Which element is known to not contain any neutrons?

Hydrogen is the element that is known to not contain any neutrons. The most common isotope of hydrogen, known as protium, has a single proton and no neutrons. This characteristic distinguishes hydrogen from other elements, which typically have at least one neutron in their atomic nuclei.

In contrast, oxygen, carbon, and helium all contain neutrons. For example, oxygen has eight protons and eight neutrons in its most stable isotope, carbon typically has six protons and six neutrons, and helium usually has two protons and two neutrons. Therefore, hydrogen stands out as the only element that can be found without any neutrons, underscoring its unique atomic structure.