How does the acid-base character of oxides change across a period?

Across a period in the periodic table, the acid-base character of oxides undergoes a systematic transition as the bonding shifts from ionic to covalent.12

As you move from left to right across a period, the oxides progress through the following stages of acid-base behavior:

• Strongly basic: Typically found at the beginning of a period (e.g., Group 1 oxides like ${\text{Na}}_2\text{O}$), these ionic oxides react with water to form strong hydroxide bases.12
• Weakly basic: As the metallic character decreases, oxides (such as $\text{MgO}$) become less basic.12
• Amphoteric: Located in the middle of the transition, these oxides can behave as either acids or bases depending on the environment.1
• Weakly acidic to strongly acidic: As the elements become more nonmetallic and the bonding becomes increasingly covalent, the oxides react with water to form acids (or oxyacids), with acidity increasing toward the right side of the period (e.g., ${\text{SO}}_3$ and ${\text{Cl}}_2{\text{O}}_7$).132

This trend reflects the underlying change in the nature of the chemical bonds and the increasing electronegativity of the elements as one moves across the period.132