AQA Periodicity Chem 5


Period 3 - Reactions with water

  • Sodium is more reactive than magnesium as it takes less energy to lose one electron instead of two.
  • Sodium reacts vigorously with cold water, forming a silvery molten ball on the surface that fizzes and produces H2 gas.

2Na(s) + 2H2O(l) --> 2NaOH(aq) + H2(g)       (strongly alkaline solution formed, ph 12-14)

  • Magnesium reacts very slowly with cold water. No reaction is visible, but it forms a weakly alkaline solution which shows that a reaction has occured:

Mg(s) + H2O(l) --> Mg(OH)2(aq) + H2(g)       ph 9-10

  • Magnesium reacts much faster when heated and reacted with steam:

Mg(s) + H2O(g) --> MgO(s) + H2(g)

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Period 3 - Reactions with oxygen

General equation is element + oxygen --> oxide

  • 2Na(s) + 1/2 O2(g) --> Na2O(s)
  • Mg(s) + 1/2 O2(g) --> MgO(s)
  • 2Al(s) + 1 1/2 O2(g) --> Al2O3(s)
  • Si(s) + O2(g) --> SiO2(s)
  • P4(s) + 2O2(g) --> P4O10(s)
  • S(s) + O2(g) --> SO2(g)
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Period 3 - Element, oxide & flame test summary

Element             Oxide                       Reaction in air                        Flame           

Na                             Na2O                                   Vigorous                                        Yellow

Mg                             MgO                                   Vigorous                                    Brilliant white

Al                              Al2O3                                                  Slow                                              N/A

Si                               SiO2                                                   Slow                                              N/A

P                              P4O10                                Spontaneously combusts                     Brilliant white

S                               SO2                                             Burns steadily                                     Blue               

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Period 3 Oxides - Boiling/Melting points

(Note: SO3 should read SO2 and be just below 0)

The difference in boiling/melting points is all to do with the differences in structure and bonding.

  • Na2O, MgO and Al2O3 all have high melting points as they form giant ionic lattices.
  • SiO2 has a higher melting point than the other non-metal oxides as it forms a macromolecular structure.
  • P4O10 and SO2 form simple covalent molecules.
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Period 3 Oxides- Melting trends in detail

  • The giant ionic lattices (of Na2O, MgO and Al2O3) have strong electrostatic attraction between the ions which results in a high amount of energy being needed to break the bonds and melt them.
  • The giant macromolecular structure (of SiO2) has strong covalent bonds holding the structure together, so lots of energy is needed to overcome these bonds and melt them.
  • Simple molecular structures (of P4O10 and SO2) are held together by weak van der Waals and dipole-dipole forces which requires little energy to overcome.
  • MgO has a higher melting point than Na2O as it forms a 2+ ion which attracts oxygen's 2- ion more strongly than sodium's 1+ ion. 
  • Al2O3 has a lower melting point than expected as the 3+ ions distort the oxygen's electron cloud - making it partially covalent (not a perfect ionic model).
  • P4O10 has a higher melting point than SO2 as it is a larger molecule and therefore it's van der Waals forces are stronger - increasing it's melting point in comparision to SO2.
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Period 3 Oxides - Physical trends summary

Oxide            Melting point (K)              Bonding                        Structure

Na2O                            1548                                    Ionic                                     Giant Ionic

MgO                             3125                                    Ionic                                     Giant Ionic

Al2O3                            2345                             Ionic/Covalent                             Giant Ionic

SiO2                             1883                                 Covalent                               Macromolecular  

P4O10                            573                                  Covalent                                   Molecular

SO3                               290                                  Covalent                                   Molecular

SO2                               200                                  Covalent                                   Molecular

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Period 3 Oxides - Reactions with water

Basic oxides

Na2O(s) + H2O(l) --> 2Na+(aq) + 2OH-(aq)             pH ~14

MgO(s) + H2O (l) --> Mg(OH)2(s) <----> Mg2+(aq) + 2OH-(aq)        pH ~9                                                                                                       Magnesium oxide is sparingly soluble in water.

