QUESTIONSHEET 1 SUBATOMIC PARTICLES

Transcription

1 QUESTIONSHEET 1 SUBATOMIC PARTICLES a) (i) Atom The smallest quantity of an element to have the properties of that element Element A substance comprising atoms with identical atomic numbers (ii) Substances consisting of atoms with same atomic number / same number of protons / of the same element but with different mass number / different numbers of nucleons / different neutron numbers (iii) Mass number The number of nucleons / neutrons and protons in an atom of an element Atomic number The number of protons in an atom of an element Difference The atoms of an element have identical atomic numbers but can have different mass numbers The atoms of an isotope have identical atomic numbers and identical mass numbers b) Names and characteristics Proton, neutron Proton has relative mass of 1 (½) and relative charge of +1 (½) Neutron has relative mass of 1 (½) and is uncharged (½) Exception Hydrogen c) Number of neutrons Number of electrons Radon = 136 (½) 86 (½) 208 Pb = 126 (½) 82 2 = 80 (½) 127 I = 74 (½) = 54 (½)

2 QUESTIONSHEET 2 a) (i) Relative abundance of a given isotope = RELATIVE ATOMIC MASS Amount of a given isotope x 100% Sum of amounts of all isotopes of that element (ii) The weighted average mass of an atom of a given element divided by 1/12 th of the mass of an atom of carbon-12 (iii) A r = (79 x 0.505) + (81 x 0.495) = 79.99% 80.0% to 1 d.p. b) (i) An amount of substance which contains the same number of particles / molecules / atoms as there are atoms in g of carbon-12 (ii) Molar mass The mass of 1 mole of a substance Relationship No.of moles = mass molar mass (iii) 15 g of carbon = 1.25 mol No. of particles = =

3 QUESTIONSHEET 3 THE MASS SPECTROMETER a) A Vacuum / to vacuum pump B Sample chamber C Ionisation chamber D Negatively charged plates E Beam of high speed cations F Magnet / magnetic field b) (i) Reduces the pressure so less thermal energy is needed to vaporise sample Removes traces of previous sample Reduces traces of air Maximum 2 marks (ii) A print-out / display from the electronics of the mass spectrometer scaled so x-axis gives mass/charge ratio and y-axis gives the percentage abundance or relative abundance (iii) Only positively charged species can be accelerated by the potential difference deflected in the correct direction by the magnetic field and counted / detected by the detector

4 QUESTIONSHEET 4 PRINCIPLES OF MASS SPECTROMETRY Ionisation A stream of high speed electrons bombards gaseous sample and forces off a further electron to form a gaseous cation Fragmentation Bond(s) rupture in a gaseous cation formed from a molecule / molecular ion to give smaller cations and free radicals Acceleration Beam of gaseous cations passes through holes / slits in two negatively charged plates / cathodes with a potential difference across them Deflection Beams of cations are deflected by magnetic field Detection Gaseous cations with a particular mass : charge ratio hit the detector The detector counts the number of cations striking it Detector signal is directly proportional to abundance / number of each type of ion

5 QUESTIONSHEET 5 RELATIVE ATOMIC MASS FROM MASS SPECTROMETRY a) (i) Relative abundance of gallium-69 = 145 mm x 100% = 60.4% (1 d.p.) (145 mm + 95 mm) Relative abundance of gallium-71 = 95 mm x 100% = 39.6 % (1 d.p.) (145 mm + 95 mm) (ii) A r (Ga ) = (60.4% x 69) + (39.6% x 71) = (2 d.p.) 100 % (iii) The conversion of Ga Ga + involves loss of an electron of negligible mass b) (i) Br + (g) (½) Br + (g) (½) Br 2 (g) 4. ( 79 Br 81 Br) + (g) Br 2+ (g) (Lose 1 mark if any +ve charge is omitted) State symbols are not essential. (ii) Peak height is directly proportional to abundance and the abundances of bromine-79 and bromine-81 in the Earth s crust are similar (iii) Peaks at 158 and 162 are of similar height because abundances of Br-79 and Br-81 are similar The mid peak is twice as high because there is twice the probability of 79 Br 81 Br occurring than 79 Br 2 or 81 Br 2

