TOPIC 9 ANSWERS & MARK SCHEMES QUESTIONSHEET 1 BASIC CONCEPTS

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1 QUESTIONSHEET 1 BASIC CONCEPTS a) (i) Increasing atomic number / number of protons (ii) Groups (iii) Periods b) (i) Alkali metals (ii) Alkaline earth metals (iii) Halogens (iv) Noble gases / inert gases c) Transition metals / d-block elements d) (i) Group number indicates the number of electrons found in an atom s outer shell (ii) In a period, atoms of all elements have the same electron core e) (i) Trend Decreases along the period Cause Increasing nuclear charge / number of protons in the nucleus (ii) Trends Increase Na Si Decrease P Ar Causes Na Si stronger bonding P Ar weaker van der Waals forces (iii) Trend Increases along the period Cause Increasing nuclear charge

2 QUESTIONSHEET 2 ATOMIC AND IONIC RADII a) (i) Atomic radii decrease Across a period, the nuclear charge / number of protons in the nucleus increases without inner shell screening changing outer shell electrons are attracted more closely Maximum 3 marks (ii) The charge increases from +1 to +3 Cations are formed by loss of outer shell electrons, which increase in number from 1 to 3 b) (i) The extent to which a cation can distort the shape of an anion (ii) Polarising power increases because cationic charge increases across a period while size decreases Or because surface charge density / charge : radius ratio increases c) Atomic radii of Period 3 elements are larger In Period 3, the outer electrons occupy a higher shell Or Period 3 elements have an additional inner shell d) (i) Greater nuclear charge for Na + / more protons in the nucleus each outer electron is subjected to a stronger attraction (ii) When Na forms Na + an electron is lost but the nuclear charge is unaffected the remaining electrons each experience a proportionally greater attraction to the nucleus When Cl forms Cl - an electron is gained each outer shell electron experiences a weaker attraction to the nucleus Or When metal atoms form cations electrons are lost and the outer shell is no longer occupied When non-metal atoms form anions electrons are gained and repulsion between the electrons causes an increase in size

3 a) QUESTIONSHEET 3 STRUCTURES Element Na Mg Al Si P S Cl Nature of Metallic Metallic Metallic Covalent Covalent Covalent Covalent Bonding Type of Metal Metal Metal Macro- Simple Simple Simple Structure molecules molecules molecules molecules (-1 for error) (8) b) Description Element (i) Can exist in a giant covalent structure where each atom is tetrahedrally bonded to four others carbon (ii) Simple molecules with six lone pairs (iii) Simple molecules containing triple bonds (iv) A gas with discrete atoms (v) A metallic structure containing +1 ions fluorine nitrogen neon lithium

4 QUESTIONSHEET 4 MELTING POINTS AND BOILING POINTS a) (i) Metallic bonding increases in strength because cations have a greater charge (ii) Melting involves breaking many / all covalent bonds in the solid (iii) Melting points are governed by the strength of van der Waals forces between molecules Such forces depend on the number of atoms per molecule 4 for white phosphorus / P 4 (½) 8 for sulfur / S 8 (½) 2 for chlorine / Cl 2 (½) 0 for argon (½) b) (i) Increasing / similar to melting points (ii) Liquid silicon contains atoms and small molecules Relatively little energy is needed to break the remaining bonds / overcome the van der Waals forces between the atoms (iii) Van der Waals forces between argon atoms are extremely weak

5 QUESTIONSHEET 5 ELECTRICAL CONDUCTIVITY a) (i) Type of bonding Metallic bonding Description Lattice of cations with delocalised valency electrons / flux (or sea ) of electrons (ii) Electrons can move under an applied p.d. throughout the entire metal (iii) Number of outer shell electrons increases from 1 to 2 to 3 Hence there are more free / delocalised / mobile electrons b) (i) Type of bonding Covalent Explanation Electrical conductivity is low very limited movement of electrons (ii) Change metallic character increases Explanation Electrical conductivity increases so mobility of electrons greater (iii) Change Great decrease in electrical conductivity Reasoning so metallic to giant covalent structure