Insoluble oxides

Al2O3 and SiO2 are insoluble in water.                                                                                                

Acidic oxides

P4O10(s) + H2O(l) --> 4H3PO4(aq)    Violent reaction. This ionises so the solution is acidic.                                                    H3PO4(aq) <----> H+(aq) + H2PO4-(aq)                 pH ~ 1

SO2(g) + H2O(l) --> H2SO3(aq)     This partially dissociates which causes the acidity of the solution.                               H2SO3(aq) <----> H+(aq) + HSO3-(aq)         pH~2

SO3(g) + H2O(l) --> H2SO4(aq) --> H+(aq) + HSO4-(aq)    Violent reaction.    pH ~1

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Period 3 Oxides - Reactions with water summary

Oxide               Bonding                      Ions present                      Approx. pH                                                              (after H2O reaction)

Na2O                        Ionic                                 Na+(aq), OH-(aq)                                13-14

MgO                         Ionic                                 Mg2+(aq), OH-(aq)                                 10

Al2O3                  Ionic/Covalent                     insoluble, no reaction                                 7

SiO2                       Covalent                          insoluble, no reaction                                 7

P4O10                     Covalent                          H+(aq) + H2PO4-(aq)                                0-1

SO2                               Covalent                           H+(aq) + HSO3-(aq)                                 2-3

SO3                               Covalent                           H+(aq) + HSO4-(aq)                                  0-1

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Period 3 Oxides - Reasons for water reactions

  • Na2O contains the O 2- ion, which is a very strong base and so readily reacts with water to produce hydroxide ions - a strongly alkaline solution.
  • MgO also contain the oxide ion but it produces a less alkaline solution than Na2O because it is less soluble than Na2O.
  • Al2O3 is ionic but the bonding is too strong to be separated, partly due to the additional covalent bonding it has.
  • SiO2 is a giant macromolecule and water will not affect it's structure.
  • P4O10, SO2 and SO3 are covalent molecules and react with water to form acid solutions.

The general trend is alkalis --> acids as we go across the period.

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Period 3 Oxides - Acids & Bases (1)

Na2O & MgO:

  • React with acids to give a salt and water only.
  • Na2O(s) + H2SO4(aq) --> Na2SO4(aq) + H2O(l)
  • MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O(l)


  • Amphoteric (reacts with both acids and alkalis).
  • Al2O3(aq) + 6HCl(aq) --> Al2Cl6(aq) + 3H2O(l)
  • Al2O3(aq) + NaOH(aq) --> 2NaAl(OH)4(aq)                                                                               (NaOH must be hot and concentrated. Sodium aluminate is formed).


  • Will act as a weak acid with strong bases.
  • SiO2(s) + 2NaOH(aq) --> Na2SiO3(aq) + H2O(l)                                                                      (NaOH must be hot and concentrated.Sodium silicate - a colourless solution - is formed).
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Period 3 Oxides - Acid & bases (2)


  • Reaction of phosphoric acid (H3PO4) with an alkali, as P4O10 forms H3PO4 when reacted with water.
  • Reacts in three stages as each H atom reacts with an OH group and is replaced by a Na ion.
  • H3PO4(aq) + NaOH(aq) --> NaH2PO4 (aq) + H2O(l)
  • NaH2PO4 (aq) + NaOH(aq) --> Na2HPO4(aq) + H2O(l)
  • Na2HPO4 (aq) + NaOH(aq) --> Na3PO4(aq) + H2O(l)
  • Overall: 3NaOH(aq) + H3PO4(aq) --> Na3PO4(aq) + 3H2O(l)


  • Adding NaOH to an aqueous SO2 first forms hydrogensulfate(IV) then sodium sulfate (IV).
  • SO2(aq) + NaOH(aq) --> NaHSO3(aq)
  • NaHSO3(aq) + NaOH(aq) --> Na2SO3(aq) + H2O(l)
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