Periodicity

Periodicity

PeriodicityThird Period elements1R.B.MahajanAtomic SizeThe trendThe diagram shows how the atomic radius changes as you go across Period 3.

The increasing number of protons in the nucleus as you go across the period pulls the bonding electrons more tightly to it. The amount of screening is constant for all of these elements because the all of these valence electrons are in 3rd level.R.B.Mahajan2

Ionic RadiiIonic radius of cation is less than its atom- one shell is lost & less screening.Ionic radius of anion is more than its atom for the contrary reasons to cation.Let's look at the radii of the simple ions formed by elements as you go across Period 3 of the Periodic Table - the elements from Na to Cl.

R.B.Mahajan3ionNaMgAlSiPSClRadius.mm0.0950.0650.0500.0410.2120.1840.181Melting PointsThe chart shows how the melting and boiling points of the elements change as you go across the period. The figures are plotted in kelvin rather than C to avoid having negative values.

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R.B.Mahajan5The trend of variation can be explained on the basis of difference in their structures.Sodium, magnesium and aluminum all have metallic structures.In sodium, only one electron per atom is involved in the metallic bond - the single 3s electron. In magnesium, both of its outer electrons are involved, and in aluminum all three.Silicon has a giant covalent structure just like diamond. A tiny part of the structure looks like this:

Phosphorus, Sulfur, Chlorine and Argon has following molecular structures

Electrical ConductivitiesSodium, magnesium and aluminum are all good conductors of electricity. Conductivity increases as you go from sodium to magnesium to aluminum.Silicon is a semiconductor.None of the rest conduct electricity.The three metals, of course, conduct electricity because the delocalized electrons (the "sea of electrons") are free to move throughout the solid or the liquid metal.In the silicon case, explaining how semiconductors conduct electricity is beyond the scope of A level chemistry courses. With a diamond structure, you mightn't expect it to conduct electricity, but it does!The rest don't conduct electricity because they are simple molecular substances. There are no electrons free to move around.

R.B.Mahajan6ElectronegativitiesElectronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7.The trend across Period 3 looks like this:

The trend is explained in exactly the same way as the trend in atomic radii.

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First Ionisation EnergyThe first ionisation energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.

It is the energy needed to carry out this change per mole of X.

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R.B.Mahajan9Notice that the general trend is upwards, but this is broken by falls between magnesium and aluminum, and between phosphorus and sulphur.Explaining the patternFirst ionisation energy is governed by:the charge on the nucleus;the distance of the outer electron from the nucleus;the amount of screening by inner electrons;whether the electron is alone in an orbital or one of a pair.

R.B.Mahajan10Name of ElementSymbolSodiumNaMagnesiumMgAluminiumAlSiliconSiPhosphorusPSulfurSChlorineClArgonArAtomic Number, z1112131415161718Electronic Configuration2,8,12,8,22,8,32,8,42,8,52,8,62,8,72,8,8Atomic Radius (picometers)186160143118110102991921stIonization Energy (kJ/mol)502744584793101710061257>1526Electronegativity (Pauling)0.931.311.611.92.192.583.16-Melting Point (oC)98639660141044113-101-189Boiling Point (oC)883109024672680280445-35-186Metallic Charactermetalmetalmetalsemi-metal (metalloid)non-metalnon-metalnon-metalnon-metalReactions with OxygenSodiumSodium burns in oxygen with an orange flame to produce a white solid mixture of sodium oxide and sodium peroxide.For the simple oxide:4 Na + O2 2Na2OFor the peroxide:2Na + O2 Na2O2MagnesiumMagnesium burns in oxygen with an intense white flame to give white solid magnesium oxide.2Mg + O2 MgO

R.B.Mahajan11Reactions with OxygenAluminumAluminum will burn in oxygen if it is powdered, otherwise the strong oxide layer on the aluminum tends to inhibit the reaction. If you sprinkle aluminum powder into a Bunsen flame, you get white sparkles. White aluminum oxide is formed.4 Al + 3O2 2Al2O3SiliconSilicon will burn in oxygen if heated strongly enough. Silicon dioxide is produced.Si + O2 SiO2

R.B.Mahajan12Reactions with OxygenWhite phosphorus catches fire spontaneously in air, burning with a white flame and producing clouds of white smoke - a mixture of phosphorus(III) oxide and phosphorus(V) oxide.The proportions of these depend on the amount of oxygen available. In an excess of oxygen, the product will be almost entirely phosphorus(V) oxide.For the phosphorus(III) oxide:P4+ 3O2 P4O6For the phosphorus(V) oxide:P4 + 5O2 P4O10SulphurSulphur burns in air or oxygen on gentle heating with a pale blue flame. It produces colourless sulphur dioxide gas.S + O2 SO2Chlorine and argonDespite having several oxides, chlorine won't react directly with oxygen. Argon doesn't react either.