6 QUESTIONSHEET 6 IONISATION ENERGIES AND ELECTRON AFFINITIES a) (i) Figure II (ii) General trend Ionisation energies generally increase across a period because of additional protons in the nucleus / greater nuclear charge while additional electrons enter the same shell Or because each outer electron is subjected to a greater effective nuclear charge due to nuclear charge increasing while the number of inner electrons remains unchanged Irregularities Value for B is lower than for Be because ionisation of B entails loss of a relatively high energy 2p electron compared with a lower energy 2s electron for Be Or 2s level shields electron in 2p with greater effect than attraction from extra proton Value for O is lower than for N because ionisation of O entails loss of a paired 2p electron and there is repulsion between paired electrons b) (i) A similar graph should be sketched on Figure II at a lower level (ii) Ionisation energies of Period 3 elements are lower (stated or implied) In Period 3, the outer electrons are in a higher shell / further from the nucleus and therefore less attracted to the nucleus The effect of increased nuclear charge is offset by the shielding effect of an additional inner shell c) (i) Na (ii) second electron from shell nearer nucleus much stronger attraction by nucleus outer electrons nearer nucleus offsets lower nuclear charge Maximum 3 marks

7 QUESTIONSHEET 7 ELECTRONEGATIVITIES AND BONDING a) (i) A measure of the power of an atom of that element, within a molecule to attract towards itself the electrons of a covalent bond (ii) Electronegativities increase because the tendency of atoms to gain electrons increases (Or electron affinities increase ) due to an increase in nuclear charge and decreased atomic radius (½) with no increase in shielding (½) Simultaneously, the tendency of atoms to lose electrons decreases (Or ionisation energies increase ) Maximum 4 marks b) Across the period, the tendency to form ionic bonds decreases at first while the tendency to form covalent bonds increases Tendency for ionic bonding (negative ion formation) increases towards the right of the period Or accept an answer based on examples (3) c) If the electronegativity difference between the bonded atoms is large / >2.1 the bonding is essentially ionic otherwise it is essentially covalent d) Bonding in PCl 3 is more ionic character/more polar because electronegativity values decrease down a group the electronegativity difference between P and Cl > than that between N and Cl

8 QUESTIONSHEET 8 OXIDATION NUMBERS a) Group 1 Group 2 (Do not accept Group 3 but do not penalise) b) +3 (½) +5 (½) (Deduct ½ mark if signs are missing) P 4 O 6 / P 2 O 3 PCl 3 P 4 O 10 / P 2 O 5 PCl 5 c) (i) +6 (ii) ground state (Ne) 3s 2 3p 4 unpairing in 3s/3p promotion to 3d giving 6 unpaired electrons (iii) -2: H 2 S or a metal sulfide, e.g. CuS +1: S 2 Cl 2 +2: SCl 2 +4: SO 2 / H 2 SO 3 or a sulfite, e.g. Na 2 SO 3 +6: SO 3 / H 2 SO 4 or a sulfate, e.g. Na 2 SO 4 (Accept any 3)

9 a) (i) QUESTIONSHEET 9 TEST QUESTION I Atomic radius / nm Labelled axes Suitable scales Correctly plotted points Curve of best fit Atomic radius to Atomic number (ii) Increasing nuclear charge / number of protons in the nucleus with electrons added to the same shell b) (i) Decrease across the period (ii) Have lower values c) (i) kj mol -1 (ii) Oxygen has an electron pair in a 2p orbital Electron-electron repulsion between paired electrons Value for N is high due to stability of a half-filled subshell d) Difference Boiling point Be > Li Explanation Be has a greater number of free / valency electrons and a greater cationic charge Be has stronger metallic bonding Maximum 2 marks

10 a) (i) F QUESTIONSHEET 10 TEST QUESTION II (ii) B (iii) E b) Element C Explanation Highest nuclear charge strongest attraction for an electron Electron is added to the same shell in all cases Maximum 2 marks c) (i) Element C Explanation Exists as a giant molecular lattice with extremely strong covalent bonds between atoms (ii) Element B Explanation B is a metal / has metallic bonding with the highest number of free electrons d) (i) Oxidation no. in DE 3 +3 Oxidation no. in DE 5 +5 (ii) D 2 O 3 / D 4 O 6 D 2 O 5 / D 4 O 10