Ch 3 introduces the fundamental concepts of atoms and molecules, laws of chemical combination, writing chemical formulae, and the mole concept for counting atoms.
Law of Conservation of Mass (Lavoisier): mass is neither created nor destroyed in a chemical reaction. Law of Constant Proportions (Proust): a compound always contains the same elements in the same ratio by mass (e.g., H₂O always has H:O = 1:8 by mass).
Atom: smallest particle retaining identity of an element. Molecule: two or more atoms bonded together. Atomicity: number of atoms in a molecule (O₂ = 2, O₃ = 3). Valency determines how atoms combine. Chemical formula shows types and numbers of atoms (H₂SO₄ = 2H + 1S + 4O).
1 mole = 6.022 × 10²³ particles (Avogadro's number). Molar mass = mass of 1 mole of a substance in grams. For atoms, molar mass = atomic mass in grams (C = 12 g/mol). Number of moles = given mass / molar mass.
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A mole is a counting unit — just as a dozen = 12, a mole = 6.022 × 10²³ (Avogadro's number). It connects the microscopic world (atoms) to the macroscopic world (grams). One mole of any substance contains 6.022 × 10²³ particles and has a mass equal to its relative mass in grams.
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