Ionic Compounds: Structure, Formation, and Nomenclature

Chapter 5: Ionic Compounds

Learning Objectives

• Define and describe ionic bonding.

• Determine the charge on ions using the periodic table and the octet rule.

• Write formulas for ionic compounds.

• Write the name of ionic compounds from a given formula and provide the formula when given the name of an ionic compound.

• Recognize common polyatomic ions and write the name and formula of compounds containing polyatomic ions.

Introduction to Bonding

Most elements combine with other elements to form compounds. The electrons involved in compound formation are called valence electrons, which are found in the outermost shell of an atom.

• Atoms join together in two general ways:

  • Ionic bonding: Valence electrons transfer from the atoms of one element (usually a metal) to the atoms of another element (usually a nonmetal) to form ionic compounds.

  • Covalent bonding: Valence electrons are shared between two atoms or among atoms of different elements to form covalent compounds.

Chemical Bonds

Chemical bonds are the forces that hold atoms of elements together in a compound. There are two main types relevant to this chapter:

• Ionic bonds:

  • Formed when a metal and a nonmetal bond together.

  • One or more electrons transfer from the metal to the nonmetal.

  • This process produces ions (charged atoms) that are attracted to one another.

  • The result is an ionic bond and the formation of an ionic compound.

• Covalent bonds:

  • Formed by the sharing of valence electrons between nonmetals.

  • Not the focus of this chapter, but important for understanding molecular compounds.

Formation of Ions

An ion is an electrically charged particle formed when electrons are gained or lost by an atom.

• Cation: A positively charged ion formed when a neutral atom loses one or more electrons.

  • Example: Sodium ion ($\text{Na}^+$) has a +1 charge.

• Anion: A negatively charged ion formed when a neutral atom gains one or more electrons.

  • Example: Chloride ion ($\text{Cl}^-$) has a -1 charge.

Periodic Table and Ionic Charges

The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their outermost shell, similar to the nearest noble gas.

• Metals (left side of the periodic table) tend to lose electrons and form cations.

• Nonmetals (right side of the periodic table) tend to gain electrons and form anions.

For example, sodium (Na) loses one electron to become $\text{Na}^+$, while chlorine (Cl) gains one electron to become $\text{Cl}^-$. The resulting ions have electron configurations similar to noble gases.

Transition Metals and Variable Charges

Many transition metals and some main group metals can form more than one type of cation, each with a different charge. For example:

• Iron can exist as $\text{Fe}^{2+}$ (iron(II)) or $\text{Fe}^{3+}$ (iron(III)).

When naming compounds with these metals, the charge is indicated using Roman numerals in parentheses.

Properties of Ionic Compounds

Ionic compounds are solids at room temperature and usually form crystals with a highly ordered structure called a crystal lattice. They share several key properties:

• High melting point: Strong electrostatic attractions between ions require high temperatures to melt the compound.

• High conductivity (when dissolved or molten): Ionic compounds conduct electricity when their ions are free to move, such as in aqueous solution or molten state.

• High solubility in water: Many ionic compounds dissolve readily in water, allowing their ions to move freely and conduct electricity.

Writing Formulas for Ionic Compounds

The formula of an ionic compound reflects the ratio of cations to anions needed to achieve electrical neutrality. The sum of all ionic charges in a compound must be zero:

• For example, sodium chloride ($\text{NaCl}$) contains one $\text{Na}^+$ and one $\text{Cl}^-$ ion.

• For magnesium chloride ($\text{MgCl}_2$):

  • Magnesium ion: $\text{Mg}^{2+}$

  • Chloride ion: $\text{Cl}^-$

  • Two chloride ions are needed to balance the +2 charge of magnesium.

General rule: The total positive charge must equal the total negative charge.

Naming Ionic Compounds

The name of an ionic compound is constructed by naming the cation first, followed by the anion.

• Main group metal cations: Use the element name (e.g., calcium ion is "calcium").

• Transition metal cations: Indicate the charge with a Roman numeral (e.g., iron(III)).

• Simple anions: Use the element name with the ending changed to "-ide" (e.g., chloride, oxide).

Examples:

• $\text{KCl}$: potassium chloride

• $\text{MgO}$: magnesium oxide

• $\text{CrCl}_2$: chromium(II) chloride

Polyatomic Ions

Polyatomic ions are groups of covalently bound atoms that carry an overall ionic charge. Many common polyatomic ions contain a nonmetal bonded to oxygen.

• Examples:

  • $\text{NO}_3^-$: nitrate

  • $\text{SO}_4^{2-}$: sulfate

  • $\text{CO}_3^{2-}$: carbonate

  • $\text{OH}^-$: hydroxide

Common Polyatomic Ions Table

Ion Name	Formula	Charge
Nitrate	NO3-	-1
Sulfate	SO42-	-2
Carbonate	CO32-	-2
Hydroxide	OH-	-1
Phosphate	PO43-	-3
Ammonium	NH4+	+1
Acetate	C2H3O2-	-1
Additional info: Table entries inferred from standard GOB Chemistry curriculum.

Writing and Naming Ionic Compounds with Polyatomic Ions

Polyatomic ions combine with other oppositely charged ions to form ionic compounds. The naming convention is similar to binary ionic compounds:

• Name the cation first, followed by the polyatomic anion.

• Use parentheses in the formula if more than one polyatomic ion is needed (e.g., $\text{Fe(NO}_3)_3$ for iron(III) nitrate).

Examples:

• $\text{Na}_2\text{SO}_4$: sodium sulfate

• $\text{Mg(OH)}_2$: magnesium hydroxide

• $\text{K}_2\text{CO}_3$: potassium carbonate

• $\text{Fe(NO}_3)_3$: iron(III) nitrate

Medical Application Example

Potassium chloride ($\text{KCl}$) is administered to patients with low potassium levels. Ionic compounds are widely used in medicine, industry, and daily life.

Summary Table: Key Concepts in Ionic Compounds

Concept	Description	Example
Ionic Bond	Transfer of electrons from metal to nonmetal	NaCl
Cation	Positively charged ion	Na+
Anion	Negatively charged ion	Cl-
Polyatomic Ion	Covalently bound group with charge	SO42-
Crystal Lattice	Ordered structure of ionic solids	NaCl
High Melting Point	Strong ionic attractions	MgO
Conductivity	Conducts electricity when dissolved or molten	NaCl(aq)
Additional info: Table entries inferred from standard GOB Chemistry curriculum.