R.B.Mahajan13Reactions with ChlorineR.B.Mahajan14Sodium- Sodium burns in chlorine with a bright orange flame. White solid sodium chloride is produced.2Na + Cl2 2NaClMagnesium- Magnesium burns with its usual intense white flame to give white magnesium chlorideMg + Cl2 MgCl2Aluminium- Aluminium is often reacted with chlorine by passing dry chlorine over aluminium foil heated in a long tube. The aluminium burns in the stream of chlorine to produce very pale yellow aluminium chloride. This sublimes (turns straight from solid to vapour and back again) and collects further down the tube where it is cooler.2Al + 3Cl2 2AlCl3 Reactions with ChlorineR.B.Mahajan15SiliconIf chlorine is passed over silicon powder heated in a tube, it reacts to produce silicon tetrachloride. This is a colourless liquid which vaporises and can be condensed further along the apparatusSi + 2Cl2 SiCl4PhosphorusWhite phosphorus burns in chlorine to produce a mixture of two chlorides, phosphorus(III) chloride and phosphorus(V) chloride (phosphorus trichloride and phosphorus pentachloride).Phosphorus(III) chloride is a colourless fuming liquid.P4 + 6Cl2 4PCl3Phosphorus(V) chloride is an off-white (going towards yellow) solid.P4 + 10Cl2 4PCl5SulphurIf a stream of chlorine is passed over some heated sulphur, it reacts to form an orange, evil-smelling liquid, disulphur dichloride, S2Cl2.2S + Cl2 S2Cl2 or S + Cl2 SCl2 Chlorine and argonIt obviously doesn't make sense to talk about chlorine reacting with itself, and argon doesn't react with chlorine.Reactions with WaterR.B.Mahajan16Sodium- Sodium reacts vigorously with water, melting, giving hydrogen gas leaving strongly alkaline solution of sodium hydroxide.2Na + 2H2O 2NaOHMagnesium- Magnesium reacts very slowly with cold water producing weakly alkaline solution with slightly soluble Magnesium hydroxide. Mg + 2H2O Mg(OH)2Hot Magnesium reacts with steam vigorously to form Magnesium oxide and hydrogen gas.Mg + 2H2O MgO +H2OxidesR.B.Mahajan17Basicoxides and hydroxides: Na2O, NaOH, MgO and Mg(OH)2Sodium oxide:Na2O (s)+H2O (l) 2NaOH (aq)Na+(aq) +OH-(aq) +H+(aq) Na+(aq) + H2O (l)Magnesium oxide:MgO (s)+2H+(aq) Mg2+(aq)+H2O (l)Mg(OH)2(s)+2H+(aq) Mg2+(aq)+2H2O (l)OxidesBasic properties:Aluminium oxide is very resistant to attack by acids and so the first reaction is slow. Aluminium hydroxide reacts easily with dilute acids.Al2O3(s)+6H+(aq)" 2Al3+(aq)+3H2O(l)Al(OH)3(s)+3H+(aq)" Al3+(aq)+3H2O (l)Acidic properties:Alkaline attack on aluminium oxide is slow unless the alkali is hot and concentrated or molten. The formula of thealuminate ion shown is accepted in exams; you may also see [Al(OH)4]-or AlO2-, though the latter only really comes from reaction with molten alkali.Al2O3(s) + 6OH(aq) + 3H2O(l)" 2 [Al(OH)6]3 (aq)Aluminium hydroxide reacts readily with sodium hydroxide solution:Al(OH)3(s)+3OH(aq)" [Al(OH)6]3 (aq)

R.B.Mahajan18Amphotericoxide and hydroxide: Al2O3, Al(OH)3.OxidesR.B.Mahajan19Acidicoxides (acid anhydrides) SiO2, P4O10, SO2, SO3, Cl2O.Silicon dioxide,silica. Silicas giant covalent lattice means it is very resistant to attack; the alkali solution must be concentrated and needs heating.SiO2(s) +2 OH(aq)" SiO32 (aq)+H2O(l)Phosphorus(V) oxide.The reaction is violent with cold water.P4O10(s) + 2H2O (l)" 4HPO3(aq)On heating the solution further reaction occurs forming phosphoric(V) acid:HPO3(aq)+H2O(l)" H3PO4(aq)Sulphur dioxide:SO2(g)+H2O (l)" H2SO3(aq)Sulphur trioxide:This reaction is violent, and is not used directly to make sulphuric acid.SO3(g)+H2O (l)" H2SO4(aq)Dichlorine oxide: Cl2O (aq)+H2O(l)" 2 HClO (aq)

ChloridesThe chlorides we'll be looking at are:NaCl, MgCl2, AlCl3, SiCl4, PCl5, PCl3, S2Cl2The structuresSodium chloride and magnesium chloride consist of giant ionic lattices at room temperatureAluminium chloride and phosphorus(V) chloride are tricky! They change their structure from ionic to covalent when the solid turns to a liquid or vapour. Melting and boiling pointsSodium and magnesium chlorides are solids with high melting and boiling points because of the large amount of heat which is needed to break the strong ionic attractions.The rest are liquids or low melting point solids. Leaving aside the aluminium chloride and phosphorus(V) chloride cases where the situation is quite complicated, the attractions in the others will be much weaker intermolecular forces such as van der Waals dispersion forces. These vary depending on the size and shape of the molecule, but will always be far weaker than ionic bonds.

R.B.Mahajan20ChloridesOxidation number of elements in chlorides, reaction with water

R.B.Mahajan21ChlorideNaMgAlSiPSOxidation number+1+2+3+4+5 or +3+2 or +1With waterDissolves

Reacts & gives of HCL fumes