6 QUESTIONSHEET 6 FIRST IONISATION ENERGY a) Definition The minimum energy required to remove one mole of electrons from one mole of gaseous atoms of a given element to form one mole of singly charged gaseous ions Equation Cl(g) Cl + (g) + e - (1 for species, 1 for state symbols) b) Fourth ionisation energy of manganese c) Endothermic Energy must be supplied to overcome the electrostatic force of attraction between negatively charged electron and the positively charged nucleus d) First ionisation energies decrease in magnitude as a group is descended This is because, whilst the nuclear charge increases down a group this is more than offset by the increase in shielding due to the increasing number of inner shells Because atomic radii increase down the group the outer shell electrons are further from the nucleus and less strongly attracted to it Maximum 4 marks

7 QUESTIONSHEET 7 CHANGES IN FIRST IONISATION ENERGIES ACROSS THE PERIODIC TABLE a) These generally increase in magnitude Across a period the nuclear charge / number of protons in the nucleus increases and there is no increase in screening because all the elements in a period have same number of shells Also, the atomic radii decrease across a period Maximum 3 marks b) (i) Be 1s 2s 2p (½) B Be + (½) (½) (ii) B + O (½) (½) N (½) O + (½) N + (½) (iii) An atom of boron loses an electron from a 2p orbital whilst an atom of beryllium loses an electon from a 2s orbital Electrons in 2s orbitals have lower energy than those in 2p orbitals (Therefore more energy is required to remove them.) A nitrogen atom has a very stable electron configuration with a half-filled 2p subshell An oxygen atom has a 2p orbital with two electrons in it mutually repelling each other O + ions have a very stable half-filled 2p subshell N + ions lack the stability of a half-filled 2p subshell (Therefore more energy is required to remove an electron from nitrogen.)

8 QUESTIONSHEET 8 SUCCESSIVE IONISATION ENERGIES a) X and Z belong to Group 2 because their atoms each have two outer shell electrons This is because there is a significant jump in the value of the third ionisation energy compared with the second, showing the third electron is in an inner shell Y belongs to Group 3 because its atom has three outer shell electrons This is shown by the significant jump from the third to the fourth ionisation energy Maximum 5 marks b) X < Y < Z (2) (Award 1 mark only if X and Y are interchanged. Note that, within a period, a Group 2 element has a higher first ionisation energy than a Group 3 element.) c) Identity of X Magnesium lg I Gradual increase Sudden increase in correct place Gradual increase No. of electrons removed d) lg I 2p 3s No. of electrons removed e) Low first ionisation energy (½) shows one electron in outer shell (½) Then a jump (½) to the next eight which increase steadily (½) showing eight electrons in second shell (½) Another jump from I 9 to I 10 (½) shows two electrons in the first shell (½) Maximum 3 marks

9 QUESTIONSHEET 9 ELECTRONIC CONFIGURATIONS OF ATOMS a) Calcium 1s 2 2s 2 2p 6 3s 2 3p 6 s Silicon 1s 2 2s 2 2p 6 3s 2 3p 2 p Sulfide 1s 2 2s 2 2p 6 3s 2 3p 6 p b) (i) Description Spherical / spherically symmetrical about the nucleus Diagram (ii) Description Dumb-bell shaped / double lobed Diagram y z x c) 1s 2p 3p 4s 3d 4p 5s 4f 5d 6p (3) Deduct 1 mark for each mistake.

10 QUESTIONSHEET 10 ELECTRON AFFINITY a) (i) The enthalpy change / energy released when each of 1 mole of gaseous oxygen atoms gains an electron to form O - (g) Allow enthalpy change for O (g) + e - O - (g) (ii) O - (g) + e - O 2- (g) (2) (1 for species ; 1 for state symbols) (iii) First electron affinity is exothermic because the positive nuclear charge attracts a negatively charged electron Second electron affinity is endothermic because O - is negatively charged and repels an incoming negatively charge electron b) (i) Down the group from chlorine the shielding effect increases / effective nuclear charge decreases the 1 st electron affinities will be decreasingly exothermic / will decrease in magnitude (ii) Chlorine is in the same period as sulphur, but in a higher group it has a larger effective nuclear charge the magnitude of electron affinity is greater for chlorine (iii) Anomalous means that fluorine does not fit in with the general trend given in b)(i) The electron affinity of fluorine is of smaller magnitude / less exothermic than for chlorine Fluorine is a very small atom The seven outer shell electrons in a fluorine atom, being negatively charged and close together, repel the incoming electron Maximum 3 marks

