Chemical Formulas and Naming

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 common naming patterns with stated scope.

A simple binary ionic compound usually names the cation first and the anion with an -ide ending: NaCl is sodium chloride. Some metals form multiple ion charges and require a Roman numeral, such as iron(III) chloride for FeCl₃. Binary molecular compounds of nonmetals can use prefixes to indicate atom counts, such as carbon dioxide for CO₂ and dinitrogen tetroxide for N₂O₄. Naming conventions have exceptions and common names; the formula and charge model must be considered. Polyatomic ions such as nitrate retain group names in many compounds. A Roman numeral gives metal ion charge, not the number of metal atoms.

Objective: Use atomic masses to compute simple relative formula masses.

Relative formula mass sums the appropriate atomic masses for each atom represented in a formula. Using approximate atomic masses H ≈ 1 and O ≈ 16, H₂O has relative molecular mass about 18; using Ca ≈ 40, Ca(OH)₂ has about 74. Molar mass has the same numerical value expressed in g/mol for ordinary isotopic composition, but relative formula mass itself is dimensionless. The periodic table’s atomic mass is an isotopic weighted average, so mass calculations are approximations at the stated precision. Percent composition by mass is component mass divided by total formula mass times 100%. Counts and parentheses must be applied before arithmetic.