Atomic Weight

Atoms are exceedingly small, so small that actual weights of atoms were not able to be determined until early in the twentieth century. The weight of an atom of oxygen-16 (an oxygen atom with eight neutrons in the nucleus) was found to be 2.657 × 10^-23 grams and an atom of carbon-12 (a carbon atom with six neutrons in the nucleus) was found to weigh 1.99 × 10^-23 grams. Because these units are so very small, they are not practical and are seldom used in everyday laboratory work. Rather, the weight of an atom is usually calculated in units other than grams, one that is closer to the size of the particle being weighed and is therefore more practical.

The table of atomic weights is based on a unit called an atomic mass unit, abbreviated u, or in older notation, amu. This unit is defined as 1/12 the mass of carbon-12 (12C) and is equal to 1.6606 × 10^-24 grams. On this scale, carbon-12 weighs exactly 12 atomic mass units. But because even the smallest amount of matter contains enormous numbers of atoms, atomic weights are usually interpreted to mean grams of an element rather than atomic mass units. When interpreted in grams, the atomic weight of an element represents 6.02 × 10²³ atoms, which is defined as one mole. Thus, the atomic weight in grams is the mass of an element that contains one mole or 6.02 × 10²³ atoms.

Atomic weights are actually atomic masses but historically they were called atomic weights because the method used to determine them was called weighing. This terminology has persisted and is more familiar to most people even though the values obtained are actually atomic masses.