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The correct order of acidic strength is:
1. SO2 < P2O3 < SiO2 < Al2O3
2. SiO2 < SO2 < Al2O< P2O
3.  Al2O< SiO2 < SO2 < P2O3
4.  Al2O< SiO2  < P2O3 < SO2 

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Correct Answer - Option 4 :  Al2O< SiO2  < P2O3 < SO2 


  • Oxides are compounds formed by the reaction of metals, non-metals with oxygen. The types of oxides are:
  • Basic oxide: It is a complex chemical substance oxide, which forms a salt with a chemical reaction with acids or acidic oxides and does not react with bases or basic oxides.
  • E.g. Copper oxide (Cu2O), Magnesium oxide (MgO)
  • Acidic oxide: It is a complex chemical substance oxide, which forms a salt with the chemical reactions with bases or basic oxides and does not react with acidic oxides.
  • E.g. Carbon dioxide (CO2), Silicon dioxide (SiO2)
  • Amphoteric oxide: It is a complex chemical substance, also oxides, which form a salt with the chemical reactions with acids (or acid oxides) and with bases (or basic oxides).
  • E.g. Copper, Zinc, Tin, Lead, Aluminum, and Beryllium.


  • Nature of oxides of the elements:
    • Metals form basic oxides and Non-metals form acidic oxides.
    • As we move from left to right across a period, the metallic character decreases and the non-metallic character increases.
    • Hence, oxides become more acidic as we move from left to right across a period.
    • On the other hand, as we move down along a group, the metallic character increases and the non-metallic character decreases.
    • Hence, moving down a group, the basic character of oxides increases and the acidic character of the oxides decreases.
  • Aluminium, Silicon, phosphorus and sulphur are elements of period 3.
  • As we move from Aluminium to Sulphur, the non-metallic character increases.
  • Thus, the acid character of oxides also increases, hence SO2 is the most acidic and Al2O3 is the least acidic.

Hence, the correct order if acidity is  Al2O< SiO < P2O< SO​.

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