What is the lightest known element?

Hydrogen is the lightest known element, with an atomic number of 1. This means it has one proton in its nucleus and no neutrons, making it the simplest and least massive of all elements. The extremely low atomic mass of hydrogen is significant in various scientific contexts, including chemistry and physics, as it plays a critical role in the formation of molecules and the universe itself.

In contrast, helium, while also a light gas, is heavier than hydrogen because it contains two protons and two neutrons. Oxygen and lithium are even heavier, with oxygen having eight protons and lithium three, contributing to their greater atomic masses. Understanding the properties of hydrogen also highlights its importance in energy production and the study of stars, where it fuels nuclear fusion.