Isotope

Etymology

From Greek "isos" (equal) plus "topos" (place) — “same place,” since isotopes share one slot on the periodic table. Frederick Soddy coined it in 1913 on a suggestion from the physician Margaret Todd.

noun · /EYE-suh-tope/
One of several versions of the same element whose atoms carry different numbers of neutrons, and so different weights.
What it names
Why one element can come in heavier and lighter varieties — some stable, some radioactive — that behave identically in chemistry but differently in the nucleus.
Example
Carbon dating works because the isotope carbon-14 decays at a known rate while ordinary carbon-12 does not.
Related idea
All isotopes of an element share its chemistry; some are radioactive, meaning their nuclei break down and emit radiation. Carbon-12 is stable, but its isotope carbon-14 is radioactive — the basis of dating.