The terms "atomic weight" and "relative atomic mass" both refer to the mass of an atom, but they have different meanings.
The atomic weight of an element is a weighted average of the masses of all the naturally occurring isotopes of that element. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Hydrogen, for example, has three isotopes: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium). These isotopes have different masses, so the atomic weight of hydrogen is the weighted average of these isotopes based on their natural abundance. The atomic weight of hydrogen is approximately 1.008 atomic mass units (u) according to the periodic table.
On the other hand, the relative atomic mass, also known as the atomic number, specifically refers to the mass of the most common isotope of an element. For hydrogen, the most abundant isotope is hydrogen-1, which consists of a single proton and no neutrons. Therefore, the relative atomic mass of hydrogen is approximately 1 atomic mass unit (u).
In summary, the atomic weight of an element takes into account the masses and relative abundances of all its isotopes, while the relative atomic mass (atomic number) refers to the mass of the most abundant isotope of that element.