9.2.3—Reactions/equations: the reactions, if any,

A chemical-periodicity comparison is only meaningful when the equations, states and products are specified. Balance atoms and charge before interpreting the trend.

Compare like reactions—such as element plus oxygen or oxide plus water—then use structure and oxidation state to explain why behaviour changes across the period.

4Na + O₂ → 2Na₂O and Si + O₂ → SiO₂ are both balanced, but their products differ from ionic/basic to giant covalent because the underlying bonding changes.

Do not infer a general reaction from one oxide or one state. Periodic trends describe families of related cases, not every possible condition.