Key facts & rules

• Protons: +1 charge, in nucleus. Neutrons: neutral, in nucleus. Electrons: −1 charge, in shells.
• Isotopes: same element (same proton number), different mass number (different neutrons). Same chemical properties.
• Electronic configuration: fill shells 2, 8, 8, 18 … (first shell max 2, second max 8).
• Group number = number of outer electrons. Period number = number of occupied shells.
• Ion: atom that has gained or lost electrons. Cation (+): lost electrons. Anion (−): gained electrons.
• Relative atomic mass (Ar): weighted average mass of an atom compared to 1/12 of ¹²C.
• Compound: two or more elements chemically bonded — fixed ratio, different properties from elements.
• Mixture: can be separated by physical means (filtration, distillation, chromatography).