Trends in Chemical properties of elements in period 3

Trends in Chemical properties of elements in period 3 :

Period 3 of the table consists of Sodium, Magnesium, Aluminum, Silicon, Phosphorus, Sulphur, Chlorine and Argon. They can be roughly divided into metals (Na, Mg and Al), metalloids (Si) and non-metals (P, S, Cl, and Ar), and therefore the elements within these groups tend to possess similar chemical properties and form similar compounds like oxides, which successively have similar reactions.

Reaction with Water:

The reactivity of the period 3 elements with water from left to right decrease up to middle then become irregular.

Sodium reacts with cold water rapidly producing sodium hydroxide and hydrogen gas. Reaction is highly exothermic reaction.

       2Na +2H2O→ 2NaOH + H2

Magnesium do not show appreciable reaction with cold water, it react with hot boiling water.

When a clean coil of magnesium is dropped in cold water it gets covered with bubbles of hydrogen which floats on the surface of water. As a result of reaction magnesium hydroxide is also produced which get deposited on the surface of magnesium and thus stops further reaction.

   Mg +2H2O→ Mg(OH)2 + H2

Magnesium burns in steam and produce magnesium oxide (white colour) and hydrogen gas.

    Mg +H2O(g) → MgO + H2

Aluminum powder reacts with steam on heating. But reaction is very slow due to oxide layer on aluminum.

   2Al + 3H2O(g) → Al2O3 +3 H2

Silicon reacts with steam at red heat.

    Si + 2H2O(g) →SiO2 +2 H2

Chlorine reacts with water to produces hydrochloric acid and hypochlorous acid. The reaction is reversible

Hypochlorous acid in presence of sunlight slowly decomposes to hydrochloric acid and oxygen gas.                             

2Cl2 +2 H2O ⇌ 2HCl +2 HOCl

2HOCl →2 HCl + O2

Overall reaction 

  2Cl2 + 2H2O → 4HCl + O2

Argon does not react with water.

Reaction with Oxygen:

Sodium (Group I-A) burns in oxygen with yellow flame and produces a mixture of sodium oxide and sodium peroxide. Both are white solids.

    4Na + O2 → 2Na2O (sodium oxide. It is normal oxide)

   2Na + O2 → Na2O2 (sodium peroxide)

Magnesium (Group II-A) burns in oxygen with intense luminous flame and produce white solid magnesium oxide.

     2Mg + O2 → 2MgO

Aluminum (Group III-A) will burn only if it is powdered, otherwise oxide layer on it prevents reaction.

     4Al +3 O2 → 2Al2O3

  Silicon (Group IV-A) burns only on very strong heating.

     Si + O2 →SiO2

White Phosphorus (Group V-A) readily catches fire in air and burns with white smoke producing phosphorus (III) oxide and phosphorus (V) oxide.

  P4 + 3O2 → P4O6    

also written as P2O3            

 P4 + 5O2 → P4O10       

 also written as P2O5

Sulphur burns in oxygen on gentle heating with a pale blue flame producing sulphur dioxide gas.

   S + O2 → SO2

Argon does not react with oxygen.

Reaction with Chlorine:

Sodium (Group I-A) burns in chlorine with bright orange flame and produces a white solid sodium chloride.

  2Na + Cl2 → 2NaCl

Magnesium (Group II-A) burns in chlorine with intense luminous flame and produce white solid magnesium chloride.

   Mg + Cl2 → MgCl2

Aluminum (Group III-A) will react with chlorine when dry chlorine is passed over aluminum foil placed in a long tube. Reaction occurs on heating. Pale yellow aluminum chloride is produced.

  2Al +3Cl2 → 2AlCl3

 Silicon (Group IV-A) reacts with chlorine when chlorine is passed over heated powder of silicon. A colourless liquid silicon tetra chloride is produced.

