Atom
Sometimes, we need a more exact measurement. The exact mass of an atom is called its atomic mass. This is usually measured with the atomic mass unit (amu), also called the dalton. One amu is exactly 1/12 of the mass of a carbon-12 atom, which is  grams. Hydrogen-1 has a mass of about 1 amu. The heaviest atom known, oganesson, has a mass of about 294 amu, or  grams. The average mass of all atoms of a particular element is called its atomic weight.