http://www.ck12.org Chapter 7. Chemical Nomenclature
7.1 Ionic Compounds
Lesson Objectives
- Define and give examples of chemical formulas for ionic compounds.
- Be able to name and write the formulas for both monatomic and polyatomic ions.
- Explain and use the Stock system for naming ionic compounds, when necessary.
- Memorize the list of polyatomic ions, including both the formulas and the charges.
- When given the chemical formula for an ionic compound, be able to write its name.
- When given the name of an ionic compound, be able to write its chemical formula.
Lesson Vocabulary
- empirical formula: The lowest whole-number ratio between two ions.
- binary ionic compound: A compound made up of a cation and an anion.
- ternary ionic compound: An ionic compound that is composed of more than two elements.
- monatomic ion: Form when a single atom gains or loses electrons.
- polyatomic ion: An ion composed of more than one atom.
- oxoanion: Anions in which one or more oxygen atoms are all bonded to a central atom of some other element.
Introduction
As we saw in the previous chapter, ions are formed when atoms gain or lose electrons. If an atom loses one or more
electrons, the resulting ion has a positive charge (more protons are present than electrons). If the atom gains one or
more electrons, the resulting ion has a negative charge (more electrons are present than protons). Positive ions are
called cations, and negative ions are called anions. Because opposite charges attract one another, cations and anions
are held together by strong electromagnetic forces. An ionic compound consists of a large three-dimensional array
of alternating cations and anions. For example, sodium chloride (NaCl) is composed of Na+and Cl−ions arranged
into a structure like the one shown inFigure7.1.
The most straightforward way to describe this structure with a chemical formula is to give the lowest whole-number
ratio between the two ions, which is known as anempirical formula. In the case of NaCl, there are equal numbers
of sodium ions and chloride ions in the salt crystal. In contrast, a crystal of magnesium chloride has twice as many
chloride ions as magnesium ions, so it has a formula of MgCl 2.
Naming Ionic Compounds
Ionic compounds are composed of one type of cation and one type of anion. The name of an ionic compound can
be formed by writing the name of the cation followed by the name of the anion. For example, NaCl is composed