3.1.1.3 Mass numbers & Isotopes 289 minutes 283 marks Page 1 of 33
Q1. (a) Define the term atomic number of an element. (b) Give the symbol, including mass number and atomic number, for an atom of an element which contains 12 neutrons and 11 electrons. (c) In terms of s and p sub-levels, give the electronic configuration of an aluminium atom. (d) How many neutrons are there in one 27 Al atom? (e) Define the term relative atomic mass of an element. (f) Parts (i) to (iv) below refer to the operation of a mass spectrometer. (i) Name the device used to ionise atoms in a mass spectrometer. Why is it necessary to ionise atoms before acceleration? (iii) What deflects the ions? (iv) What is adjusted in order to direct ions of different mass to charge ratio onto the detector? (4) Page 2 of 33
(g) A meteorite was found to contain three isotopes of element X. A mass spectrometer gave the following information about these isotopes. m/z 24.0 25.0 26.0 Relative abundance 64.2 20.3 15.5 (i) Calculate the relative atomic mass of X. Using the Periodic Table, suggest the most likely identity of element X. (iii) Suggest one reason why the relative atomic mass of X, given in the Periodic Table, differs from your answer to part (g)(i). (5) (Total 16 marks) Q2. (a) State the relative charge and relative mass of a proton, of a neutron and of an electron. In terms of particles, explain the relationship between two isotopes of the same element. Explain why these isotopes have identical chemical properties. (7) (b) Define the term relative atomic mass. An element exists as a mixture of three isotopes. Explain, in detail, how the relative atomic mass of this element can be calculated from data obtained from the mass spectrum of the element. (7) (Total 14 marks) Q3. (a) Ionisation is the first of the four main stages involved in obtaining the mass spectrum of a sample of gaseous titanium atoms. Explain how ionisation is achieved. Name the remaining three stages and, in each case, state how each stage is achieved. Explain why it would be difficult to distinguish between 48 Ti 2+ and 24 Mg + ions using a mass spectrometer. (10) (b) State any differences and similarities in the atomic structure of the isotopes of an element. State the difference, if any, in the chemistry of these isotopes. Explain your answer. (4) Page 3 of 33
(c) The table below gives the percentage abundance of each isotope in the mass spectrum of a sample of titanium. m/z 46 47 48 49 50 % abundance 8.02 7.31 73.81 5.54 5.32 Define the term relative atomic mass of an element. Use the above data to calculate the value of the relative atomic mass of titanium in this sample. Give your answer to two decimal places. (4) (Total 18 marks) Q4. (a) Complete the following table. Particle Relative charge Relative mass Proton Neutron Electron (3) (b) An atom of element Z has two more protons and two more neutrons than an atom of. Give the symbol, including mass number and atomic number, for this atom of Z. (c) Complete the electronic configurations for the sulphur atom, S, and the sulphide ion, S 2. S 1s 2... S 2 1s 2... (d) State the block in the Periodic Table in which sulphur is placed and explain your answer. Block... Explanation... Page 4 of 33
(e) Sodium sulphide, Na 2 S, is a high melting point solid which conducts electricity when molten. Carbon disulphide, CS 2, is a liquid which does not conduct electricity. (i) Deduce the type of bonding present in Na 2 S and that present in CS 2 Bonding in Na 2 S... Bonding in CS 2... By reference to all the atoms involved explain, in terms of electrons, how Na 2 S is formed from its atoms. (iii) Draw a diagram, including all the outer electrons, to represent the bonding present in CS 2 (iv) When heated with steam, CS 2 reacts to form hydrogen sulphide, H 2 S, and carbon dioxide. Write an equation for this reaction. (7) (Total 16 narks) Q5. (a) Give the relative charge and relative mass of an electron. Relative charge... Relative mass... (b) Isotopes of chromium include 54 Cr and 52 Cr (i) Give the number of protons present in an atom of 54 Cr Page 5 of 33
Deduce the number of neutrons present in an atom of 52 Cr (iii) Apart from the relative mass of each isotope, what else would need to be known for the relative atomic mass of chromium to be calculated? (3) (c) In order to obtain a mass spectrum of a gaseous sample of chromium, the sample must first be ionised. (i) Give two reasons why it is necessary to ionise the chromium atoms in the sample. Reason 1... Reason 2... State what is adjusted so that each of the isotopes of chromium can be detected in turn. (iii) Explain how the adjustment given in part (c) enables the isotopes of chromium to be separated. (4) (d) (i) State what is meant by the term empirical formula. A chromium compound contains 28.4% of sodium and 32.1% of chromium by mass, the remainder being oxygen. Calculate the empirical formula of this compound. (4) (Total 13 marks) Page 6 of 33
