10.2 Periodicity in Chemical Properties

10.2    Periodicity in Chemical Properties

Reaction with oxygen gas, O2

  • Sodium burns on heating with an orange-yellow flame to form white sodium oxide

4Na(s) + O2(g) → 2Na2O(s)


  • Magnesium burns on heating with a brilliant white flame to form white magnesium oxide

2Mg(s) + O2(g) → 2MgO(s)


  • An oxide layer will form on the aluminium when it is exposed to air, this oxide layer prevents aluminium from However, if powdered aluminium is used, it burns on heating with white flames to form white aluminium oxide.

4Al(s) + 3O2(g) → 2Al2O3(s)


  • Silicon burns slowly at red heat to form silicon(Vl) oxide or silicon dioxide

Si(s) + O2(g) → SiO2(s)


  • Phosphorus burns on heating with a white flame to form clouds of white covalent oxides, phosphorus(III) oxide and phosphorus(V) chloride

P4(s) + 3O2(g) → P4O6(s)     ; if oxygen gas is limited P4(s) + 5O2(g) → P4O10(s)               ; if oxygen gas is in excess

  • Sulfur burns on heating with a blue flame to form sulfur dioxide gas. S(s) + O2(g) → SO2(g)

Under suitable conditions, sulfur dioxide can be converted to sulfur trioxide.(See also the Contact process)

2SO2(g) + O2(g) ⇌ 2SO3(g)


  • Chlorine forms several oxides(Cl2O and Cl2O7), but it will not react directly with oxygen


  • Argon does not react with oxygen to form any oxides.


  • Going across Period 3, the reactivity towards oxygen decreases because the reducing power(tendency to be oxidised) of the elements decreases


Reaction with chlorine gas, Cl2

  • Sodium burns on heating in chlorine gas with an orange-yellow flame to form white sodium chloride

2Na(s) + Cl2(g) → 2NaCl(s)

  • Magnesium burns on heating in chlorine gas with a brilliant white flame to form white magnesium chloride.

Mg(s) + Cl2(g) → MgCl2(s)

  • Aluminium burns on heating to form ionic aluminium

2Al(s) + 3Cl2(g) → 2AlCl3(s)

At temperature about 180 °C, aluminium chloride converts to a molecular form, Al2Cl6, a dimer of covalent AlCl3. At even higher temperature, Al2Cl6 breaks into simple AlCl3 molecules.

2Al(s) + 3Cl2(g) → Al2Cl6(g)

  • Silicon burns slowly in chlorine gas at red heat to form covalent silicon(IV) chloride or silicon tetrachloride, a colourless liquid which vaporises

Si(s) + Cl2(g) → SiCl4(l)

  • Phosphorus burns in chlorine gas to produce a mixture of two chlorides, phosphorus(III) chloride, PCl3 and phosphorus(V) chloride, PCl5. In excess chlorine gas, PCl5 is the major product

P4(s) + 6Cl2(g) → 4PCl3(l)       ; if chlorine gas is limited P4(s) + 10Cl2(g) → 4PCl5(s)                ; if chlorine gas is in excess

PCl3 is a fuming liquid while PCl5 is an off-white solid.

  • Sulfur burns in chlorine gas to produce disulfur dichloride, an orange, evil- smelling liquid

2S(s) + Cl2(g) → S2Cl2(l)

  • Chlorine obviously does not react with chlorine gas
  • Argon does not react with chlorine gas to form any chlorides


Reaction with water, H2O

  • Sodium catches fire in cold water and a violently exothermic reaction occurs to form sodium hydroxide and hydrogen gas

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)


  • Magnesium reacts very slowly with cold water, taking several days to collect a test tube of hydrogen gas and a weakly alkaline magnesium hydroxide solution

Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g)     ; very slow However, it reacts rapidly with steam to produce magnesium oxide and hydrogen gas.

Mg(s) + H2O(g) → MgO(s) + H2(g)      ; very fast


Variation in oxidation number of  Period 3 oxides and chlorides

  • Oxidation number of a Period 3 oxide or chloride corresponds to the number of electrons used for bonding. It is always positive because oxygen is more electronegative than any of the element


  • The maximum oxidation number is the same as Group number. This corresponds to the total number of valence electrons

  • In the oxides, the maximum oxidation number increases from +1 in Na to +6 in S.
  • In the chlorides, the maximum oxidation number increases from +1 in Na to +5 in P.


  • Phosphorus and sulfur show several oxidation numbers because they can expand their octet through the excitation of electrons to the empty 3d orbitals


  • For example:
    1. In SO2, S has oxidation number +4 because only four electrons are used for bonding
    2. In SO3, S has oxidation number +6 because all six electrons are used for bonding
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