUPAC. This lookup takes that number and returns the element's name, its one- or two-letter symbol, and its standard atomic mass (the weighted average of its naturally occurring isotopes), giving you the same identification you'd get from reading a periodic table cell, just by number instead of by symbol or position. It covers all 118 elements; for an element with no stable isotope the mass is the mass number of its longest-lived or best-known isotope, in square brackets.

Chemistry students, teachers preparing worksheets, and anyone working from a dataset or problem set that references elements only by atomic number use it to quickly confirm which element and mass they're dealing with without scanning a full 118-element chart. It's also a handy cross-check when studying periodic trends — since atomic number increases left-to-right and top-to-bottom across the table, the lookup helps connect a raw number to the element's actual position and properties.

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Comments (1)

  • Editorial team

    A practical use for the mass line is building molar masses by hand. Look up each element in the formula and add: sodium chloride is 22.990 for Na plus 35.45 for Cl, which gives 58.44 g/mol, and water is 2 × 1.008 plus 15.999, or 18.015 g/mol. This lookup covers only 50 elements, so check that every atom in your formula is in it. Which element numbers did you search for that came back empty?

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