Hydrogen is unique compared to other elements in the periodic table because it has only one proton, which gives it an atomic number of 1. The atomic number of an element is defined as the number of protons in its nucleus.
The mass number of an element is the total number of protons and neutrons in the nucleus. For the most common isotope of hydrogen, known as protium, there are no neutrons, so the mass number is also 1 (1 proton + 0 neutrons = 1).
Thus, for hydrogen:
- Atomic number = 1 (1 proton)
- Mass number = 1 (1 proton + 0 neutrons)
This relationship holds true specifically for the most abundant isotope of hydrogen, but hydrogen does have other isotopes (deuterium with 1 neutron and tritium with 2 neutrons) that have different mass numbers. However, when we refer to hydrogen's mass number being the same as its atomic number, we are usually discussing the most common isotope, protium.