Polyatomic Ions, Molecular Compounds and Acids

A polyatomic ion is a group of atoms bonded tightly together that carries an overall charge. In nitrate, ${\text{NO}}_3^{-}$, one nitrogen and three oxygen atoms share electrons with each other, and the whole group has one extra electron. In a compound, a polyatomic ion behaves like a single unit: it keeps its name, its formula and its charge. You write and name these compounds with exactly the same charge-balancing rules you used for binary ionic compounds. Most of these ions have to be memorized, so learn the table below in small groups.

1. Write each ion with its charge, metal (or ammonium) first. Calcium hydroxide: ${\text{Ca}}^{2+}$ and ${\text{OH}}^{-}$.
2. Find how many of each ion make the total charge zero: one ${\text{Ca}}^{2+}$ needs two ${\text{OH}}^{-}$.
3. If you need more than one of a polyatomic ion, put it in brackets and write the subscript outside: ${\text{Ca(OH)}}_2$. Never change the subscripts inside the ion.
4. If only one polyatomic ion is needed, leave out the brackets: NaOH, ${\text{KNO}}_3$, ${\text{CaCO}}_3$.
5. To name, write the cation name then the polyatomic ion's name unchanged: ${\text{Al}}_2{({\text{SO}}_4)}_3$ is aluminum sulfate.

When two non-metals combine, neither one gives up electrons; instead they share electrons, forming a molecule. Because there are no ions, there are no charges to balance, and the same two elements can often combine in more than one ratio. Carbon and oxygen form both CO, a poisonous gas, and ${\text{CO}}_2$, the gas in soft drinks. So the name has to say exactly how many atoms of each element are in the molecule, and it does this with Greek prefixes.

• Name the first element in full, with a prefix only if there are two or more atoms of it. "Mono" is never used on the first element: CO is carbon monoxide, not monocarbon monoxide.
• Name the second element with a prefix (including mono) and the -ide ending: ${\text{CO}}_2$ is carbon dioxide, ${\text{CCl}}_4$ is carbon tetrachloride.
• When a prefix ending in a or o comes before "oxide", drop that vowel: monoxide, tetroxide, pentoxide. So ${\text{N}}_2{\text{O}}_4$ is dinitrogen tetroxide and ${\text{P}}_2{\text{O}}_5$ is diphosphorus pentoxide.
• A few molecular compounds keep common names you should know: ${\text{H}}_2\text{O}$ water, ${\text{NH}}_3$ ammonia, ${\text{CH}}_4$ methane and ${\text{H}}_2{\text{O}}_2$ hydrogen peroxide.

An acid is a compound that releases hydrogen ions, ${\text{H}}^{+}$, when it dissolves in water. You can usually spot one because its formula starts with H and it carries the state symbol (aq), meaning aqueous, or dissolved in water. The state symbol matters: HCl(g) is hydrogen chloride gas, and only when it is dissolved in water, HCl(aq), is it called hydrochloric acid. Acids are named by two rules. An acid made of hydrogen and one other element (a binary acid) is named hydro- + root + -ic acid. An acid containing a polyatomic ion with oxygen (an oxyacid) changes the ion's ending: -ate becomes -ic acid and -ite becomes -ous acid.