Variants of a chemical element that have the same number of protons and occupy the same position in the periodic table, but have a different number of neutrons; often used in medical diagnosis, biological tracing, and geological dating.

two or more words or phrases used together in a sentence to refer to the same person or thing

angles in the same relative position at each intersection where a straight line crosses two others; specifically, if the two lines are parallel, these angles are equal

radioactive isotope; radioisotope

the process of separating different isotopes of the same chemical element