11 QUESTIONSHEET 11 ATOMIC RADII a) Atomic shielding Reduction of the attractive force between the electrons in the outer shell of an atom and the nucleus by the presence of inner shell electrons Nuclear charge A positive charge which is directly proportional to the number of protons in the nucleus of an atom Effective nuclear charge The force of attraction experienced by outer shell electrons It is affected by the nuclear charge, radius of the atom and the shielding effect and therefore it is smaller than the nuclear charge Maximum 2 marks b) (i) Across a period nuclear charge increases but shielding does not alter as no more inner shell electrons are added Therefore the effective nuclear charge increases and outer shell electrons are pulled more closely to the nucleus and atomic radii decrease Maximum 3 marks (ii) Down a group nuclear charge increases but shielding increases even more so the effective nuclear charge is reduced Therefore outer shell electrons are attracted less strongly and atomic radii increase Maximum 3 marks c) (i) Potassium atom has a larger radius than the argon atom The potassium atom has more shells of electrons than the argon atom This outweighs the increase in nuclear charge (ii) The sodium atom has a larger radius than the magnesium atom They occupy the same period and have the same number of shells of electrons The effective nuclear charge increases / number of protons in the nucleus increases Quality of language: at least one well constructed answer with a logical link between two statements.

12 QUESTIONSHEET 12 COMPARISONS OF ATOMIC AND IONIC RADII a) Sodium has a larger radius than chlorine They occupy the same period, and across a period atomic radii decrease because effective nuclear charge increases / number of protons in the nucleus increases b) Rubidium ion has a larger radius than sodium ion The rubidium ion has more shells of electrons This outweighs the increase in nuclear charge c) Radius of Mg is larger than that of Mg 2+ The nuclear charges of Mg and Mg 2+ are the same Mg 2+ lost the outer shell d) Radius of Cl - is larger than that of Cl The nuclear charges of Cl and Cl - are the same, but there are more outer shell electrons in Cl - Repulsion between these electrons causes an increase in size e) Radius of H - is larger than that of H + H + has no outer shell electrons H - has two f) Radius of O 2- is larger than that of O - O 2- and O - have same nuclear charge, but O 2- has more outer shell electrons Repulsion between these electrons causes an increase in size g) N 3- has a larger radius than Mg 2+ They have the same number of electrons The nuclear charge is greater in Mg 2+ Quality of language: at least one well constructed answer with a logical link between two statements.

13 QUESTIONSHEET 13 IONISING RADIATIONS a) (i) Radiation Nature of radiation Relative penetrative power 4 2 Alpha Helium nucleus / / 2 He + Thin sheet of paper Beta Emission of electrons Thin sheet of aluminium Gamma High energy electromagnetic Lead block radiation (ii) γ -ve plate (cathode) α β +ve plate (anode) Source containing emitters of α, β and γ radi Sketch γ - unaffected β - deflected to +ve plate to a greater extent than α α - deflected to ve plate; larger radius than β due to its greater mass b) (i) The stone contains radio-isotopes which eventually decay to form a radio-isotope of radon This enters the lungs and can result in the development of a tumour (ii) The lining of the lungs is a very thin tissue (to permit gaseous exchange during breathing) Radon is a gas 4 He 2

14 QUESTIONSHEET 14 RADIOACTIVITY a) (i) An isotope with an unstable nucleus which decays, emitting ionising radiation (ii) Time for the number of unstable nuclei / atoms to decay by 50% Or Time for the radiation emitted by a radio-isotope to fall to 50% of its original rate of emission (iii) Th He + Ra (2) (iv) Ra e + Ac (2) (For (iii) and (iv) deduct 1 mark for 1 mistake) (v) For any suggestion involving loss of 1 α particle and 2 β particles, e.g Th He + Ra Ra e + Ac Ac e + Th Allow other sequences (lose 1 mark for 1 mistake) b) (i) Radon is in group 0, therefore it is a noble gas with a complete octet of outer shell electrons and is chemically unreactive Its nucleus is unstable, therefore it will emit radioactivity and form an isotope of a different element (ii) 3 days and 20 hours = 92 hours 19 days and 4 hours = 460 hours 460/92 = 5 t ½ t ½ t ½ t ½ t ½ t ½ 100% 50% 25% 12.5% 6.25% 3.13% Answer = 3.13 % t ½ t ½ t ½ t ½ (iii) 100% 50% 25% 12.5% 6.25% 4 t ½ 4 52 s = 208 s = 3 min 28 s

