Core definitions and the mole concept

The mole concept underpins every quantitative calculation in chemistry.

Key Definition An atom is the smallest particle of an element. An ion is a charged particle formed when an atom (or group of atoms) gains or loses electrons. A molecule is two or more atoms chemically (covalently) bonded together, and a compound is a substance containing two or more different elements chemically combined.

Empirical formulae give the simplest whole-number ratio of atoms of each element in a compound; molecular formulae give the actual number of atoms of each element present.

Key Definition The mole is the SI unit for amount of substance: the amount of substance that contains as many particles as there are atoms in 12 g of carbon-12. This number is given by the Avogadro constant, L = 6.02 × 10²³ mol⁻¹.

The number of particles in a sample is calculated by multiplying the amount in moles by L, while the amount in moles is obtained from mass via n = m / M, where m is the mass in grams and M is the molar mass.

Balanced full equations conserve both mass and charge. Ionic equations remove spectator ions to show only the species that change. State symbols (s), (l), (g) and (aq) are essential for full marks.

Relative atomic, molecular and formula masses are all measured on the carbon-12 (¹²C) scale and have no units. Molar mass equals Mᵣ numerically but carries the unit g mol⁻¹. Parts per million (ppm) expresses very dilute concentrations, particularly for atmospheric gases.

Concentration is calculated using c = n / V, where n is the amount in moles and V is the volume in dm³. Conversion between mol dm⁻³ and g dm⁻³ uses the molar mass (concentration in g dm⁻³ = concentration in mol dm⁻³ × M).

These foundations connect every later topic. Equation balancing supports stoichiometry, the mole links to gas volumes and titrations, and concentration calculations enable practical analysis.

Exam Tip Examiners frequently penalise unit errors and forgotten state symbols, so always check both before finalising any answer.