Chemistry 1.25 - 1.26 - Names and ionic formulae

Names and constituent ions

The positive ion is named first and the negative ion second. The ending of the second name gives useful information, but it must be read carefully.

The ending -ide

A simple negative ion made from one element takes the ending -ide: chlorine becomes chloride, oxygen becomes oxide, and bromine becomes bromide. Compounds such as sodium chloride and magnesium oxide therefore contain two elements.

A halide is any Group 7 ion in a compound: fluoride $\ce{F-}$, chloride $\ce{Cl-}$, bromide $\ce{Br-}$ or iodide $\ce{I-}$.

The ending -ate

The named ions nitrate, carbonate and sulfate all contain oxygen joined with another element. In these examples, an ionic compound ending in -ate contains oxygen and at least two other elements: potassium nitrate, for example, contains potassium, nitrogen and oxygen.

Do not turn this into the false rule that every -ide compound has only two elements. Hydroxide, $\ce{OH-}$, is the important exception in this lesson: it is a group of oxygen and hydrogen atoms with an overall $1-$ charge.

The constituent-ion formulae needed for formula deductions are:

The small number already inside a polyatomic ion formula is part of that ion. For example, $\ce{NO3-}$ contains one nitrogen atom and three oxygen atoms and behaves as one ion with charge $1-$. A formula deduction must preserve that internal composition.

Deducing neutral formulae

An ionic compound has no overall charge. Its formula gives the smallest whole-number ratio of ions for which total positive charge equals total negative charge.

Use this charge-balance method: