Which element is known to not contain any neutrons?

Hydrogen is the element that is known to not contain any neutrons in its most common isotope, which is known as protium. Protium is the simplest and lightest hydrogen isotope, consisting of only one proton and one electron, thus making it neutronless.

It’s important to note that while hydrogen can exist in other isotopes, such as deuterium and tritium, these isotopes include one and two neutrons respectively. However, when referring to the most prevalent form of hydrogen, it does not have any neutrons.

In contrast, the other elements listed do contain neutrons in their most common isotopes. For instance, helium typically has two protons and two neutrons, lithium contains three protons and three or four neutrons, and oxygen usually has eight protons and eight neutrons.