15 QUESTIONSHEET 15 APPLICATIONS OF RADIO-ISOTOPES a) Age of sample One half-life because counts reduced by 50% Assumptions Fresh wood at the time it was alive contained the same proportion of carbon-14 as fresh wood does today The relic was originally made from fresh wood The counts had been corrected for background radiation Maximum 2 marks b) Any example e.g. iodine-131 Thyroid gland in neck accumulates iodine Patient takes aqueous iodine-131. Rate of uptake can be measured by a radiation counter c) (i) Sketch counter sheet source of radiation Explanation A source of radiation is placed just after the rollers Radiation is focused at the sheet Radiation passing through is counted If the count is too low, the steel is too thick / the rollers are closed up but if the count is too high, the steel is too thin / the rollers are opened up Maximum 4 marks. The first 3 marks can be obtained from the diagram. (ii) Long half-life A radioactive isotope source with a short half-life would have to be replaced regularly A decrease in the count could be due either to excessive steel thickness or deterioration of the source The equipment would have to be recalibrated Highly penetrating radiation is needed Use a γ emitter (accept β but not α emitter) Maximum 5 marks

16 QUESTIONSHEET 16 TEST QUESTION I a) (i) Number of protons Number of neutrons Number of electrons Sulfur Sulfur Sulfur (ii) S S S (iii) Ar = ( ) + ( ) + ( ) = = significant figures b) (i) The addition of a neutron to the nucleus of an atom would have the following effect on the element: increase the atomic mass True increase the atomic number False alter the chemical properties False (ii) Different isotopes of the same elements differ in their: atomic masses True atomic numbers False chemical properties False c) (i) What are the differences between 1 H 2 O and 1 H 2 O 2? 1 H 2 O 2 contains 1 more O atom than 1 H 2 O So it has different chemical properties (ii) What are the differences between 1 H 2 O and 2 H 2 O? 2 H contains a neutron/ is heavier than 1 H So the molecule has a larger M r /is heavier (iii) What are the differences between 12 CO and 14 CO 2? 14 CO 2 contains 1 more O atom than 12 CO So it has different chemical properties 14 C contains more neutrons/ is heavier than 12 C

17 QUESTIONSHEET 17 TEST QUESTION II a) (i) 1s 2 2s 2 2p 5 (ii) Group 7 (iii) 19 (iv) X consists of one isotope only (v) 19 F + and 19 F 2 + (vi) As successive electrons are removed, the ion which remains has an increasing positive charge so that more energy is needed to remove further electrons / there is a general increase in ionisation energy Discontinuity between I 7 and I 8 as electrons start to be removed from the inner shell b) (i) Group 3 (ii) Relative abundance (iii) Mass/charge ratio Lg I No. of electrons removed c) YX 3

18 QUESTIONSHEET 18 THE HYDROGEN SPECTRUM a) (i) 1s 2 2s 2 2p 5 a) Each line is caused by an electronic transition / jump towards the nucleus from one fixed energy level to another releasing a fixed amount of energy corresponding to radiation of single wavelength / frequency and hence a line in the spectrum or if there were no fixed energy levels there would be a band spectrum Maximum 4 marks b) (i) Lines getting closer together (ii) Towards the short wavelength / high frequency / blue end NOT from left to right (iii) Energy levels become progressively closer together away from the nucleus E values / amounts of energy released on transition become closer together when E is high and λ is short / n is high Maximum 3 marks c) Cause Transitions into n = 1 Explanation E is high (½) ν is high / λ is low (½) d) (i) Ionisation energy is the energy required for the promotion of an electron from n = 1 to n = Radiation at the series limit of the Lyman series is energy released in the reverse process / as a result of electronic transition from n = to n = 1 (ii) Planck constant Avogadro constant e) Spectrum line Energy level transition 1. A 2. E 3. B 4. G 5. D 6. F 7. C Maximum 4 marks (-1 for each incorrect answer)