Chemistry
Because atoms of different elements react with one another in very specific proportions but atoms of different elements have different weights, chemists often describe the number of different elements and compounds in terms of the number of "moles". A "mole" of any element contains the same number of atoms: 602,214,150,000,000,000,000,000 atoms. The atomic mass of an element can be used to see how much of the element makes a mole. For example, the atomic mass of copper is about 63.55. That means about 63.55 grams of copper metal has a mole of atoms. The atomic mass of chlorine is about 35.45. That means 35.45 grams of chlorine has a mole of atoms in it.