Periodic table with atomic masses

What atomic mass means

Almost all of an atom's mass is in its nucleus, in protons and neutrons that each weigh close to 1 u. One atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom. The value shown for each element is its standard atomic weight: the average mass of its natural isotopes, weighted by how common each one is.

One mole of an element has a mass in grams equal to its atomic mass, so the same number is also its molar mass. Carbon's atomic mass is 12.011 u, so one mole of carbon atoms has a mass of 12.011 g. On the table, masses are shown with up to three decimal places below 100 and up to two above.

Why atomic masses are not whole numbers

Most elements are a mixture of isotopes, atoms with the same number of protons but different numbers of neutrons. Chlorine, for example, is about 76% chlorine-35 and 24% chlorine-37. The weighted average of the two is 35.45, which is why chlorine's atomic mass falls between them instead of being a whole number.

Many radioactive elements, such as technetium, radon and every element after uranium, have no natural isotope mixture to average. For these, the table gives the mass number of the longest-lived or best-known isotope as a whole number, for example 98 for technetium and 222 for radon.

The trend and its exceptions

Atomic mass increases with atomic number almost everywhere, because each new element adds a proton and usually one or more neutrons. In a few places the order is reversed: argon is heavier than potassium, cobalt is heavier than nickel, and tellurium is heavier than iodine. Their most common isotopes carry extra neutrons. The table is ordered by atomic number, the number of protons, not by mass, so each element still sits in the group that matches its chemistry. Mendeleev placed tellurium before iodine for exactly this reason, trusting chemical properties over mass.

Worked example: the molar mass of water

To find the molar mass of a compound, add up the atomic masses of all its atoms. Water, H₂O, has two hydrogen atoms at 1.008 each and one oxygen atom at 15.999: 2 × 1.008 + 15.999 = 18.015 g/mol. So 18.015 g of water contains one mole of molecules, about 6.022 × 10²³ of them. For longer formulas, the molar mass calculator does the same sum for you.