# 3: Ionic Compounds


When you think of bonding, you may not think of ions or molecules. Like most of us, you probably think of bonding between people. Like people, molecules bond — and some bonds are stronger than others. It's hard to break up a mother and baby, or a molecule made up of one oxygen and two hydrogen atoms! A chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom that may be involved in chemical interactions. Valence electrons are the basis of all chemical bonds.

• 3.1: Ions
Atoms can gain or lose electrons to form positively charged or negatively charged species called ions.
• 3.2: Ions and the Octet Rule
Atoms tend to gain or lose electrons to achieve an octet (8 valence electrons). Electron configurations can be used to show how many electrons are needed to complete an octet and form an ion.
• 3.3: Ions of Some Common Elements
The periodic table can be used to determine ion charge.
• 3.4: Periodic Properties and Ion Formation
The energy changes associated with ion formation are called ionization energy and electron affinity. Ionization energy is the energy required to remove an electron and electron affinity is the energy released when an atom gains an electron. This is a more quantitative way to determine which atoms will form anions and which atoms will form cations.
• 3.5: Naming Monoatomic Ions
Cations and anions formed from one element (monoatomic ions) are assigned names based on nomenclature rules used by chemists. Cations are named using the element name plus the word "ion". In cases where an atom forms ions of variable charge, Roman numerals are used to specify each ion. Anions are named using the element name with the ending changed to "ide".
• 3.6: Polyatomic Ions
• 3.7: Ionic Bonds
The attraction of oppositely charged ions caused by electron transfer is called an ionic bond. The strength of ionic bonding depends on the magnitude of the charges and the sizes of the ions.
• 3.8: Formulas of Ionic Compounds
Although ionic compounds are made up of ions with charges, the overall charge must be zero, because matter is electrically neutral. Therefore, proper chemical formulas for ionic compounds can be determined by balancing the total positive charge with the total negative charge.
• 3.9: Naming Ionic Compounds
Each ionic compound has its own unique name that comes from the names of the ions that make up the formula unit. By convention, the cation is always named first followed by the anion. Because the ratio of ions in a compound is determined by the charge on each ion, no prefixes are needed to indicate how many of each ion are present. When a compound contains an ion with variable charge, a Roman numeral is used to specify the charge.
• 3.10: Some Properties of Ionic Compounds
Ionic compounds are composed of cations and anions that are strongly attracted to each other. Hence, ionic solids have very high melting points and are extremely hard. When dissolved in water, the ions separate from each other, allowing them to form electrolyte solutions.
• 3.11: H⁺ and OH⁻ Ions - An Introduction to Acids and Bases
Hydrogen ions (H+) and hydroxide ions (OH–) are two important ions discussed later in this text as acidic and basic ions respectively. Both of these ions can form compounds that you will see in organic and biological chemistry.

Thumbnail: The crystal structure of sodium chloride, NaCl, a typical ionic compound. The purple spheres represent sodium cations, $$\ce{Na^{+}}$$, and the green spheres represent chloride anions, $$\ce{Cl^{−}}$$. (Public Domain; Benjah-bmm27 via Wikipedia)

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