Whether two elements bond, and how, is set by the interaction between their valence electrons and nuclei. Elements in the same column have the same valence configuration, so they form analogous compounds: if sodium forms Na₂O, then potassium forms K₂O.

The typical charge an atom takes in an ionic compound is predicted by its position on the table — main-group elements gain or lose electrons to reach the electron count of the nearest noble gas:

• Group 1 → +1 · Group 2 → +2 · Group 13 → +3
• Group 15 → −3 · Group 16 → −2 · Group 17 → −1
• Group 18 → does not typically form ions

Formulas follow from charge balance: the compound must be electrically neutral overall. Al³⁺ with O²⁻ gives Al₂O₃ (two +3 and three −2 sum to zero). This is the "criss-cross" shortcut, but the reasoning is neutrality, not a trick.

Reactivity trends follow the same logic:

• Metals get more reactive down a group — lower ionization energy makes electron loss easier. Cesium reacts violently with water; lithium reacts calmly.
• Nonmetals get more reactive up a group — higher electronegativity and more negative electron affinity make electron gain easier. Fluorine is the most aggressive.

Transition metals do not follow a single rule; they commonly show multiple oxidation states, and the exam will supply the charge or the formula when you need it.