To understand living organisms, therefore, it is crucial to know how the chemical bonds that hold atoms together in molecules are formed. The characteristics of substances other than pure elements—including the materials from which living cells are madedepend on which atoms they contain and the way the atoms are linked together in groups to form molecules. The dense, positively charged nucleus contains most of the atom's mass. The much lighter and negatively charged electrons occupy space around the nucleus. The chemical properties of an atom are determined by the number and arrangement of its electrons: it is most stable when its outer electron shell is completely filled. Many scientists simply had trouble believing that molecules heavier than about 4000 daltons—the largest compound that had been synthesized by organic chemists—could exist at all. Researchers tried to estimate its size by breaking it down into its chemical components.