* Highly ionic oxides (Group 1 and 2) are basic.

* Highly covalent oxides (Groups 14-17) are acidic.

* The "amphoteric line" moves further to the right of Period as Groups are descended

* For elements of variable oxidation state, acidity increases with oxidation state.

 cf N₂O₃ / N₂O₅     SO₂ / SO₃        Cl₂O / ClO₂         PbO / PbO₂

Explanation:
 

* Concept of "Polarizing Power" explains the trends
 

Consider the reaction: EO_n/2 + (6 + ⁿ/2)H₂O  E(H₂O)₆ⁿ⁺ + nOH^-
 

~ Eⁿ⁺ large (Group 1 and 2), E(H₂O)₆ⁿ⁺ ion stable, oxide is basic

 ~ Eⁿ⁺ small, eg. B³⁺, Si⁴⁺, => Hydrolysis  E(OH)n + nH₂O neutral

eg. Al(H₂O)₆³⁺ is amphoteric Si(OH)₄, or B(OH)₃ are weakly acidic.

 ~ Eⁿ⁺ very small, e.g. S⁶⁺, =? further hydrolysis, and strong acid properties.

 ~ Explains why acidity increases with oxidation state.

* Alternative viewpoint, as Eⁿ⁺ gets smaller preference for