HELP on ATOMIC WEIGHT (RELATIVE ATOMIC MASSES) (AW) (in atomic mass units, amu)
The mass of the atoms of an element. The mass of an isotope is not simply the sum of the masses of all the constituent protons and neutrons within the nucleus, because when they are all gathered together inside the nucleus, some of the mass is 'lost' and this loss is called the binding energy. For natural elements with more than one isotope, the atomic weight is the average value for the mixture of isotopes. Atomic weight increases almost linearly with Atomic number (number of protons), but not quite: elements with high atomic number have a greater proportion of neutrons than protons, and so are proportionately heavier. The range varies from 1.008 for hydrogen, to 238.0 for uranium, and beyond. (1 atomic mass unit=1.660 10éçîkg). By definition, the atomic mass, M of the isotope åçC is 12 exactly. Atomic mass numbers from 1 to 238 are found naturally on Earth except for masses 5 and 8. The atomic weights given for elements too radioactive to be found naturally on Earth are quoted for the longest lived isotope.
