What is the lightest chemical element?

The title of lightest chemical element belongs to the one with the simplest atomic structure imaginable. With an atomic number of 1, its nucleus contains just a single proton and, in its most common form, no neutrons at all. An electron orbits this tiny nucleus, but it's the contents of the nucleus that account for nearly all of an atom's mass. Every other element on the periodic table has a more crowded nucleus containing more protons and neutrons, which makes them inherently heavier.

This elemental simplicity is a direct result of its origin story. Forged in the immediate aftermath of the Big Bang, it was the very first element to exist. Consequently, it is by far the most abundant element, making up roughly 75% of all normal matter in the universe. It serves as the primary fuel for stars, including our own sun, which fuses its atoms together to create helium in the process that powers our solar system.

On Earth, it's most famously known as a component of water, which is reflected in its name, derived from Greek words meaning "water-former." While heavier versions called isotopes exist (such as deuterium, which has one neutron), the fundamental, single-proton version reigns as the undisputed lightweight champion of the periodic table.