Q6. A gaseous sample of chromium can be analysed in a mass spectrometer. Before deflection, the chromium atoms are ionised and then accelerated. (a) Describe briefly how positive ions are formed from gaseous chromium atoms in a mass spectrometer. (b) What is used in a mass spectrometer to accelerate the positive ions? (c) What is used in a mass spectrometer to deflect the positive ions? (d) The mass spectrum of a sample of chromium shows four peaks. Use the data below to calculate the relative atomic mass of chromium in the sample. Give your answer to two decimal places. m/z 50 52 53 54 Relative abundance / % 4.3 83.8 9.5 2.4 (Total 6 marks) Q7. (a) One isotope of sodium has a relative mass of 23. (i) Define, in terms of the fundamental particles present, the meaning of the term isotopes. Explain why isotopes of the same element have the same chemical properties. Page 7 of 33
(iii) Calculate the mass, in grams, of a single atom of this isotope of sodium. (The Avogadro constant, L, is 6.023 10 23 mol 1 ) (5) (b) Give the electronic configuration, showing all sub-levels, for a sodium atom. (c) Explain why chromium is placed in the d block in the Periodic Table. (d) An atom has half as many protons as an atom of 28 Si and also has six fewer neutrons than an atom of 28 Si. Give the symbol, including the mass number and the atomic number, of this atom. (Total 9 marks) Q8. (a) Complete the following table. Relative mass Relative charge Proton Electron (b) An atom of element Q contains the same number of neutrons as are found in an atom of 27 A1. An atom of Q also contains 14 protons. (i) Give the number of protons in an atom of 27 A1. Deduce the symbol, including mass number and atomic number, for this atom of element Q. (3) Page 8 of 33
(c) Define the term relative atomic mass of an element. (d) The table below gives the relative abundance of each isotope in a mass spectrum of a sample of magnesium. m/z 24 25 26 Relative abundance (%) 73.5 10.1 16.4 Use the data above to calculate the relative atomic mass of this sample of magnesium. Give your answer to one decimal place. (e) State how the relative molecular mass of a covalent compound is obtained from its mass spectrum. (Total 10 marks) Q9. (a) Define the terms (i) mass number of an atom, relative molecular mass. (3) Page 9 of 33
(b) (i) Complete the electron arrangement for a copper atom. 1s 2... Identify the block in the Periodic Table to which copper belongs. (iii) Deduce the number of neutrons in one atom of 65 Cu (3) (c) A sample of copper contains the two isotopes 63 Cu and 65 Cu only. It has a relative atomic mass, A r, less than 64. The mass spectrum of this sample shows major peaks with m/z values of 63 and 65, respectively. (i) Explain why the A r of this sample is less than 64. Explain how Cu atoms are converted into Cu + ions in a mass spectrometer. (iii) In addition to the major peaks at m/z = 63 and 65, much smaller peaks at m/z = 31.5 and 32.5 are also present in the mass spectrum. Identify the ion responsible for the peak at m/z = 31.5 in the mass spectrum. Explain why your chosen ion has this m/z value and suggest one reason why this peak is very small. Identity of the ion... Explanation for m/z value... Reason why this peak is very small... (6) (Total 12 marks) Page 10 of 33
Q10. A sample of iron from a meteorite was found to contain the isotopes 54 Fe, 56 Fe and 57 Fe. (a) The relative abundances of these isotopes can be determined using a mass spectrometer. In the mass spectrometer, the sample is first vaporised and then ionised. (i) State what is meant by the term isotopes. Explain how, in a mass spectrometer, ions are detected and how their abundance is measured. How ions are detected... How abundance is measured... (5) (b) (i) Define the term relative atomic mass of an element. The relative abundances of the isotopes in this sample of iron were found to be as follows. m/z 54 56 57 Relative abundance (%) 5.8 91.6 2.6 Use the data above to calculate the relative atomic mass of iron in this sample. Give your answer to one decimal place. (4) (c) (i) Give the electron arrangement of an Fe 2+ ion. State why iron is placed in the d block of the Periodic Table. Page 11 of 33
