Naming compounds and writing formulas: name and write formulas for ionic compounds (including polyatomic ions), binary molecular compounds and simple acids.

What this topic is asking

Standard CH.3 requires fluent nomenclature: writing a correct formula from a name and a correct name from a formula. Virginia expects you to handle ionic compounds (including those with polyatomic ions and transition metals), binary molecular compounds, and simple acids. A correct formula is the starting point for molar mass, balancing equations and every calculation that follows.

Ionic compounds

For example, aluminum ($\text{Al}^{3+}$) and oxygen ($\text{O}^{2-}$) cross over to $\text{Al}_2\text{O}_3$. To name an ionic compound, write the metal name, then the nonmetal name ending in -ide (sodium chloride, magnesium oxide). The charge of a main-group metal is fixed by its group, so no roman numeral is needed.

Polyatomic ions and transition metals

Transition metals can form more than one charge, so the name uses a roman numeral to state it: iron(II) is $\text{Fe}^{2+}$ and iron(III) is $\text{Fe}^{3+}$. Reading a formula in reverse, you find the metal's charge from the charge needed to balance the anion.

Binary molecular compounds

A binary molecular compound contains two nonmetals and is named with Greek prefixes that state the number of each atom: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5). The first element keeps its name (and drops mono- if it is one), the second ends in -ide. So $\text{CO}$ is carbon monoxide, $\text{CO}_2$ is carbon dioxide, and $\text{P}_2\text{O}_5$ is diphosphorus pentoxide. Prefixes are used only for molecular compounds, never for ionic ones.

Simple acids

An acid produces hydrogen ions in water and is named from its anion. If the anion has no oxygen (a binary acid), use "hydro...ic acid": $\text{HCl}$ is hydrochloric acid. If the anion is a polyatomic oxyanion, drop the ending and add -ic (for -ate) or -ous (for -ite): sulfate gives sulfuric acid ($\text{H}_2\text{SO}_4$), nitrate gives nitric acid ($\text{HNO}_3$).

Try this

Q1. Write the formula for magnesium nitrate (magnesium $\text{Mg}^{2+}$, nitrate $\text{NO}_3^-$). [1 point]

• Cue. $\text{Mg}(\text{NO}_3)_2$; one $\text{Mg}^{2+}$ needs two nitrate ions, which take parentheses.

Q2. Name the molecular compound $\text{SF}_6$. [1 point]

• Cue. Sulfur hexafluoride (hexa- for six fluorine, second element ends in -ide).