Oxides of Third Row Elements

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  • A binary compound of an element with oxygen, in which the oxygen atom is electronegative is called an oxide. e.g. MgO, Al2O3 etc.
  • The oxide in which Oxygen exhibits the normal oxidation state of -2 is called normal oxide.

Classification of Oxides:

  • Depending upon the chemical behaviour, oxides of third-row elements are Classified into three types, namely i) acidic oxides  Na2O, MgO ii) basic oxides SiO2, SO3, Cl2O7, P2O5  and iii) amphoteric oxides Al2O3.

Basic Oxides:

  • The oxides which react with water and produce alkali and can neutralise acids forming salt and water are called basic oxides.
  • Na2O of sodium and MgO of magnesium of the third row are basic oxides. Because both oxides produce alkali when treated with water and can neutralise acid producing salt and water.

Na2O    +     H2O        →    2 NaOH (strong base)

Na2O    +   2 HCl        →   2 NaCl    +   H2O

MgO    +   2 H2O        →   Mg (OH)2 (base)

MgO    +   2 HCl        →   MgCl2    +   H2O



  • Na and Mg have bigger atomic size. 1 and 2 valence electrons respectively and very low ionisation potential value.

Amphoteric Oxide:

  • The oxide which acts as an acid as well as base and neutralises acid as well as base to give salt and water is called an amphoteric oxide.
  • Al2O3 of aluminium is an amphoteric oxide. It neutralises acid like HCl as well as a base like NaOH.

Al2O3 is acidic in nature because it reacts with a base to give salt and water.

Al2O3    +    2 NaOH  →   2 NaAlO2  (sodium aluminate)   +      H2O

Al2O3 is acidic in nature because it reacts with a base to give salt and water.

Al2O3    +    6 HCl  → 2 AlCl3    +   3 H2O



  • Aluminium has higher I.P. than sodium and its electronegativity is greater than Mg and Na metals. So Al-O bond in  Al2O3  shows amphoteric in nature.

Acidic Oxides:

  • The oxides of electronegative elements which give acid when treated with water and can neutralise bases to produce salt and water called acidic oxides.
  • Oxides of Phosphorous, Sulphur, Chlorine are acidic.

SiO2   +  2 NaOH →   Na2SiO3   + H2O

P4O10 +   6 H2O  → 4 H3PO4

SO3   +  H2O    →     H2SO4

SO3    +  2 Na OH  → Na2SO4  +  H2O

Cl2O7   + H2O →  2 HClO(perchloric acid)



Cl2O7  + 2 Na OH  →  2 NaClO4 +   H2O

  • Si, P ,S, and Cl have greater ionisation potential , electronegativity , tendency to attract electrons.  They have smaller atomic size and more valency electrons.

Trend in Acidic and Basic Character:

Trend:

  • Generally, the oxides of metals are basic in nature while that of non-metals are acidic.
  • As we move from Na to Cl along the third period, acidic character of elements goes on gradual increasing while basic character goes on decreasing.

Reasons:

  • In third row elements, those elements who have low ionisation potential, bigger atomic size, less electronegativity, less number of valence electrons form basic oxides as their oxides give alkalies when treated with water.
  • On the other hand, those elements who have greater ionisation potential, smaller atomic size, greater electronegativity, more valency electrons form acidic oxides, Such oxides give acids when treated with water.
  • The trend is so because from Na to Cl while going along the third period, ionisation potential gradually increases, atomic size decreases, the number of valence electrons increases.

Examples:

  • Na2O and MgO of Na and Mg respectively are highly basic in nature. They produce alkalies when treated with water.  They neutralise acids. Na and Mg have very low ionisation potential amongst third-row elements.

Na2O    +     H2O        →    2 NaOH (strong base)

Na2O    +   2 HCl        →   2 NaCl    +   H2O

MgO    +   2 H2O        →   Mg (OH)2 (base)

MgO    +   2 HCl        →   MgCl2    +   H2O



  • The oxides of electronegative elements such as Si, P, S. and Cl are acidic in nature.  Their oxides give acids when treated with water can neutralise base producing salt and water. Si, P, S and Cl elements have greater ionisation potential, more valency electrons, greater electronegativity.

SiO2   +  2 NaOH →   Na2SiO3   + H2O

P4O10 +   6 H2O  → 4 H3PO4

SO3   +  H2O    →     H2SO4

SO3    +  2 Na OH  → Na2SO4  +  H2O

Cl2O7   + H2O →  2 HClO(perchloric acid)



Cl2O7  + 2 Na OH  →  2 NaClO4 +   H2O

  • Al2O2 of aluminium is amphoteric because it neutralises acid as well as base producing salt and water. Due to its dual nature, it is called amphoteric.

Al2O3 is acidic in nature because it reacts with a base to give salt and water.

Al2O3    +    2 NaOH  →   2 NaAlO2  (sodium aluminate)   +      H2O

Al2O3 is acidic in nature because it reacts with a base to give salt and water.



Al2O3    +    6 HCl  → 2 AlCl3    +   3 H2O

  • The oxides of third-row elements are summarised in the following table.
Element Na Mg Al Si P S Cl

Hydroxy Compound

NaOH MgOH) 2 Al(OH)3 Si(OH)3 P(OH)3

PO(OH)3

SO(OH)2

SO2(OH)2

ClO3(OH)

ClO2(OH)



Nature very  strong base strong   base Amphoteric very weakly Acidic weakly Acidic strongly acidic very strongly acidic

Scientific Reasons:

Sodium oxide is more basic than Magnesium oxide. 

  • A binary compound of an element with oxygen, in which Oxygen atom is electronegative is called an oxide. The oxides which react with water and produce alkali and can neutralise acids forming salt and water are called basic oxides.
  • Na2O, MgO are basic oxides. These oxides produce alkali when treated with water and can neutralise acid producing salt and water.
  • Sodium is more electropositive element than Magnesium. Sodium has lower ionisation potential and lower electronegativity than Magnesium.
  • Due to the above reasons Sodium loses its electron very easily to oxygen than Magnesium and acts as more basic than Magnesium.

Previous Topic: Crystal Structure of Phosphorous, Sulphur, Chlorine, and Argon

Next Topic: Hydroxides of the Third Row Elements

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