Atoms, Molecules, and Ions: Chemical Formulas and Nomenclature

Atoms, Molecules, and Ions

Chemical Formulas

Chemical formulas are symbolic representations that indicate the types and numbers of atoms present in a chemical substance. They are fundamental to understanding the composition and properties of compounds in chemistry.

Types of Chemical Formulas

• Empirical Formula: Shows the simplest whole-number ratio of atoms of each element in a compound. It does not necessarily represent the actual number of atoms in a molecule, but rather the lowest ratio.

• Molecular Formula: Indicates the exact number of atoms of each element in a molecule of the compound. It provides more specific information than the empirical formula.

• Structural Formula: Illustrates how atoms are bonded to one another, showing the connectivity and arrangement of atoms within the molecule. It does not depict the three-dimensional shape.

• Perspective Drawings, Ball-and-Stick Models, Space-Filling Models: These models represent the three-dimensional arrangement of atoms in a molecule, aiding in visualizing molecular geometry.

Comparison of Empirical and Molecular Formulas

Compound	Molecular Formula	Empirical Formula
Water	H2O	H2O
Glucose	C6H12O6	CH2O
Ozone	O3	O
Hydrazine	N2H4	NH2

Key Point: If the molecular formula is known, the empirical formula can be determined. The reverse is not always possible without additional information.

Examples of Standard Types of Formulas and Models

Compound	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Hydrogen	H2	H—H	[model]	[model]
Water	H2O	H—O—H	[model]	[model]
Ammonia	NH3	H—N—H      |     H	[model]	[model]
Methane	CH4	H—C—H      |     H     H	[model]	[model]

Additional info: Models are typically shown as images; here, they are referenced as [model].

Formulas of Ionic Compounds

Ionic compounds are composed of positive ions (cations) and negative ions (anions). Their formulas reflect the ratio of ions required to achieve electrical neutrality.

• Ionic compounds do not exist as discrete molecules; instead, they form extended lattice structures.

• The formula unit of an ionic compound is usually the same as its empirical formula.

• Charge Balance: The sum of the charges on the cations and anions in each formula unit must be zero.

Example: For ZnBr2:

• Zn2+ + 2Br-

• $(2+) + 2(-1) = 0$

Examples: Formulas of Ionic Compounds

Compound	Cation	Anion	Formula	Charge Calculation
Aluminum oxide	Al3+	O2-	Al2O3	2 × (+3) + 3 × (−2) = +6 + (−6) = 0
Calcium bromide	Ca2+	Br-	CaBr2	1 × (+2) + 2 × (−1) = +2 + (−2) = 0
Sodium carbonate	Na+	CO32-	Na2CO3	2 × (+1) + 1 × (−2) = +2 + (−2) = 0

Key Point: The subscripts in ionic formulas are determined by the charges of the ions, ensuring the overall charge is zero.