OCR A Jan 2009 Paper 1 Q4 (with explained solutions) |

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OCR A Jan 2009 Paper 1 Q4

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The table below shows the melting points and atomic radii of the elements in Period 3, Na to Cl. 10 element melting point / C Na 98 atomic radius / pm 186 Mg 639 160 Al 660 143 Si 1410 118 44 110 113 102 Cl 101 99 1 pm = 11012 m (a) (i) Explain the difference in melting point for the elements Na and Mg.[3] (ii) Sulfur exists as S8 molecules and chlorine as Cl 2 molecules. Use this information to explain the difference in their melting points.[2] (b) Explain the decrease in the atomic radii across the period from Na to Cl. In your answer, you should use appropriate technical terms, spelt correctly.[3] [Total: 8]OCR 2009<br />

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F321 Question i 4 Mark Scheme January 2009 Marks Expected Answers Magnesium ions have a greater charge(cid:57) Magnesium has more (delocalised OR outer) electrons(cid:57) Magnesium has greater attraction between ions and electrons OR has stronger metallic bonds (cid:57) ii Cl2 OR S8 has intermolecular OR van der Waals' forces (cid:57) S8 has stronger intermolecular forces OR van der Waals' forces than Cl2 OR S8 has more electrons (cid:57) Additional Guidance USE annotations with ticks, crosses, ecf, etc for this part. ALLOW REVERSE ARGUMENT e.g. sodium ions have a smaller charge ALLOW Mg2+ / Mg ion / Na ion / Na+ ion ALLOW charge density as alternative to charge ALLOW REVERSE ARGUMENT e.g. sodium has fewer electrons ALLOW REVERSE ARGUMENT e.g. sodium has less attractions between ions and electrons OR has weaker metallic bonds (cid:57) ALLOW REVERSE ARGUMENT ie Cl2 has weaker intermolecular forces OR van der Waals' forces DO NOT ALLOW comparison involving covalent bonds ALLOW REVERSE ARGUMENT Cl2 has fewer electrons<br />
Mark Scheme Marks F321 Question Expected Answers nuclear charge increases/ protons increase (cid:57) electrons added to the same shell OR screening OR shielding remains the same (cid:57) greater attraction OR greater pull (cid:57) Total January 2009 Additional Guidance USE annotations with ticks, crosses, ecf, etc for this part. Nuclear OR proton(s) OR nucleus spelt correctly ONCE IGNORE atomic number increases IGNORE nucleus gets bigger charge increases is not sufficient ALLOW effective nuclear charge increases OR shielded nuclear charge increases IGNORE reference to atomic radius staying the same ALLOW shielding is similar DO NOT ALLOW extra shielding A comparison must be included: i.e. greater pull, more pull, held more tightly;<br />

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