Principles of Chemistry (Section 1) is the largest section in Edexcel IGCSE Chemistry — covering particle theory, atomic structure, bonding, the periodic table, and quantitative chemistry (moles and stoichiometry).
Atom: protons (+ charge, relative mass 1), neutrons (no charge, mass 1), electrons (− charge, mass ≈ 0). Atomic number = protons. Mass number = protons + neutrons. Isotopes: same protons, different neutrons. Electron configuration: 2, 8, 8 pattern. Ionic bonding: metal loses electrons, non-metal gains → oppositely charged ions attract (giant ionic lattice). Properties: high MP, conduct when molten/dissolved. Covalent bonding: atoms share electron pairs. Simple covalent: low MP, don\'t conduct. Giant covalent (diamond, graphite, silicon dioxide): very high MP. Metallic: positive ions in sea of delocalised electrons; conducts electricity, malleable.
Relative formula mass (Mr): sum of Ar values. Moles: n = mass / Mr. Molar volume of gas at room temperature = 24 dm³/mol. Concentration: c = n/V (mol/dm³). Balanced equations show mole ratios. Percentage yield = (actual/theoretical) × 100%. Atom economy = (Mr of useful product / Mr of all products) × 100%. Empirical formula: simplest whole number ratio. Molecular formula: actual number of atoms (n × empirical).
Ionic bonding involves the transfer of electrons from a metal to a non-metal, forming positive cations and negative anions held together by electrostatic attraction in a giant lattice. Example: NaCl — Na loses 1 electron to become Na⁺, Cl gains it to become Cl⁻. Covalent bonding is the sharing of electron pairs between non-metal atoms. Each shared pair counts as one covalent bond. Example: H₂O — each H shares one electron with O, forming two O-H bonds. Key property differences: ionic compounds have high melting points and conduct electricity when molten or in solution. Simple covalent molecules have low melting points and don\'t conduct.
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