Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Formula Mass
Formula Mass
Formula Mass: When Molecules Don't Exist
Some substances, such as sodium chloride, do not contain discrete molecules as their constituent units. In solid NaCl there is no such thing as a single "molecule of NaCl": the crystal is a vast, continuous three-dimensional structure of positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl⁻) arranged alternately, as shown in Fig. 1.10.
Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.
Fig. 1.10 is a simplified own-drawn schematic of the point made in the text: sodium chloride has no single discrete molecule, only a continuous three-dimensional lattice of alternating Na+ and Cl- ions. Every Na+ ion is surrounded by six neighbouring Cl- ions, and every Cl- ion is surrounded by six neighbouring Na+ ions -- an arrangement that repeats throughout the crystal, which is exactly why the text …
It may be noted that in sodium chloride, one Na⁺ ion is surrounded by six Cl⁻ ions, and vice-versa.
Because there is no single molecular entity, the formula NaCl — the simplest ratio of the ions — is used to calculate the formula mass instead of a molecular mass, since in the solid state sodium chloride does not exist as a single entity. The formula mass is the sum of the atomic masses of the atoms in one formula unit, computed exactly as the book prints it:
Do not use the term "molecular mass" for ionic compounds like NaCl — they do not form molecules. Use "formula mass" for such substances. …