Citation:
The weight of a hydrogen atom is approximately 1 atomic mass unit (amu), as it consists of one proton and one electron (the neutron count in a common hydrogen atom is zero).
Variants of an element that have different numbers of neutrons but same number of protons.
Deuterium: An isotope of hydrogen with one neutron, making its atomic weight approximately 2 amu.
Tritium: Another isotope of hydrogen with two neutrons and one proton, giving it an atomic weight around 3 amu.