The modern periodic table is ordered by which property?

The modern periodic table is ordered by atomic number, the number of protons in an atom’s nucleus.

Atomic number gives every element a unique integer identity: hydrogen is 1, helium is 2, carbon is 6, and uranium is 92. In the standard table, elements are arranged from left to right in increasing atomic number, with each new row beginning when a new electron shell starts filling.

This was not always the rule. Dmitri Mendeleev’s 1869 table was organized largely by atomic mass and recurring chemical properties. That arrangement worked remarkably well, but it produced apparent exceptions, such as tellurium and iodine. In the early 20th century, Henry Moseley’s X-ray experiments showed that nuclear charge—not atomic mass—determines the correct sequence.

Atomic mass still appears on most periodic tables, but it is a weighted average of an element’s naturally occurring isotopes. Isotopes can have different numbers of neutrons, yet they remain the same element because their proton count—and therefore atomic number—does not change.