Ionic and Covalent Compounds: Structure, Naming, and Lewis Dot Representations

Chapter 3: Ionic and Covalent Compounds

Overview of Elements and Compounds

Pure substances in chemistry are classified as either elements or compounds. Elements can exist as atomic or molecular forms, while compounds are formed from two or more elements joined by chemical bonds. Compounds are further categorized based on the type of bonding: ionic or covalent.

• Elements: Pure substances consisting of only one type of atom.

• Atomic Elements: Exist as single atoms (e.g., noble gases).

• Molecular Elements: Exist as molecules, often diatomic (e.g., H2, N2, O2).

• Compounds: Substances formed from two or more elements chemically bonded together.

• Covalent Compounds: Atoms share electrons (typically nonmetals).

• Ionic Compounds: Atoms transfer electrons, forming cations and anions (typically metal + nonmetal).

Ionic and Covalent Bonds

Chemical bonds form because atoms achieve lower potential energy when bonded than when separate. Atoms bond by losing, gaining, or sharing valence electrons to achieve a stable electron configuration, often an octet (eight valence electrons).

• Ionic Bonds: Formed by the transfer of electrons from a metal (cation) to a nonmetal (anion).

• Covalent Bonds: Formed by the sharing of electrons between two nonmetals.

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve eight valence electrons.

Example: Sodium (Na) transfers an electron to chlorine (Cl) to form NaCl (ionic bond). Two chlorine atoms share electrons to form Cl2 (covalent bond).

Compounds Containing Monatomic Ions

Ionic compounds are composed of positive and negative ions (cations and anions) arranged so that the total charge is zero. The ratio of ions is reflected in the subscripts of the formula unit.

• Monatomic Ions: Ions consisting of a single atom with a positive or negative charge.

• Cation: Positively charged ion (e.g., Na+, Ca2+).

• Anion: Negatively charged ion (e.g., Cl-, O2-).

• Formula Unit: The simplest ratio of ions that results in a neutral compound.

Example: In NaCl, the ratio is 1:1 for Na+ and Cl-.

Monatomic Cations and Anions

Monatomic ions are formed by the transfer of electrons. When dissolved in water, ionic compounds conduct electricity. The periodic table helps identify common monatomic ions.

• Common Monatomic Cations: H+, Li+, Na+, K+, Mg2+, Ca2+, Fe2+, Fe3+, Cu2+, Ag+

• Common Monatomic Anions: F-, Cl-, Br-, I-, O2-, S2-

Example: Sodium (Na) loses one electron to form Na+; chlorine (Cl) gains one electron to form Cl-.

Formula Unit for an Ionic Compound

The formula unit of an ionic compound shows the simplest whole-number ratio of cations to anions that results in a neutral compound.

• Write the symbol for the metal and its charge, followed by the symbol for the nonmetal and its charge.

• The magnitude of the charge on each ion (without the sign) becomes the subscript for the other ion.

• Reduce subscripts to the smallest whole-number ratio.

• Ensure the sum of the charges of the cations exactly cancels the sum of the charges of the anions.

Example: For Mg2+ and Cl-, the formula unit is MgCl2.

Table: Common Monatomic Ions

Cation	Anion
Na+ (Sodium)	Cl- (Chloride)
Ca2+ (Calcium)	O2- (Oxide)
Fe2+ (Iron(II))	S2- (Sulfide)
Ag+ (Silver)	F- (Fluoride)
Cu2+ (Copper(II))	Br- (Bromide)

Key Equations

• Charge Balance in Ionic Compounds:

$\text{Total positive charge} + \text{Total negative charge} = 0$

• Example for MgCl2:

$\text{Mg}^{2+} + 2\text{Cl}^{-} \rightarrow \text{MgCl}_2$

Additional info:

• Polyatomic ions and covalent compounds are covered in subsequent sections (not included in the provided images).

• Lewis dot structures and naming conventions for covalent compounds are also important topics in this chapter.