Atoms form chemical bonds to achieve more stable electron configurations — typically a full outer shell of 8 electrons (the octet rule). The type of bond depends on the electronegativity difference between atoms: large differences produce ionic bonds (electron transfer), small differences produce covalent bonds (electron sharing), and metals form a special electron-sea metallic bond.

The mole is the chemist's counting unit. One mole of any substance contains exactly 6.022 × 10²³ particles (Avogadro's number, N?). Since atoms are unimaginably tiny — a hydrogen atom weighs 1.67 × 10?²4 grams — the mole allows chemists to work with amounts that can be weighed on a balance. The molar mass of any element (in g/mol) equals its atomic mass in amu.

Stoichiometry — from the Greek for "element measure" — is the quantitative relationship between reactants and products in a chemical reaction. A balanced equation is a recipe: coefficients give molar ratios, enabling calculation of theoretical yield, limiting reagent, and percent yield.

Matter exists in different states depending on the balance between the kinetic energy of its particles and the intermolecular forces holding them together. In a solid, particles vibrate in fixed positions. In a liquid, particles flow past each other while remaining close. In a gas, they move rapidly and independently, filling all available space. Phase transitions occur when energy input or removal tips the balance between kinetic energy and attractive forces.

Thermodynamics governs the flow of energy in chemical systems. The First Law states energy is always conserved: ?U = q + w (internal energy = heat + work). The Second Law states the entropy of the universe always increases for any spontaneous process — disorder is the natural direction of change.

The decisive quantity for spontaneity is Gibbs free energy: ?G = ?H - T?S. When ?G < 0, a reaction is spontaneous. Enthalpy (?H) captures heat released or absorbed; entropy (?S) captures the change in disorder. Temperature determines which factor dominates when they conflict.

A thermodynamically favourable reaction can still be effectively impossible if the activation energy barrier is enormous. Diamond is thermodynamically unstable relative to graphite (?G < 0 for diamond ? graphite), yet diamonds persist for billions of years because the kinetic barrier is insurmountable at room temperature. Kinetics and thermodynamics are independent — both must be considered.

Most chemical reactions are reversible — products can react to regenerate reactants. At equilibrium, the forward and reverse reactions occur at equal rates, so concentrations remain constant. Equilibrium is dynamic, not static: reactions continue in both directions. The equilibrium constant K tells us where equilibrium lies — whether reactants or products are favoured.

When a system at equilibrium is disturbed, it shifts to oppose the change and reach a new equilibrium.

The Brønsted-Lowry definition is most useful: an acid is a proton (H?) donor; a base is a proton acceptor. Every acid-base reaction involves conjugate pairs — the acid loses a proton to become its conjugate base; the base gains a proton to become its conjugate acid. Acid strength is quantified by Ka: the larger the Ka (smaller the pKa), the stronger the acid.

Electrochemistry connects chemistry with electricity through oxidation-reduction (redox) reactions — reactions involving electron transfer. The mnemonic OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons) — is essential. In a galvanic (voltaic) cell, a spontaneous redox reaction generates electricity. In an electrolytic cell, electricity drives a non-spontaneous reaction.

Organic chemistry is the chemistry of carbon compounds. Carbon is unique: it forms 4 covalent bonds, bonds strongly to itself (C–C, C=C, C=C), and creates chains, branches, and rings of virtually unlimited complexity. Over 10 million organic compounds are known — more than all other chemical compounds combined. They include fuels, plastics, drugs, DNA, proteins, and all biochemical molecules.

The key to organic chemistry is functional groups — specific arrangements of atoms that give characteristic chemical properties regardless of the rest of the molecule. Knowing a functional group's properties tells you almost everything about how a molecule will react.

Nuclear chemistry involves changes in the nucleus rather than the electron shell. These reactions involve energies millions of times greater than chemical reactions — and can transform one element into another (transmutation). Radioactivity was discovered by Henri Becquerel in 1896 and studied by Marie Curie, who coined the term and discovered polonium and radium — becoming the first person to win two Nobel prizes in different sciences.

Chemistry is not confined to the laboratory. Every product of modern civilisation — medicines, materials, fuels, food, fertilisers, electronics — exists because of chemistry. And every major challenge facing humanity — climate change, antibiotic resistance, renewable energy, clean water — requires chemistry for its solution. The 12 Principles of Green Chemistry, formulated by Paul Anastas and John Warner in 1998, guide chemists to design processes that reduce hazardous substances, minimise waste, and use renewable resources.