Atomic Structure and Chemical Bonding
Atomic structure and bonding is where the paper stops asking only for recall and starts asking you to write: electronic configurations, ion charges, formulae and balanced equations.
Sub-atomic particles and numbers
An atom has protons (positive, in the nucleus), neutrons (neutral, in the nucleus) and electrons (negative, in shells). Atomic number Z = number of protons; mass number A = protons + neutrons; so neutrons = A − Z. Isotopes have the same Z but different A, as in chlorine-35 and chlorine-37. In a neutral atom, electrons = protons.
Electronic configuration and valency
Shells fill 2, 8, 8. The outermost electrons decide valency: with 1–4 electrons the atom tends to lose or share that number; with 5–8 it gains (8 − n). Sodium (2,8,1) has valency 1; magnesium (2,8,2) has 2; aluminium (2,8,3) has 3; chlorine (2,8,7) has 1; oxygen (2,6) has 2. Noble gases have a complete octet and valency 0.
Ions and formulae
Metals lose electrons to form cations (Na⁺, Mg²⁺, Al³⁺); non-metals gain electrons to form anions (Cl⁻, O²⁻, N³⁻). Write a formula by the criss-cross rule: the valency of one species becomes the subscript of the other, then simplify.
Bonding and equations
Ionic bonding transfers electrons (NaCl); covalent bonding shares them (H₂, O₂, CH₄). A chemical equation must be balanced - the atoms of each element must be equal on both sides. Change coefficients only; never change subscripts.
Worked example
Write the formula of aluminium sulphate. The ions are Al³⁺ and SO₄²⁻. Criss-cross gives Al₂(SO₄)₃, which does not simplify. Check neutrality: 2 × (+3) = +6 and 3 × (−2) = −6.
Traps
Writing CaCl instead of CaCl₂; giving the mass number as the number of neutrons; saying a covalent compound 'contains ions'; forgetting to simplify Mg₂O₂ to MgO.