It is a short step from using a symbol for an element to using a formula for a compound. However, it is often forgotten that die very act of writing the formula for a compound assumes the law of constant composition. HgO originally meant "some of a compound containing equal numbers of mercury and oxygen atoms". A formula used in this way is referred to as an empirical formula, for it contains only the cxpcrimantally determined ratio of the relative numbers of atoms of different elements in the compound, but makes no statement about how many atoms there are in one molcculc. An empirical formula should be written with brackets (HgO)x to show that it makes no claim to represent one molecule of the substance; but for inorganic solids where dicrc is seldom a method of determining jc, the brackets arc never used.
A molecule of a compound is defined as the smallest part of a compound that can exist as a free and separate substance. For crystalline solids in which one atom is surrounded by several equivalent neighbours, the word "molecule" has no well-defined meaning. Thus, the formula HgO refers to a gram-molecular weight of mercuric oxide, whereas the gram-molecular weight is the sum of gram-atomic weights of the elements in the proportions in which they occur in the compound. For example, zinc has a gram-atomic weight of 65 g and chlorine a gram atomic weight of 35.5 g, so the gram- molecular weight of zinc chloride (ZnCl2) Is 135 g.
For gases, the molcculc is a perfectly real entity, and die molecular weight of any volatile substance defined as