  Si + 2Cl2 →SiCl4

White Phosphorus (Group V-A) burns in chlorine producing phosphorus (III) chloride and phosphorus (V) chloride.

 P4 + 6Cl2 → 4PCl3               

P4 + 10Cl2 → 4PCl5                          

Sulphur reacts with chlorine if chlorine is passed over heated Sulphur and disulphur chloride (Sulphuryl chloride) is produced which is orange coloured liquid having foul smell.                       

2S + Cl2 → S2Cl2

Argon does not react with chlorine.

Compounds of Third Period:

Oxides of Third Period:

        Binary compounds of oxygen are called oxides.

Na2O   MgO     Al2O3    SiO2   P4O10    SO3  Cl2O7       

–        –         –               –     P4O6      SO2    Cl2O

Structure of oxides:

Oxides of sodium, magnesium and aluminum are giant ionic structures, oxide of silicon is giant molecular structure, oxide of phosphorus is dimer, while oxides of sulphur and chlorine are discrete molecules.                                 

Melting and Boiling points of oxides:

Melting and boiling points of oxides of sodium, magnesium and aluminum are high due to their giant ionic structures. They have strong ionic bonds so high energy is required to break these bonds, thus their melting and boiling points are high.   

Melting and boiling points of oxide of silicon are high due to its giant molecular structure. It has strong covalent bonds so high energy is required to break these bonds, thus its melting point and boiling point are high.

Melting and boiling points of oxides of phosphorus, sulphur and chlorine are low because they exist as discrete molecule. They have weak van der Waal forces so little energy is required to overcome these forces, thus their melting and boiling points are low. Strength of Van der Waal forces depends upon size, shape and polarity of molecules.               

Na2O   MgO   Al2O3   SiO2   P4O10   P4O6   SO3 SO2  Cl2O7   Cl2O     

Melting Points (oC)

1280   2900   2040   1610    340     23.8    -17  

-72    -92   -120.6

Boiling Points (oC)  

  1950    3600     2977       2230     360       173.1     45      

    -10        82      2

Electrical conductivity of oxides:

The covalent oxides do not conduct electricity as they do not possess free electrons also they do not have ions. However ionic oxides in molten state or aqueous state conduct electricity due to movement of their ions.

Acidic and basic behaviour of oxides:

Across the period basic strength of oxides decreases while acid strength increases. Thus oxides of Na and Mg are basic. Oxide of Al is amphoteric while oxides of Si, P, S, and Cl are acidic.

Sodium oxide reacts vigorously with water and produces alkaline solution.

   Na2O(s) + H2O(l) → 2Na+(aq) + 2OH(aq)  

   Or      O2 + H2O → 2OH

Sodium oxide being base also reacts with acids and produces salt and water.

  Na2O(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l)

Magnesium oxide is slightly soluble in water. It also produces alkaline solution.

  MgO(s) + H2O(l) ⇌ Mg+2(aq) + 2OH(aq)

Magnesium oxide being base also reacts with acids and produces salt and water.

  MgO(s) + 2HCl(aq) ⇌ MgCl2(aq) + H2O(l)

CaO also show same behaviour   CaO(s) + 2HCl(aq) ⇌ MgCl2(aq) + H2O(l)

Aluminium oxide being amphoteric oxide reacts both with dilute acids and dilute alkalis.

 Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l)

Al2O3(s) + 2NaOH(aq) → 2NaAlO2(aq) + H2O(l)

                                 Sodium aluminate                                      

Silicon oxide does not react with water however it reacts with alkalis as it is a weak acid

SiO2(s) + 2NaOH(aq) → Na2SiO3 + H2O(i)

However CO2 can react with water  CO2(g) + H2O(l) → H2CO3(aq)   (Carbon belongs to 2nd period)

Oxide of nitrogen reacts with water and produces two acids.

  2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)

Oxides of P, S and Cl react with water and produce strongly acidic solution.

