Which is the lightest element known?

The lightest element known is hydrogen. Its atomic number is 1, which means it has the simplest and smallest atomic structure, consisting of only one proton and one electron. Because of this minimal structure, hydrogen has the lowest atomic mass of all elements, making it the lightest.

In comparison, helium, although also a light element with an atomic number of 2, is heavier than hydrogen because it contains two protons and two neutrons. Similarly, lithium and beryllium have even higher atomic numbers and more complex atomic structures, thus having greater atomic masses than hydrogen.