Atomic mass is defined as the mass of one atom of the clement compared with the mass of one atom of C12 (the stable light isotope of carbon). Thus one atom of hydrogen which weighs approximately -j- the mass of one atom of carbon (1/2) has an atomic mass of 1(1/2×12=1). It may be noted that atomic mass is a ratio and hence has no units. Thus an atomic mass may be given in any unit of measure i.e. grams, pounds, ounces and so on. Generally atomic masses are expressed in atomic mass units (a.m.u). One atomic mass unit, therefore equals exactly one twelfth the mass of a carbon —12 atom.
MOLECULAR MASS AND FORMULA MASS
Every molecule must have a miss since it is composed of atoms. For instance the molecule of water is composed of the elements hydrogen and oxygen. The atoms of these elements form molecules; each consisting of two hydrogen atoms bonded to one oxygen atom and the molecule of water is represented by the molecular formula, H2O. The mass of a molecule is referred as its molecular mass and is defined as the sum of the atomic masses of the atoms of all the elements present in a molecule shown by its molecular formula. For example, the molecular mass of water (H2O) is (1+1+l6)18 a.m.u.
In many substances atoms do not aggregate into molecules. For instance in sodium chloride (NaC1), the atoms are not organized into discrete molecules, but are bonded to one another in a network structure. The same is true for silicon carbide (SiC). Such compounds are represented by their simplest (empirical) formulas. We can not assign molecular mass to such compounds because there are no discrete molecules; we, therefore, assign a quantity called formula mass for such compounds. Hence formula mass is the sum of the atomic masses as given in the simplest (empirical) formula of the substance.
Sodium chloride is represented by the simplest formula NaCl which means that in its crystalline form, the sodium ions (Na+) and chloride ions (CL-) occur in the ratio 1:1. Thus the formula mass of NaC1 is 23+35.5=58.5 a.m.u.