Final answer:
The only element whose most common isotope has no neutrons is hydrogen, specifically the isotope known as protium. The atomic number, which is the number of protons in an atom's nucleus, identifies the element. Hydrogen's atomic number is 1, making it unique among elements.
Step-by-step explanation:
The element that is unique in that it's the only atom without a neutron is hydrogen. Specifically, the most common isotope of hydrogen, known as protium, has one proton and no neutrons.
Every atom of an element has a distinctive number of protons, known as the atomic number, and hydrogen's atomic number is 1. This number is crucial as it defines the element: no other element besides hydrogen has exactly one proton in its atoms.
While the mass of a neutron is slightly greater than the mass of a proton, their diameters are approximately the same. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Hydrogen has isotopes as well, such as deuterium with one proton and one neutron, and tritium with one proton and two neutrons, but these are less common than protium.