Chemical Combinations and Atomic Theory

Laws of Chemical Combinations

• Five fundamental laws govern how elements combine to form compounds:
• Law of Conservation of Mass: In all physical and chemical changes, the total mass of reactants equals the total mass of products. Matter is neither created nor destroyed.
• Law of Definite Proportions: A pure compound always contains the same elements in a fixed proportion by weight.
• Law of Multiple Proportions: If two elements form multiple compounds, the ratio of the masses of one element that combines with a fixed mass of the other element is a simple whole number.
• Gay-Lussac's Law of Gaseous Volumes: Under similar conditions of temperature and pressure, the volumes of gases that react or are produced in a chemical reaction are in simple whole number ratios.
• Avogadro's Law: Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

Dalton's Atomic Theory

• Dalton proposed a theory about atoms in 1808:
• Matter is made of indivisible atoms.
• Atoms of the same element have identical properties (including mass). Atoms of different elements have different properties and masses.
• Compounds are formed from atoms combining in fixed ratios.
• Chemical reactions involve the rearrangement of atoms; atoms are neither created nor destroyed.

Atomic Mass

• Atomic mass is the number of times an atom of an element is heavier than 1/12th the mass of a carbon-12 atom (the relative atomic mass).
• One atomic mass unit (amu) is 1/12th of the mass of a carbon-12 atom.

Average Atomic Mass

• Most elements exist as isotopes (different atoms of the same element with different masses).
• Average atomic mass is the average relative mass of an element's atoms compared to the mass of carbon-12 atoms.

Molecular Mass and Formula Mass

• Molecular mass: The sum of the atomic masses of all atoms in a molecule.
• Formula mass: The sum of the atomic masses of all atoms in a formula unit of an ionic compound.

Mole Concept

• A mole of any substance contains Avogadro's number of particles (6.022 × 10²³).
• One mole of an element contains one gram atomic mass of the element.

Percentage Composition

• Percentage composition expresses the percentage by mass of each element in a compound.

Empirical and Molecular Formula

• Empirical formula: The simplest whole-number ratio of atoms in a compound.
• Molecular formula: The actual number of atoms of each element in a molecule of the compound.
• Molecular formula = n × Empirical formula, where n is a whole number.

Stoichiometry

• Stoichiometry is the quantitative study of reactants and products in chemical reactions.
• Balanced chemical equations are used for stoichiometric calculations.

Limiting Reactant

• The limiting reactant is the reactant that is completely consumed in a reaction, determining the amount of product formed.

Reactions in Solutions

• Various ways to express the concentration of a substance in a solution:
• Mass percent (w/w%)
• Mole fraction
• Molarity (moles of solute/liter of solution)
• Molality (moles of solute/kg of solvent)

States of Matter and Classification of Matter

• Matter exists as solids, liquids, or gases.
• Matter is classified into elements, compounds, and mixtures.
• Elements contain only one type of particle (atom or molecule).
• Compounds are formed from two or more elements in fixed ratios.
• Mixtures contain two or more substances in varying proportions.

Scientific Notation and Significant Figures

• Scientific notation is used for expressing very large or small numbers conveniently.
• Significant figures represent the precision of a measurement.

Dimensional Analysis

• Dimensional analysis is a technique to convert between units of measurement.