(iii) State the difference, if any, in the chemical properties of isotopes of the same element. Explain your answer. Difference... Explanation... (4) (Total 13 marks) Q11. (a) State, in terms of the fundamental particles present, the meaning of the term isotopes. (b) An atom contains one more proton than, but the same number of neutrons as, an atom of 36 S. Deduce the symbol, including the mass number and the atomic number, of this atom. (c) The table below gives the relative abundance of each isotope in a mass spectrum of a sample of germanium, Ge. m/z 70 72 74 Relative abundance (%) 24.4 32.4 43.2 (i) Complete the electron arrangement of a Ge atom. 1s 2... Use the data above to calculate the relative atomic mass of this sample of germanium. Give your answer to one decimal place. Page 12 of 33
(iii) State what is adjusted in a mass spectrometer in order to direct ions with different m/z values onto the detector. Explain your answer. Adjustment... Explanation... (iv) One of the isotopes of Ge, given in the table in part (c), has an ion that forms a small peak in the mass spectrum which is indistinguishable from a peak produced by 36 S + ions. Identify this Ge ion and explain your answer. Ion... Explanation... (8) (Total 11 marks) Q12. In one model of atomic structure, the atom has a nucleus surrounded by electrons in levels and sub-levels. (a) Define the term atomic number. (b) Explain why atoms of an element may have different mass numbers. (c) The table below refers to a sample of krypton. Relative m/z 82 83 84 86 Relative abundance / % 12 12 50 26 (i) Name an instrument which is used to measure the relative abundance of isotopes. Define the term relative atomic mass. Page 13 of 33
(iii) Calculate the relative atomic mass of this sample of krypton. (5) (d) Give the complete electronic configuration of krypton in terms of s, p and d sub-levels. (e) In 1963, krypton was found to react with fluorine. State why this discovery was unexpected. (f) Use a suitable model of atomic structure to explain the following experimental observations. (i) The first ionisation energy of krypton is greater than that of bromine. The first ionisation energy of aluminium is less than the first ionisation energy of magnesium. (4) (Total 13 marks) Q13. In 1913 Niels Bohr proposed a model of the atom with a central nucleus, made up of protons and neutrons, around which electrons moved in orbits. After further research, the model was refined when the existence of energy levels and sub-levels was recognised. (a) Complete the following table for the particles in the nucleus. Particle Relative charge Relative mass proton neutron (b) State the block in the Periodic Table to which the element tungsten, W, belongs. Page 14 of 33
(c) Isotopes of tungsten include 182 W and 186 W (i) Deduce the number of protons in 182 W Deduce the number of neutrons in 186 W (d) In order to detect the isotopes of tungsten using a mass spectrometer, a sample containing the isotopes must be vaporised and then ionised. (i) Give two reasons why the sample must be ionised. 1... 2... State what can be adjusted in the mass spectrometer to enable ions formed by the different isotopes to be directed onto the detector. (e) State and explain the difference, if any, between the chemical properties of the isotopes 182 W and 186 W Difference... Explanation... (f) The table below gives the relative abundance of each isotope in the mass spectrum of a sample of tungsten. m/z 182 183 184 186 Relative abundance /% 26.4 14.3 30.7 28.6 Use the data above to calculate a value for the relative atomic mass of this sample of tungsten. Give your answer to 2 decimal places. (Total 12 marks) Page 15 of 33
Q14. (a) (i) Define the term relative atomic mass (A r ) of an element. A sample of the metal silver has the relative atomic mass of 107.9 and exists as two isotopes. In this sample, 54.0% of the silver atoms are one isotope with a relative mass of 107.1 Calculate the relative mass of the other silver isotope. State why the isotopes of silver have identical chemical properties. (4) Page 16 of 33
(b) The isotopes of silver, when vaporised, can be separated in a mass spectrometer. Name the three processes that occur in a mass spectrometer before the vaporised isotopes can be detected. State how each process is achieved. (6) (c) State the type of bonding involved in silver. Draw a diagram to show how the particles are arranged in a silver lattice and show the charges on the particles. (3) Page 17 of 33
