Ionic and Molecular Compounds

Types of Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. The two main types are ionic bonds and covalent bonds. Ionic bonds involve the transfer of electrons from metals to nonmetals, forming ions, while covalent bonds involve the sharing of electrons between nonmetals to form molecules.

• Ionic Compounds: Composed of positive and negative ions held together by electrostatic forces.

• Molecular Compounds: Consist of molecules formed by atoms sharing electrons through covalent bonds.

Ions and the Octet Rule

Formation of Ions

Atoms form ions to achieve a stable electron arrangement, often resembling the nearest noble gas configuration. This is described by the Octet Rule, where atoms tend to have eight electrons in their outer shell.

• Cations: Positively charged ions formed when metals lose electrons.

• Anions: Negatively charged ions formed when nonmetals gain electrons.

Formation of Cations

Metals in Groups 1, 2, and 13 lose valence electrons to form cations with a positive charge.

Formation of Anions

Nonmetals in Groups 15, 16, and 17 gain electrons to form anions with a negative charge. The name of the anion ends with "-ide" (e.g., chloride, oxide).

Ionic Charges from Group Numbers

The charge of an ion formed by a representative element can often be predicted from its group number:

• Group 1A: +1

• Group 2A: +2

• Group 3A: +3

• Group 5A: -3

• Group 6A: -2

• Group 7A: -1

Properties and Formulas of Ionic Compounds

Formation and Properties

Ionic compounds form when electrons are transferred from metals to nonmetals, resulting in a strong electrostatic attraction between oppositely charged ions. These compounds typically have high melting points and are solid at room temperature.

Writing Chemical Formulas

The chemical formula of an ionic compound shows the lowest whole-number ratio of ions that results in a neutral compound. The cation is always written first, followed by the anion. Subscripts are used to indicate the number of each ion needed to balance the charges.

• Example: combines with to form .

• Example: combines with two ions to form .

Examples of Ionic Compounds

Naming Ionic Compounds

Rules for Naming

• The name of the metal (cation) is written first and does not change.

• The name of the nonmetal (anion) is written second and ends with "-ide".

• Subscripts are not included in the name.

Metals with Variable Charges (Transition Metals)

Many transition metals can form more than one type of positive ion. The charge is specified using a Roman numeral in parentheses after the metal name.

• Examples: Fe2+ is iron(II), Fe3+ is iron(III).

• Zn2+, Cd2+, and Ag+ are exceptions and have only one common charge.

Determining Variable Charge

To name compounds with metals of variable charge:

1. Determine the charge of the cation from the charge of the anion and the overall neutrality of the compound.

2. Name the cation with its elemental name and the charge as a Roman numeral in parentheses.

3. Name the anion with the "-ide" suffix.

4. Write the cation name first, followed by the anion.

Example: For MnF2, the charge on Mn is +2, so the name is manganese(II) fluoride.

Polyatomic Ions

Definition and Common Polyatomic Ions

A polyatomic ion is a group of covalently bonded atoms that carries an overall charge. Most are negatively charged (anions), except for ammonium (NH4+).

• Most polyatomic ions end in "-ate" or "-ite" depending on the number of oxygen atoms.

• "Per-" and "hypo-" prefixes indicate more or fewer oxygen atoms, respectively.

Examples:

• ClO4-: Perchlorate

• ClO3-: Chlorate

• ClO2-: Chlorite

• ClO-: Hypochlorite

Writing Formulas with Polyatomic Ions

When writing formulas for compounds containing polyatomic ions, balance the total positive and negative charges. Use parentheses around the polyatomic ion if more than one is needed.

• Example: Magnesium nitrate: and combine to form .

Naming Compounds with Polyatomic Ions

Follow the same rules as for simple ionic compounds: write the cation first, then the name of the polyatomic ion. Recognition of polyatomic ions is essential for correct naming.

Summary Table: Common Polyatomic Ions

Practice Problems

• Learning Check #1: Determine the charge and name for: CdBr2, Cu2S, SnCl2, FeO.

• Learning Check #2: Write the formula for: Magnesium chloride, Nickel (III) oxide, Tin (IV) chloride, Copper (I) oxide.

• Learning Check #3: Write the formula for sodium phosphate.

• Learning Check #4: Name the following: Ca(NO3)2, SnSO4.