A Level Physics Practice Exam 2026 - Free A Level Physics Practice Questions and Study Guide

Atoms of the same element that have different numbers of neutrons are referred to as isotopes. This term originates from the Greek words "isos," meaning "equal," and "topos," meaning "place," indicating that these atoms occupy the same position in the periodic table — they have the same number of protons (which defines the element) but differ in their neutron counts. This results in different atomic masses for the isotopes of a given element.

For example, carbon can exist as carbon-12 (with 6 neutrons) and carbon-14 (with 8 neutrons), both being carbon but having different mass numbers. The distinct neutrons can lead to varying physical properties and stability, which is why isotopes can play significant roles in phenomena such as radioactive decay and applications in medicine and dating organic materials.

The other terms provided do not accurately describe this phenomenon. Isomers refer to molecules with the same molecular formula but different structures. Ionization relates to the process where atoms or molecules gain or lose electrons. Homologs are typically used in organic chemistry to describe compounds that differ by a specific structural unit, often a -CH2- group.