Helium does not have the lowest mass of all atoms. In fact, helium has a relatively low mass compared to many other elements, but it is not the lightest element.
The lightest atom in terms of atomic mass is hydrogen (H), which has a mass of approximately 1 atomic mass unit (amu). Hydrogen has a single proton and usually no neutrons in its nucleus, making it the lightest element.
Helium (He) is the second lightest element and has an atomic mass of approximately 4 amu. It consists of two protons and usually two neutrons in its nucleus, giving it a slightly higher mass than hydrogen. The extra mass in helium comes from the presence of two protons and neutrons.
It is important to note that when comparing atomic masses, we are considering the average mass of the atoms in a naturally occurring sample of an element, taking into account the various isotopes and their abundances. The atomic mass listed on the periodic table is an average value.
While helium has a low mass compared to many other elements, it is not the lightest atom in terms of atomic mass. Hydrogen holds that distinction.