Writing Formulas

Objective: Count atoms and formula units in expressions.

In a chemical formula, a subscript gives the count of atoms of an element within one molecule or formula unit; parentheses group a polyatomic unit so an outside subscript multiplies every atom inside. Ca(OH)₂ contains one Ca, two O, and two H in each formula unit. A coefficient in an equation multiplies the entire formula: 3H₂O represents six H atoms and three O atoms. Changing a coefficient changes the number of units, while changing a subscript changes the chemical formula and often the substance. An absent subscript means one. Count atoms carefully before balancing or comparing mass.

Objective: Apply writing formulas to calculations, evidence, and specified situations.

Translate ion names to symbols and charges, then find the lowest ratio that gives zero total charge. Reduce any common factor in ionic subscripts. When more than one polyatomic ion is needed, enclose that ion in parentheses and place the multiplier outside. Subscripts indicate atom or ion ratios; coefficients indicate numbers of whole formula units. Once a compound’s identity is established, its subscripts cannot be changed simply to balance a reaction.