One is for potentially dating fossils (once-living things) using carbon-14 dating, and the other is for dating rocks and the age of the earth using uranium, potassium and other radioactive atoms.
The atomic number corresponds to the number of protons in an atom.
In 1947, Libby obtained the first age determination using the carbon-14 dating technique and won the Nobel Prize for his discovery in 1960.
Scientists use a technique called radiometric dating to estimate the ages of rocks, fossils, and the earth.
Atomic mass is a combination of the number of protons and neutrons in the nucleus.