(d) Silver reacts with fluorine to form silver fluoride (AgF). Silver fluoride has a high melting point and has a structure similar to that of sodium chloride. State the type of bonding involved in silver fluoride. Draw a diagram to show how the particles are arranged in a silver fluoride lattice and show the charges on the particles. Explain why the melting point of silver fluoride is high. (5) (Total 20 marks) Q15. A mass spectrometer can be used to investigate the isotopes in an element. (a) Define the term relative atomic mass of an element. Page 18 of 33
(b) Element X has a relative atomic mass of 47.9 Identify the block in the Periodic Table to which element X belongs and give the electron configuration of an atom of element X. Calculate the number of neutrons in the isotope of X which has a mass number 49 (3) (c) The mass spectrum of element Z is shown below. Use this spectrum to calculate the relative atomic mass of Z, giving your answer to one decimal place. Identify element Z. (4) Page 19 of 33
(d) State how vaporised atoms of Z are converted into Z + ions in a mass spectrometer. State and explain which of the Z + ions formed from the isotopes of Z in part (c) will be deflected the most in a mass spectrometer. (4) (e) Explain briefly how the relative abundance of an ion is measured in a mass spectrometer. (Total 15 marks) Q16. (a) Define the term relative atomic mass. An organic fertiliser was analysed using a mass spectrometer. The spectrum showed that the nitrogen in the fertiliser was made up of 95.12% 14 N and 4.88% 15 N Calculate the relative atomic mass of the nitrogen found in this organic fertiliser. Give your answer to two decimal places. (4) Page 20 of 33
(b) In a mass spectrometer, under the same conditions, 14 N + and 15 N + ions follow different paths. State the property of these ions that causes them to follow different paths. State one change in the operation of the mass spectrometer that will change the path of an ion. (c) Organic fertilisers contain a higher proportion of 15 N atoms than are found in synthetic fertilisers. State and explain whether or not you would expect the chemical reactions of the nitrogen compounds in the synthetic fertiliser to be different from those in the organic fertiliser. Assume that the nitrogen compounds in each fertiliser are the same. (Total 8 marks) Q17. Define the term mass number of an atom. The mass number of an isotope of nitrogen is 15. Deduce the number of each of the fundamental particles in an atom of 15 N.................. (Total 3 marks) Page 21 of 33
Q18. Indium is in Group 3 in the Periodic Table and exists as a mixture of the isotopes 113 In and 115 In. (a) Use your understanding of the Periodic Table to complete the electron configuration of indium. 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6... (b) A sample of indium must be ionised before it can be analysed in a mass spectrometer. (i) State what is used to ionise a sample of indium in a mass spectrometer. Write an equation, including state symbols, for the ionisation of indium that requires the minimum energy. (iii) State why more than the minimum energy is not used to ionise the sample of indium. (iv) Give two reasons why the sample of indium must be ionised. Reason 1... Reason 2... (c) A mass spectrum of a sample of indium showed two peaks at m/z = 113 and m/z = 115. The relative atomic mass of this sample of indium is 114.5 (i) Give the meaning of the term relative atomic mass. Page 22 of 33
Use these data to calculate the ratio of the relative abundances of the two isotopes. (d) State and explain the difference, if any, between the chemical properties of the isotopes 113 In and 115 In Difference in chemical properties... Explanation... (e) Indium forms a compound X with hydrogen and oxygen. Compound X contains 69.2% indium and 1.8% hydrogen by mass. Calculate the empirical formula of compound X. (3) (Total 15 marks) Q19. Mass spectrometry can be used to identify isotopes of elements. (a) (i) In terms of fundamental particles, state the difference between isotopes of an element. State why isotopes of an element have the same chemical properties. Page 23 of 33
(b) Give the meaning of the term relative atomic mass. (Extra space)... (c) The mass spectrum of element X has four peaks. The table below gives the relative abundance of each isotope in a sample of element X. m/z 64 66 67 68 Relative abundance 12 8 1 6 (i) Calculate the relative atomic mass of element X. Give your answer to one decimal place. (3) Use the Periodic Table to identify the species responsible for the peak at m/z = 64 (d) Suggest one reason why particles with the same mass and velocity can be deflected by different amounts in the same magnetic field. Page 24 of 33
