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Third Period elements 1 R.B.Mahajan

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The trend The 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 3 rd level. R.B.Mahajan 2

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Ionic 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.Mahajan 3 ionNaMgAlSiPSCl Radius. mm 0.0950.0650.0500.0410.2120.1840.181

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The 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. R.B.Mahajan 4

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5 The 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

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Sodium, 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.Mahajan 6

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Electronegativity 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. R.B.Mahajan 7

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The 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. R.B.Mahajan 8

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9 Notice that the general trend is upwards, but this is broken by falls between magnesium and aluminum, and between phosphorus and sulphur. Explaining the pattern First 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.

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R.B.Mahajan 10 Name of Element Symbol Sodium Na Magnesium Mg Aluminium Al Silicon Si Phosphorus P Sulfur S Chlorine Cl Argon Ar Atomic Number, z 1112131415161718 Electronic Configuration 2,8,12,8,22,8,32,8,42,8,52,8,62,8,72,8,8 Atomic Radius (picometers) 18616014311811010299192 1 st Ionization Energy (kJ/mol) 502744584793101710061257>1526 Electronegativity (Pauling) 0.931.311.611.92.192.583.16- Melting Point ( o C) 98639660141044113-101-189 Boiling Point ( o C) 883109024672680280445-35-186 Metallic Character metal semi-metal (metalloid) non-metal

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Sodium Sodium 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 + O 2 2Na 2 O For the peroxide: 2Na + O 2 Na 2 O 2 Magnesium Magnesium burns in oxygen with an intense white flame to give white solid magnesium oxide. 2Mg + O 2 MgO R.B.Mahajan 11

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Aluminum Aluminum 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 + 3O 2 2Al 2 O 3 Silicon Silicon will burn in oxygen if heated strongly enough. Silicon dioxide is produced. Si + O 2 SiO 2 R.B.Mahajan 12

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White 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: P 4 + 3O 2 P 4 O 6 For the phosphorus(V) oxide: P 4 + 5O 2 P 4 O 10 Sulphur Sulphur burns in air or oxygen on gentle heating with a pale blue flame. It produces colourless sulphur dioxide gas. S + O 2 SO 2 Chlorine and argon Despite having several oxides, chlorine won't react directly with oxygen. Argon doesn't react either. R.B.Mahajan 13

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R.B.Mahajan 14 Sodium- Sodium burns in chlorine with a bright orange flame. White solid sodium chloride is produced. 2Na + Cl 2 2NaCl Magnesium- Magnesium burns with its usual intense white flame to give white magnesium chloride Mg + Cl 2 MgCl 2 Aluminium- 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 + 3Cl 2 2AlCl 3

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R.B.Mahajan 15 Silicon If 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 apparatus Si + 2Cl 2 SiCl 4 Phosphorus White 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. P 4 + 6Cl 2 4PCl 3 Phosphorus(V) chloride is an off-white (going towards yellow) solid. P 4 + 10Cl 2 4PCl 5 Sulphur If a stream of chlorine is passed over some heated sulphur, it reacts to form an orange, evil-smelling liquid, disulphur dichloride, S 2 Cl 2. 2S + Cl 2 S 2 Cl 2 or S + Cl 2 SCl 2 Chlorine and argon It obviously doesn't make sense to talk about chlorine reacting with itself, and argon doesn't react with chlorine.

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R.B.Mahajan 16 Sodium- Sodium reacts vigorously with water, melting, giving hydrogen gas leaving strongly alkaline solution of sodium hydroxide. 2Na + 2H 2 O 2NaOH Magnesium- Magnesium reacts very slowly with cold water producing weakly alkaline solution with slightly soluble Magnesium hydroxide. Mg + 2H 2 O Mg(OH) 2 Hot Magnesium reacts with steam vigorously to form Magnesium oxide and hydrogen gas. Mg + 2H 2 O MgO +H 2

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R.B.Mahajan 17 Basic oxides and hydroxides: Na 2 O, NaOH, MgO and Mg(OH) 2 Sodium oxide: Na 2 O (s) + H 2 O (l) 2NaOH (aq) Na + (aq) + OH - (aq) + H + (aq) Na + (aq) + H 2 O (l) Magnesium oxide: MgO (s) + 2H + (aq) Mg 2+ (aq) + H 2 O (l) Mg(OH) 2 (s) + 2H + (aq) Mg 2+ (aq) + 2H 2 O (l)

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Basic properties: Aluminium oxide is very resistant to attack by acids and so the first reaction is slow. Aluminium hydroxide reacts easily with dilute acids. Al 2 O 3 (s) + 6H + (aq) " 2Al 3+ (aq)+3H 2 O(l) Al(OH) 3 (s) + 3H + (aq) " Al 3+ (aq) + 3H 2 O (l) Acidic properties: Alkaline attack on aluminium oxide is slow unless the alkali is hot and concentrated or molten. The formula of the aluminate ion shown is accepted in exams; you may also see [Al(OH) 4 ] - or AlO 2 -, though the latter only really comes from reaction with molten alkali. Al 2 O 3 (s) + 6OH (aq) + 3H 2 O(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.Mahajan 18 Amphoteric oxide and hydroxide: Al 2 O 3, Al(OH) 3.

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R.B.Mahajan 19 Acidic oxides (acid anhydrides) SiO 2, P 4 O 10, SO 2, SO 3, Cl 2 O. Silicon dioxide, silica. Silicas giant covalent lattice means it is very resistant to attack; the alkali solution must be concentrated and needs heating. SiO 2 (s) + 2 OH (aq) " SiO 3 2 (aq) + H 2 O(l) Phosphorus(V) oxide. The reaction is violent with cold water. P 4 O 10 (s) + 2H 2 O (l) " 4HPO 3 (aq) On heating the solution further reaction occurs forming phosphoric(V) acid: HPO 3 (aq) + H 2 O(l) " H 3 PO 4 (aq) Sulphur dioxide: SO 2 (g) + H 2 O (l) " H 2 SO 3 (aq) Sulphur trioxide: This reaction is violent, and is not used directly to make sulphuric acid. SO 3 (g) + H 2 O (l) " H 2 SO 4 (aq) Dichlorine oxide: Cl 2 O (aq) + H 2 O(l) " 2 HClO (aq)

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The chlorides we'll be looking at are: NaCl, MgCl 2, AlCl 3, SiCl 4, PCl 5, PCl 3, S 2 Cl 2 The structures Sodium chloride and magnesium chloride consist of giant ionic lattices at room temperature Aluminium 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 points Sodium 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.Mahajan 20

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Oxidation number of elements in chlorides, reaction with water R.B.Mahajan 21 ChlorideNaMgAlSiPS Oxidation number +1+2+3+4+5 or +3+2 or +1 With waterDissolvesReacts & gives of HCL fumes