Matter, Atomic Structure, and Chemical Reactions
What to know
Matter is classified as pure substances (elements, compounds) or mixtures (homogeneous, heterogeneous). Distinguish physical changes (state, shape, dissolving) from chemical changes (new substances, bonds broken and formed). Know the states of matter and the particle model: solids vibrate in fixed positions, liquids flow but stay in contact, gases move freely and fill their container. Phase changes - melting, freezing, vaporization, condensation, sublimation, deposition - involve energy transfer without a temperature change during the transition.
Atomic structure: protons (positive, in the nucleus, define the element), neutrons (neutral, nucleus, define isotopes), electrons (negative, in shells/orbitals, determine bonding). Atomic number = protons; mass number = protons + neutrons; ions form by gaining or losing electrons. The periodic table organizes elements by increasing atomic number; groups share valence-electron counts and thus similar properties; periods are rows. Metals lose electrons, nonmetals gain or share, and noble gases are stable with full valence shells.
Reactions
Balance equations so atoms are conserved; coefficients multiply whole formulas, subscripts do not change. Recognize synthesis, decomposition, single replacement, double replacement, and combustion. Conservation of mass means total reactant mass equals total product mass. Exothermic reactions release energy; endothermic reactions absorb it.
Traps
Questions confuse mass number with atomic number, treat a subscript as a coefficient, or assume dissolving always means a chemical reaction. Watch for phase-change questions where temperature stays constant while energy is added.
Rule of thumb and worked example
Balance CH₄ + O₂ → CO₂ + H₂O. Carbon is balanced (1 each). Hydrogen: 4 on the left, so put 2 before H₂O. Oxygen: right side now has 2 + 2 = 4, so put 2 before O₂. Result: CH₄ + 2O₂ → CO₂ + 2H₂O.