(e) Explain how the detector in a mass spectrometer enables the abundance of an isotope to be measured. (Extra space)... (Total 12 marks) Page 25 of 33
Q20. The mass spectrum of a sample of krypton taken from a meteorite is shown below. (a) Use this spectrum to calculate the relative atomic mass of this sample of krypton. Give your answer to one decimal place. Explain why the value you have calculated is slightly different from the relative atomic mass given in the Periodic Table................. (Extra space)......... (4) Page 26 of 33
(b) State how krypton is ionised in the mass spectrometer. Write an equation, including state symbols, to show the reaction that occurs when the first ionisation energy of Kr is measured. Sometimes the mass spectrum of Kr has a very small peak with an m/z value of 42. Explain the occurrence of this peak................. (Extra space)........... (5) (Total 9 marks) Q21. The element rubidium exists as the isotopes 85 Rb and 87 Rb (a) State the number of protons and the number of neutrons in an atom of the isotope 85 Rb Number of protons... Number of neutrons... (b) (i) Explain how the gaseous atoms of rubidium are ionised in a mass spectrometer........ Page 27 of 33
Write an equation, including state symbols, to show the process that occurs when the first ionisation energy of rubidium is measured... (c) The table shows the first ionisation energies of rubidium and some other elements in the same group. Element sodium potassium rubidium First ionisation energy / kj mol 1 494 418 402 State one reason why the first ionisation energy of rubidium is lower than the first ionisation energy of sodium....... (d) (i) State the block of elements in the Periodic Table that contains rubidium... Deduce the full electron configuration of a rubidium atom... (e) A sample of rubidium contains the isotopes 85 Rb and 87 Rb only. The isotope 85 Rb has an abundance 2.5 times greater than that of 87 Rb Calculate the relative atomic mass of rubidium in this sample. Give your answer to one decimal place......... (3) Page 28 of 33
(f) By reference to the relevant part of the mass spectrometer, explain how the abundance of an isotope in a sample of rubidium is determined. Name of relevant part... Explanation....... (g) Predict whether an atom of 88 Sr will have an atomic radius that is larger than, smaller than or the same as the atomic radius of 87 Rb. Explain your answer. Atomic radius of 88 Sr compared to 87 Rb... Explanation......... (3) (Total 16 marks) Q22. (a) State the meaning of the term mass number of an isotope....... (b) Give the symbol of the element that has an isotope with a mass number of 68 and has 38 neutrons in its nucleus... Page 29 of 33
(c) The following shows a simplified diagram of a mass spectrometer. (i) State what happens to the sample in the parts labelled P and Q. P... Q... In a mass spectrometer, the isotopes of an element are separated. Two measurements for each isotope are recorded on the mass spectrum. State the two measurements that are recorded for each isotope. Measurement 1... Measurement 2... (d) A sample of element R contains isotopes with mass numbers of 206, 207 and 208 in a 1:1:2 ratio of abundance. (i) Calculate the relative atomic mass of R. Give your answer to one decimal place........... (3) Identify R... Page 30 of 33
(iii) All the isotopes of R react in the same way with concentrated nitric acid. State why isotopes of an element have the same chemical properties..... (Extra space)..... (Total 11 marks) Q23. The mass spectrum of the isotopes of element X is shown in the diagram. m / z (a) Define the term relative atomic mass......... Page 31 of 33
(b) Use data from the diagram to calculate the relative atomic mass of X. Give your answer to one decimal place............. (3) (c) Identify the ion responsible for the peak at 72.. (d) Identify which one of the isotopes of X is deflected the most in the magnetic field of a mass spectrometer. Give a reason for your answer. Isotope... Reason... (e) In a mass spectrometer, the relative abundance of each isotope is proportional to the current generated by that isotope at the detector. Explain how this current is generated......... (f) X and Zn are different elements..... Explain why the chemical properties of 70 X and 70 Zn are different. (Total 11 marks) Page 32 of 33
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