The atomic weight of an atom is a measure of how much mass an atom has. The atomic weight is calculated by adding the number of protons and neutrons together. Atomic masses are not listed as whole numbers on the periodic table because atoms can come in forms with different amounts of neutrons. The atomic weight reported for any particular element is an average weight of all the known forms of that element.
The weighted average of the atomic masses of the different isotopes of an element. A single 12C atom, for example, has a mass of 12 amu, but naturally occurring carbon also contains a 1.1% 13C. The atomic weight of carbon is therefore 12.011 amu.
atomic mass. The average mass of an atom of an element, usually expressed in atomic mass units. The terms mass and weight are used interchangeably in this case. The atomic weight given on the periodic table is a weighted average of isotopic masses found in a typical terrestrial sample of the element.
The nominal atomic weight of an isotope is given by the sum of the number of neutrons and protons in each nucleus. The exact atomic weight differs fractionally from that whole number because neutrons are slightly heavier than protons and the mass of the nucleus is also affected by the binding energy.
As listed on the periodic table, the atomic weight is a weighted average of the masses of stable isotopes of an element that occur in nature. Given in grams, the atomic weight is the weight of one mole of atoms of an element. Atomic weight is often used interchangeably with atomic mass.
The average weight (or mass) of all the isotopes of an element, as determined from the proportions in which they are present in a given element, compared with the mass of the 12 isotope of carbon (taken as precisely 12.000), that is the official international standard; measured in daltons.