01Predicting charges
Main-group atoms gain or lose electrons to reach the electron count of the nearest noble gas. Group 1 → +1, group 2 → +2, group 16 → −2, group 17 → −1.
Ionic formulas balance total charge to zero: Al³⁺ and O²⁻ → Al₂O₃.
02Notebook box
Criss-cross the charges, then reduce. Ca²⁺ + Cl⁻ → CaCl₂.
Worked example: Element X forms XO. In which group is X if O is −2? → X is +2 → group 2.