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The Period 3 Elements

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Introduction

The Period 3 Elements Element Na Mg Al Si P4 S8 Cl2 Ar Structure Metals Metalloid Non-metals (simple molecules) As you move from left to right the elements in period 3 go from highly electropositive metals through metalloids with giant structures to the simple molecular structure of non-metals. The Formulae of the Period 3 Oxides Across Period 3, the formulae of compounds show a regular pattern, which depends upon the number of electrons in the outer shell. The oxidation state of an element indicates the number of electrons involved in bonding. The relationship of the formulae of the Period 3 oxides with electronic structure and oxidation state is shown below. Element Na Mg Al Si P S Cl Electrons in outer shell 1 2 3 4 5 6 7 Formula of oxide Na2O MgO Al2O3 SiO2 P4O6 P4O10 SO2 SO3 Cl2O7 Oxidation state of element in oxide +1 +2 +3 +4 +3 +5 +4 +6 +7 The amount of oxygen atoms per mole of the element increases steadily across Period 3. For each element in the Period, there is an increase of 0.5 mole of O per mole of element. Na1O0.5 Mg1O1 Al1O1.5 Si1O2 P1O2.5 S1O3 Cl1O3.5 This periodicity is also seen across Period 2 and 4: Period 2 Li2O BeO Be2O3 CO2 Period 3 Na2O MgO Al2O3 SiO2 Period 4 K2O CaO Ga2O3 GeO2 Preparation of Oxides from the Elements Oxides of the Period 3 elements Na to Cl can be prepared by heating each element in oxygen. ...read more.

Middle

The Action of Water on the Period 3 Oxides Reactions of sodium and magnesium oxides: Metal oxides form alkalis in water. Na2O (s) + 2H2O (l) ---> 2NaOH (aq) pH=14 (Basic - hydrolysed by water to form a strongly alkaline solution) MgO (s) + 2H2O (l) ---> Mg(OH)2 (aq) pH=11 (Very low solubility due to metal's greater charge density. Basic, reacts with acids to form salts) Each hydroxide dissociates in water, releasing OH- ions into solution: NaOH (aq) ---> Na+ (aq) + OH- (aq) Oxides of Aluminium and Silicon Al2O3 and SiO2 have very strong lattices, which cannot be broken down by water. Consequently these compounds are insoluble in water. Reactions of Non-Metal Oxides All non-metal oxides form acids in water. Equations for the reactions of P4O10(s), SO2(g), SO3 (l) and Cl2O7 (l) with water are shown below: P4O10 (s) + 6H20 (l) ---> 4H3PO4 (aq) (phosphoric acid) SO2 (g) +H20 (l) ---> H2SO3 (aq) (sulphurous acid) SO3 (l) + H2O (l) ---> H2SO4 (aq) (sulphuric acid) Cl2O7 (l) + H20 (l) ---> 2HClO4 (aq) (chloric acid) - the strongest acid known Each acid dissociates in the water, releasing H+ ions into solution. E.g. H2SO4 (aq) ---> H+ (aq) + HSO4- (aq) An important rule is: * Metal oxides are basic - when soluble, metal oxides form alkaline solutions in water. * Non-metal oxides are acidic - when soluble, non-metal oxides form acidic solutions in water. ...read more.

Conclusion

* Structure changes from giant to simple molecular between SiO2 and P4O10. For chlorides, both bonding and structure change at the same point, between MgCl2 and Al2Cl6: * Bonding changes from ionic to covalent and; * Structure changes from giant to simple molecular. The Action of Water on the Chlorides of Period 3 Dissolving ionic chlorides The ionic chlorides dissolve in water forming a neutral or very weakly acidic solution: NaCl(s) + aq ---> Na+ (aq) + Cl- (aq) pH = 7 MgCl2 + aq ---> Mg2+ (aq) + 2Cl- (aq) pH = 6 Reactions of covalent chlorides The covalent chlorides are hydrolysed by water in a vigorous reaction. Strong acid solutions are formed containing hydrochloric acid. Equations for the reactions of Al2Cl6 (s), SiCl4 (l) and PCl5 (s) with water are shown below: Al2Cl6 (s) + 6H2O (l) ---> 2Al(OH)3 (s) + 6HCl (aq) SiCl4 (l) + 2H2O (l) ---> SiO2 (s) + 4HCl (aq) PCl5 (s) + 4H2O (l) ---> H3PO4 (aq) + 5HCl (aq) An important rule to note is that: * Ionic chlorides form neutral solutions in water, whereas * Covalent chlorides form acidic solutions in water. The action of water on the Period 3 oxides is summarised below: Chloride NaCl MgCl2 AlCl3 SiCl4 PCl5 Action of water on chloride Neutral solution Very weak acid Strong acid Strong acid Strong acid To conclude, across Period 3: * Bonding changes from ionic to covalent. * Basic metal oxides change to acidic non-metal oxides. * Change from giving alkaline solutions to acidic solutions. Chemistry Nick Curum Period 3 Elements 1 23/04/2007 ...read more.

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