    P4O10(s) + 6H2O(l) → 4H3PO4(aq)        

(Strong acid)

   SO3(g) + H2O(l) → H2SO4(aq) 

(Strong acid)

  Cl2O7(l) + H2O(l) → 2HClO4(aq)   

             (Strong acid)

However Cl2O produce comparatively weak acid.

Cl2O(g) + H2O(l) → 2HClO(aq)       

              (weak acid)

SO2 also react with water  

SO2(g) + H2O(l) → H2SO3(aq)

Chlorides of Third Period:

Chlorides of Na and Mg are giant ionic, having ionic bonds which are strong so their melting and boiling points are high. While chlorides of Al, Si, P and S are discrete molecules having weak van der Waal forces so their melting and boiling points are low.

Hydroxide of Third Period:

Hydroxides are the compounds in which OH (hydroxide) ion is attached to metals through ionic bonds or OH (neutral) group attached to non metals through covalent bonds.

The ionic hydroxides are basic in nature e.g. NaOH and Mg(OH)2.

Aluminum hydroxide Al(OH)3 is amphoteric.

The covalent hydroxides are acidic in nature e.g. Si(OH)4 or H4SiO4 , H3PO4, H2SO4 and HClO4.

                                                                                   (Ortho Silicic acid)

Sodium Hydroxide (NaOH):


Sodium hydroxide is prepared by electrolysis of brine (NaCl(aq)) in Nelson Cell.

   2NaCl + 2H2O → 2NaOH + H2 + Cl2


  • It is white solid.
  • It is ionic in nature.
  • It has a soapy touch i.e. slippery touch.
  • It has corrosive action for skin.
  • It has bitter taste.
  • It is hygroscopic.
  • It is highly soluble in water and the process is highly exothermic due to formation of hydrates.
  • It is soluble in alcohol.


  • It is used in soap industry.
  • It is used for refining of petroleum.
  • It is used for reclaiming rubber.
  • It is used for making mercerized cotton.
  • Its aqueous solution is used to open blocked drainage pipes.
  • It is used in acid base titration.

Magnesium Hydroxide [Mg(OH)2]:


Magnesium hydroxide is obtained as white precipitate when caustic potash (KOH) solution is added to solution of soluble magnesium salt.

 KOH(aq) + MgCl2(aq) → Mg(OH)2(s) + 2KCl(aq)


  • It is white solid.
  • It is ionic in nature.
  • It has soapy touch.
  • It has corrosive action for skin.
  • It has bitter taste.
  • It is hygroscopic.
  • It is soluble in water.
  • Its solubility in water tremendously increases by adding ammonium chloride to its solution. This is due to the fact that ammonium ions remove OH ions. So the equilibrium of Mg(OH)2 gets disturbed and according to Le Chatelier’s Principle equilibrium shifts to the right.

Mg(OH)2 ⇌ Mg2+ + 2OH

NH4Cl ⇌ NH4+ + Cl

NH4+ + OH → NH4OH

Aluminum Hydroxide [Al(OH)3]:


  • When some alkali solution is added to aqueous aluminum salt solution, aluminum hydroxide is obtained as white gelatinous precipitate.

Al2(SO4)3 + 6NH4OH → 2Al(OH)3 + 3(NH4)2SO4

  • It can also be prepared by hydrolysis of AlCl3

AlCl3 + 3H2O → Al(OH)3 + 3HCl


  • It is soluble in acids and alkalis.
  • It is amphoteric.

2Al(OH)3 + 3H2SO4 → Al2(SO4)3 + 6H2O

2Al(OH)3 + 2NaOH → 2Na[Al(OH)4]


It is used to absorb various dyes forming a colouring matter known as “lakes”. 

Silicon Hydroxide:

Silicon hydroxide is prepared by reaction of SiO2 with water at 800oC.

SiO2 + 2H2O ⇌ Si(OH)4 Silicon hydroxide is unstable so it is polymerized.

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