Chemical Compounds: Composition, Classification, and Nomenclature

Chemical Compounds: Composition, Classification, and Nomenclature

Law of Constant Composition

The Law of Constant Composition (also known as the Law of Definite Proportions) states that a given chemical compound always contains its component elements in a fixed ratio by mass, regardless of the source or method of preparation.

• Key Point: The composition of a compound is always the same; for example, water (H2O) always contains hydrogen and oxygen in a mass ratio of approximately 1:8.

• Example: Any sample of carbon dioxide (CO2) contains carbon and oxygen in a mass ratio of 12:32 (or 3:8).

Writing the Symbol of a Compound

Chemical compounds are represented by chemical formulas that use element symbols and numerical subscripts to indicate the types and numbers of atoms present.

• Key Point: The symbol for each element is taken from the periodic table, and the subscript indicates the number of atoms of that element in the compound.

• Example: The formula NaCl represents sodium chloride, with one sodium atom and one chlorine atom per formula unit.

Chemical Formulas

A chemical formula provides information about the elements present in a compound and the relative number of atoms of each element.

• Types of Chemical Formulas:

  • Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound.

  • Molecular Formula: Shows the actual number of each type of atom in a molecule.

  • Structural Formula: Shows how atoms are connected in a molecule (not always included in basic formulas).

• Example: Glucose has the molecular formula C6H12O6 and the empirical formula CH2O.

Empirical and Molecular Formulas

The empirical formula gives the simplest ratio of elements, while the molecular formula gives the actual number of atoms in a molecule.

• Key Point: The empirical formula may be the same as or different from the molecular formula, depending on the compound.

• Example: Hydrogen peroxide has the molecular formula H2O2 and the empirical formula HO.

Classification of Elements and Compounds

Elements and compounds can be classified based on their atomic or molecular nature and the types of bonds present.

• Elements:

  • Atomic Elements: Exist in nature with single atoms as their basic units (e.g., Na, He).

  • Molecular Elements: Exist as molecules composed of two or more atoms of the same element bonded together (e.g., O2, N2).

• Compounds:

  • Ionic Compounds: Composed of cations (positively charged ions) and anions (negatively charged ions) held together by ionic bonds.

  • Molecular Compounds: Composed of nonmetals bonded together by covalent bonds.

Atomic Elements and Molecular Elements

Atomic elements are elements whose basic units are single atoms, while molecular elements are elements whose basic units are molecules made up of two or more atoms of the same element.

• Examples of Atomic Elements: Sodium (Na), Iron (Fe), Helium (He).

• Examples of Molecular Elements: Oxygen (O2), Nitrogen (N2), Chlorine (Cl2).

Ionic Compounds and Molecular Compounds

Ionic compounds are formed from metals and nonmetals through the transfer of electrons, resulting in ions. Molecular compounds are formed from nonmetals sharing electrons.

• Ionic Compounds: Typically solid at room temperature, high melting points, conduct electricity when dissolved in water.

• Molecular Compounds: Can be gases, liquids, or solids; lower melting points; do not conduct electricity in solution.

• Example: Sodium chloride (NaCl) is ionic; carbon dioxide (CO2) is molecular.

Writing Formulas for Ionic Compounds

To write the formula for an ionic compound, combine the cation and anion in the smallest whole-number ratio that results in a neutral compound.

• Steps:

  1. Write the symbol and charge for the cation and anion.

  2. Balance the charges so the total positive and negative charges are equal.

  3. Write the formula without showing the charges.

• Example: Magnesium chloride: Mg2+ and Cl- combine to form MgCl2.

Nomenclature

Nomenclature is the system of naming chemical compounds. Different rules apply to different types of compounds.

Naming Binary Ionic Compounds

• Name the cation (metal) first, then the anion (nonmetal) with an "-ide" ending.

• Example: NaCl is named sodium chloride.

Naming Compounds with Metals with More Than One Ion

• Use a Roman numeral in parentheses to indicate the charge of the metal cation.

• Example: FeCl2 is iron(II) chloride; FeCl3 is iron(III) chloride.

Naming Compounds with Polyatomic Ions

• Name the cation, then the polyatomic ion as it appears in the table of common ions.

• Example: NaNO3 is sodium nitrate; CaSO4 is calcium sulfate.

Naming Molecular Compounds

• Use prefixes to indicate the number of each type of atom (mono-, di-, tri-, etc.).

• The first element keeps its name; the second element ends with "-ide." "Mono-" is usually omitted for the first element.

• Example: CO2 is carbon dioxide; P2O5 is diphosphorus pentoxide.

Naming Binary Acids and Oxyacids

• Binary acids (hydrogen + nonmetal): Use the prefix "hydro-", the root of the nonmetal, and the suffix "-ic acid."

• Example: HCl (aq) is hydrochloric acid.

• Oxyacids (hydrogen + polyatomic ion containing oxygen):

  • If the polyatomic ion ends in "-ate", the acid name ends in "-ic acid" (e.g., HNO3 is nitric acid).

  • If the polyatomic ion ends in "-ite", the acid name ends in "-ous acid" (e.g., HNO2 is nitrous acid).

Formula Mass of a Molecule and Formula Unit

The formula mass is the sum of the atomic masses of all atoms in a chemical formula. For molecular compounds, it is called molecular mass; for ionic compounds, it is called formula unit mass.

• Formula: $\text{Formula mass} = \sum (\text{number of atoms of each element} \times \text{atomic mass of element})$

• Example: The formula mass of NaCl is: $1 \times 22.99\ (\text{Na}) + 1 \times 35.45\ (\text{Cl}) = 58.44\ \text{amu}$

Table: Comparison of Compound Types

Type	Constituents	Bonding	Example
Ionic Compound	Metal + Nonmetal	Ionic Bonds	NaCl
Molecular Compound	Nonmetal + Nonmetal	Covalent Bonds	CO2

Additional info: Academic context and examples have been added to clarify and expand upon the original points for completeness and self-contained study.