The Period 3 Elements

Si(s) + O2 (g) ———> SiO2 (s)                

(Vigorous reaction with silicon powder covalent giant molecular silicon dioxide formed.)

4P (s) + 5O2 (g) ———> P4O10 (s)                

(Ignites spontaneously in oxygen - white solid produced, white flame.)

S (s) + O2 (g) ———> SO2 (g)                

(Burns with a lilac flame to give a choking gas, which fumes in moist air covalent molecules of sulphur dioxide formed, blue flame.)

SO2 reacts further with oxygen in the presence of a catalyst (V2O5):

2SO2 (g) + O2 (g) ———> 2SO3 (l)

• P4O10, SO2 and SO3 are covalent compounds, at room temperature and pressure P4O10 is a white solid, SO2 is a colourless gas and SO3 is a colourless liquid.

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Trend in the Boiling Points of the Period 3 Oxides

The table below compares the boiling points of some Period 3 oxides with their structure and bonding.

Looking at the table, Magnesium Oxide has the highest boiling point because the magnesium ions and the oxide ions in the ionic lattice formation have the larger charge densities, and they have an extremely strong attraction for each other.  Hence, it is difficult to separate the magnesium ions from the oxide ions in the crystal lattice of magnesium oxide.  So this oxide has a very high melting and boiling point.  Magnesium oxide is therefore widely used as a resilient material, used in the linings of high temperature furnaces.

Giant Structures and Boiling Point

The oxides with giant structures (Na2O to SiO2):

• Have high boiling points.
• Require a large amount of energy to break the strong forces between their particles.

The particles making up a giant lattice can be different.

• Na2O and MgO have ionic bonding and the strong forces are electrostatic attractions acting between positive and negative ions.
• Al2O3 has bonding intermediate between ionic and covalent.  Strong forces act between particles intermediate between ions and atoms.
• SiO2 has covalent bonding and the strong forces are ‘shared pairs of electrons’ acting between the atoms.

Simple Molecular Structures and Boiling Point

The oxides with simple molecular structures (P4O10 to SO3):

• Have low boiling points.
• Require a small amount of energy to break the weak van der Waals’ forces acting between the molecules.

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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.

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The Formulae of the Period 3 Chlorides

The relationship of the formulae of some Period 3 chlorides with electron structure and oxidation state is shown below.

The amount of chlorine atoms per mole of the element increases steadily across Period 3.

For each element in the Period, there is an increase of 1 mole of Cl per mole of element.

As with the Period 3 oxides, this periodicity in formulae is also seen as a repeating trend across Periods 2 and 4.  The table below shows the formulae of the chlorides from Period 2 to Period 4 of the Periodic Table.

Preparation of Chlorides from the Elements

Chlorides of Period 3 can be prepared by heating the elements in chlorine.  As with the Period 3 oxides, these are redox reactions.

Metal Chlorides

Equations for the reactions of chlorine with the metals sodium, magnesium and aluminium are shown below:

2Na(s) + Cl2 (g) ———> 2NaCl(s)

Mg(s) + Cl2 (g) ———> MgCl2(s)

2Al(s) + 3Cl2(g) ———> 2Al2Cl6(s)

At room temperature and pressure:

• NaCl and MgCl2 are white ionic compounds with giant structures;
• Al2Cl6 is a white compound but, surprisingly; it has covalent bonding and a simple molecular structure.

Although aluminium has the characteristic properties of a metal, its compounds often show covalent or intermediate bonding, like that of Al2O3.  This property results from the large polarising effect of the very small cation Al3+.

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Non-metal Chlorides

Equations for the reactions of silicon and phosphorus with chlorine are shown below:

Si (s) +2Cl2 (g) ———> SiCl4 (l)

2P(s) + 5Cl2(g) ———> 2PCl5(s)

• SiCl4 and PCl5 are covalent compounds.

Trend in the Boiling Points of the Period 3 Chlorides

The table below compares the boiling points of some Period 3 chlorides with structure and bonding:

Giant structures and boiling points

The chlorides with giant structures (NaCl and MgCl2)

• Have high boiling points;
• Require a large amount of energy to break the strong forces between their particles;
• NaCl and MgCl2 have ionic bonding and the strong forces are electrostatic attractions acting between positive and negative ions.

The chlorides with simple molecular structures (Al2Cl6 to PCl5)

• Have low boiling points;
• Require a small amount of energy to break the weak van der Waals’ forces acting between the molecules.

Comparison between the Oxides and Chlorides of Period 3

The structure and bonding of the Period 3 oxides and chlorides are compared below:

For oxides, bonding and structure change at different points.

• Bonding changes from ionic to covalent between MgO and SiO2 with Al2O3 having intermediate bonding.
• 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.

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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:

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.