Hydrogen has an atomic number of 1, which means it has one proton in its nucleus. The mass number of an atom is defined as the total number of protons and neutrons in its nucleus.
For the most common isotope of hydrogen, known as protium, there are no neutrons present, so the mass number is:
- Mass number = Number of protons + Number of neutrons
- For hydrogen (protium): Mass number = 1 proton + 0 neutrons = 1
Thus, for this isotope of hydrogen, both the atomic number (which reflects the number of protons) and the mass number are equal to 1.
Hydrogen does have two other isotopes: deuterium (which has 1 proton and 1 neutron, giving it a mass number of 2) and tritium (which has 1 proton and 2 neutrons, giving it a mass number of 3). However, when referring specifically to hydrogen, it's typically in the context of the most common isotope (protium), where the mass number and atomic number are the same.