Ionic and Molecular Compounds: Classification, Nomenclature, and Law of Definite Proportions

Ionic and Molecular Compounds

Classification of Compounds

Compounds can be classified as either ionic or molecular (covalent) based on the types of elements involved and the nature of the bonding.

• Ionic Compounds: Formed from the electrostatic attraction between cations (usually metals) and anions (usually nonmetals).

• Molecular (Covalent) Compounds: Formed by the sharing of electrons between nonmetal atoms.

Example: NaCl is ionic (metal + nonmetal), while CO2 is molecular (nonmetal + nonmetal).

Law of Definite Proportions

Definition and Application

The Law of Definite Proportions states that a chemical compound always contains exactly the same proportion of elements by mass.

• Formula: $\text{Mass ratio} = \frac{\text{mass of element 1}}{\text{mass of element 2}}$

• Application: Used to determine the mass ratio of elements in a compound, such as nitrogen to hydrogen in ammonia (NH3).

Example: If 17.0 g of NH3 contains 14.0 g N and 3.0 g H, the mass ratio of N to H is $\frac{14.0}{3.0} = 4.67$.

Writing and Naming Ionic Compounds

Formulas of Ionic Compounds

Ionic compounds are formed by combining cations and anions in ratios that yield a neutral compound.

• Steps:

  1. Write the symbol and charge for each ion.

  2. Balance the charges to get a neutral formula.

  3. Write the formula with the cation first, then the anion.

Example: Silver ion (Ag+) and chloride ion (Cl-) combine to form AgCl.

Naming Ionic Compounds

• Binary Ionic Compounds: Name the cation first, then the anion (with -ide ending).

• Transition Metals: Indicate the charge of the metal with Roman numerals in parentheses.

• Polyatomic Ions: Use the name of the polyatomic ion as is.

Example Table: Naming Ionic Compounds

Formula	Name
Mg3N2	Magnesium nitride
CuI	Copper(I) iodide
CuNO3	Copper(I) nitrate
Pb(C2H3O2)2	Lead(II) acetate
Ba(OH)2	Barium hydroxide
CoSO4·7H2O	Cobalt(II) sulfate heptahydrate

Writing and Naming Molecular (Covalent) Compounds

Formulas of Molecular Compounds

Molecular compounds are formed by nonmetals sharing electrons. Prefixes are used to indicate the number of each atom.

• Common Prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, etc.

• Formula: The less electronegative element is written first.

Example: Carbon monoxide (CO), dinitrogen tetroxide (N2O4).

Naming Molecular Compounds

• Use prefixes to indicate the number of each atom.

• The second element ends with -ide.

Example Table: Naming Molecular Compounds

Formula	Name
CO	Carbon monoxide
NI3	Nitrogen triiodide
SiCl4	Silicon tetrachloride
N2Se4	Dinitrogen tetraselenide

Acids: Naming and Formulas

Definition and Types

Acids are compounds that release hydrogen ions (H+) when dissolved in water. They are named based on their anion component.

• Binary Acids: Contain hydrogen and one other nonmetal. Named as "hydro-___-ic acid".

• Oxyacids: Contain hydrogen, oxygen, and another element (usually a nonmetal). Named based on the polyatomic ion.

Example Table: Naming Acids

Formula	Name
HCl	Hydrochloric acid
H2SO4	Sulfuric acid
HF	Hydrofluoric acid
HNO3	Nitric acid
H3PO4	Phosphoric acid

Summary Table: Classification and Naming

Type	How to Identify	Naming Rule
Ionic	Metal + Nonmetal or Polyatomic Ion	Cation + Anion (-ide or polyatomic name)
Molecular	Nonmetal + Nonmetal	Prefixes + -ide ending
Acid	H + Anion	"Hydro-___-ic acid" or "___ic/ous acid"

Additional info:

• Some questions require students to write formulas from names and vice versa, reinforcing the importance of understanding both nomenclature and chemical formulas.

• Hydrates (e.g., CoSO4·7H2O) are named by stating the compound name followed by the prefix for the number of water molecules and the word "hydrate".