Matter, Atoms, and Chemical Reactions
This domain covers the classification of matter, atomic structure, bonding, the mole concept, and the accounting of atoms in reactions. Items ask you to classify a substance, interpret an isotope or ion symbol, balance an equation, compute a molar quantity, or decide whether a described change is physical or chemical.
Structure and classification
- Atomic number (Z) = protons, and it defines the element. Mass number (A) = protons + neutrons. Isotopes share Z but differ in neutrons; ions differ in electrons.
- Pure substances are elements or compounds; mixtures are homogeneous (solution) or heterogeneous. Dissolving, melting and boiling are physical changes; burning and rusting are chemical.
- Ionic bonds transfer electrons between metal and non-metal; covalent bonds share electrons between non-metals. Electronegativity rises across a period and falls down a group.
- Reaction types: synthesis, decomposition, single replacement, double replacement, combustion, and neutralization. Acids give H⁺, bases accept H⁺; pH is logarithmic, so pH 3 is ten times more acidic than pH 4.
The mole and balancing
1 mol contains 6.022 × 10²³ particles. Molar mass is the sum of atomic masses in g/mol. Balance equations before using coefficients as mole ratios; the balanced equation enforces conservation of mass and of each element.
Worked example
How many grams of CO₂ form when 4.0 g of methane burns completely in excess oxygen?
Equation: CH₄ + 2O₂ → CO₂ + 2H₂O. M(CH₄) = 12.01 + 4(1.008) = 16.04 g/mol, so n = 4.0 ÷ 16.04 = 0.249 mol. The 1:1 ratio gives 0.249 mol CO₂; M(CO₂) = 44.01 g/mol, so mass = 0.249 × 44.01 = 11.0 g, reported as 11 g to two significant figures. The reasoning step that carries the marks is the mole ratio, which only exists after balancing.
Traps that cost marks
- Using coefficients from an unbalanced equation as mole ratios.
- Confusing atomic number with mass number, or counting electrons when asked for neutrons.
- Assuming all atoms of an element have identical mass, ignoring isotopes.
- Treating dissolving as a chemical change.
- Changing a subscript to balance an equation instead of a coefficient, which alters the substance.
- Forgetting that pH is a logarithmic scale.