Answer is: 2Zn + O₂ → 2ZnO (2Zn²⁺ + 2O²⁻).
Unbalanced chemical reaction: Zn + O₂ → ZnO.
Zn → Zn²⁺ + 2e⁻ / ×2; 2Zn → 2Zn²⁺ + 4e⁻.
4e⁻ + O₂ → 2O²⁻.
Zinc lost two electrons and became Zn²⁺, molecule of oxygen gain four electrons and become 2O²⁻. Multiply first reaction with two to have same number of electrons and put coefficients in chemical reaction.
