A hydrogen atom consists of a single proton and a single electron. Neutrons, on the other hand, are electrically neutral particles found in the nuclei of atoms (except for hydrogen-1, which does not have any neutrons).
The atomic nucleus is formed by protons and neutrons, which are collectively known as nucleons. Protons have a positive charge, while neutrons have no electric charge. The number of protons in an atom determines its atomic number and defines what element it is. For hydrogen, the atomic number is 1, indicating that it has one proton.
In most naturally occurring isotopes of hydrogen, there are also one or two neutrons in the nucleus, resulting in the isotopes deuterium (hydrogen-2) and tritium (hydrogen-3), respectively. However, these isotopes are less common than hydrogen-1, which is simply a proton and an electron.
So, the absence of a neutron in a hydrogen atom is a defining characteristic of the element. It distinguishes hydrogen from other elements and contributes to its unique properties.