Fundamental Concepts in Introduction to Chemistry: Law of Constant Composition, Chemical Formulas, and Classification of Compounds

Law of Constant Composition and Chemical Formulas

Law of Constant Composition

The Law of Constant Composition states that all samples of a given chemical compound have the same proportions of their constituent elements by mass. This law is fundamental to understanding chemical formulas and the nature of compounds.

• Definition: The law asserts that a chemical compound always contains exactly the same proportion of elements by mass.

• Discovery: Joseph Proust formulated this law in the late 18th century.

• Example: Water (H2O) always consists of 2 hydrogen atoms for every 1 oxygen atom, regardless of the source.

Chemical Formulas: Molecular and Empirical

Chemical formulas represent the composition of compounds. There are two main types: molecular formulas and empirical formulas.

• Molecular Formula: Shows the actual number of atoms of each element in a molecule (e.g., $C_6H_{12}O_6$ for glucose).

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound (e.g., $CH_2O$ for glucose).

• Example: Hydrogen peroxide has a molecular formula $H_2O_2$ and an empirical formula $HO$.

Identifying Elements in Chemical Formulas

In a chemical formula, the element listed first is typically the one with the more metallic character or lower electronegativity.

• Example: In $NaCl$, sodium (Na) is listed before chlorine (Cl).

Classification of Elements and Compounds

Molecular vs. Atomic Elements

Elements can exist as single atoms or as molecules composed of two or more atoms.

• Atomic Elements: Exist as single atoms (e.g., helium, argon).

• Molecular Elements: Exist as molecules, often diatomic (e.g., $O_2$, $N_2$, $Cl_2$).

• Example: Oxygen in nature is found as $O_2$ (diatomic molecule).

Periodic Table: Metals Forming Multiple Types of Ions

Some metals can form more than one type of ion, often transition metals.

• Example: Iron can form $Fe^{2+}$ and $Fe^{3+}$ ions.

Polyatomic Ions and Ionic Compounds

Polyatomic ions are ions composed of two or more atoms covalently bonded, acting as a single charged entity in compounds.

• Examples: Acetate ($CH_3COO^-$), Chromate ($CrO_4^{2-}$), Phosphate ($PO_4^{3-}$), Cyanide ($CN^-$).

• Ionic Compounds: Formed from the combination of cations and anions, including polyatomic ions (e.g., $Ba(NO_3)_2$, $KClO_3$).

Decomposition and Law of Constant Composition

Decomposition of Compounds

When compounds decompose, the masses of the constituent elements can be measured to verify the law of constant composition.

• Example: If two samples of sodium chloride decompose to produce different masses of sodium and chlorine, the ratio of their masses should be the same in both samples.

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

Writing and Naming Chemical Compounds

Formulas for Ionic Compounds

Ionic compounds are formed from metals and nonmetals, or from ions. The formula is determined by balancing the charges of the ions.

• Example: Potassium acetate: $KCH_3COO$

• Example: Potassium chromate: $K_2CrO_4$

• Example: Potassium phosphate: $K_3PO_4$

• Example: Potassium cyanide: $KCN$

Naming Ionic Compounds with Polyatomic Ions

When naming ionic compounds containing polyatomic ions, the cation is named first, followed by the name of the polyatomic ion.

• Examples:

  • $Ba(NO_3)_2$: barium nitrate

  • $Pb(C_2H_3O_2)_2$: lead(II) acetate

  • $NH_4I$: ammonium iodide

  • $KClO_3$: potassium chlorate

  • $CoSO_4$: cobalt(II) sulfate

  • $NaClO_4$: sodium perchlorate

Formulas for Molecular Compounds

Molecular compounds are formed from nonmetals. Their formulas reflect the actual number of atoms present.

• Examples:

  • Carbon monoxide: $CO$

  • Disulfur tetrafluoride: $S_2F_4$

  • Dichlorine monoxide: $Cl_2O$

  • Phosphorus pentafluoride: $PF_5$

  • Boron tribromide: $BBr_3$

  • Diphosphorus pentasulfide: $P_2S_5$

Classification of Elements and Compounds

Atomic vs. Molecular Elements

Elements and compounds can be classified based on their basic units.

Substance	Basic Unit
helium	molecules
CCl4	formula units
K2SO4	diatomic molecules
bromine	single atoms

Additional info: The table above classifies substances by their basic units. Helium is actually a monatomic element, not a molecule; K2SO4 is an ionic compound and should be classified as formula units, not diatomic molecules. These corrections are based on standard chemical knowledge.

Classifying Elements and Compounds

• Atomic Elements: Argon, hydrogen

• Molecular Elements: Chlorine (as $Cl_2$), cobalt (as $Co_2$ in some forms)

• Ionic Compounds: KI, CuO, PCl3

• Molecular Compounds: CS2

Summary Table: Types of Chemical Species

Type	Examples	Basic Unit
Atomic Element	He, Ar	Single atoms
Molecular Element	O2, N2, Cl2	Molecules
Molecular Compound	CO2, H2O	Molecules
Ionic Compound	NaCl, K2SO4	Formula units

Additional info:

• Some questions in the file refer to decomposition experiments and mass ratios, which are classic applications of the law of constant composition.

• Classification of compounds and elements is foundational for understanding chemical nomenclature and formula writing.

• Polyatomic ions are a key topic in introductory chemistry, as they appear frequently in ionic compounds.