Margaret Georgina Todd was a Scottish medical doctor and writer. She coined the term isotope in 1913 in a suggestion to chemist Frederick Soddy.

Isotopes are distinct nuclear species of the same chemical element. They have the same atomic number and position in the periodic table, but differ in nucleon numbers due to different numbers of neutrons in their nuclei. While all isotopes of a given element have similar chemical properties, they have different atomic masses and physical properties.