Chemistry: Materials, Reactions, and Atomic Structure
Atomic Structure and the Periodic Table
Atoms have protons and neutrons in the nucleus and electrons in shells. Atomic number is the proton count and defines the element; mass number is protons plus neutrons. Isotopes share protons but differ in neutrons. Electrons fill shells in order, and the outermost (valence) electrons drive bonding and reactivity. Groups share valence counts; periods share shell number. Metals lose electrons, nonmetals gain or share them, and noble gases are stable with full outer shells.
Bonding and Materials
Ionic bonds transfer electrons between metals and nonmetals, forming lattices with high melting points that conduct when molten or dissolved. Covalent bonds share electrons between nonmetals, giving molecules or giant networks such as diamond. Metallic bonding explains conductivity, malleability, and lustre. Properties such as hardness, solubility, and conductivity follow from structure, not just formula.
Reactions and Conservation
Chemical equations must balance because atoms are conserved. Know synthesis, decomposition, combustion, and displacement patterns, plus signs of reaction (gas, precipitate, colour change, heat). Acids release hydrogen ions and react with bases to neutralise, forming salt and water; pH below 7 is acidic, above 7 basic. Exothermic reactions release energy; endothermic ones absorb it.
Traps and a Worked Example
Traps include counting molecules instead of atoms when balancing, confusing atomic number with mass number, and assuming all covalent substances are poor conductors (graphite conducts). Worked example: balance CH₄ + O₂ → CO₂ + H₂O. Carbon is balanced; hydrogen needs 2H₂O, giving 4 oxygen on the right, so use 2O₂: CH₄ + 2O₂ → CO₂ + 2H₂O. Rule of thumb: balance one element at a time, saving hydrogen and oxygen for last.