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Topic 2 Atomic Structure 293 Version 1 2012 Definitions Learn these off by Heart Anion: A negatively charged ion. Atom: The smallest unit of matter Atomic emission spectra: The characteristic line spectrum that occurs as a result of energy being released by individual elements. Coloured lines on a black background. Atomic absorption spectra: The characteristic line spectrum that occurs as a result of energy being absorbed by individual elements. Black lines on a continuum (coloured) background. Atomic number: Number of protons in the nucleus. Atomic Mass (Mass number): Number of nucleons (protons and neutrons) in the nucleus Cation: A positively charged ion Electron: A negatively charged particle. Orbits the nucleus of an atom. Ion: A charged atom with unequal numbers of protons and electrons. Isotopes: Atoms that contain the same number of protons but a different number of neutrons. Mass Spectrometer: A device that can identify that can separate and detect particles with different masses. Neutron: A sub atomic particle within the nucleus of an atom. Has no charge (neutral) Nucleus: The centre of an atom. Contains protons and neutrons. Proton: A sub-atomic particle found in the nucleus of an atom. Positively charged. Relative abundances: The percentage of natural occurrence of an isotope of an element. Relative atomic mass: The weighted mean of all the naturally occurring isotopes of the element relative to 12C. No units. Valence electrons: The electrons in the highest main energy level. 294 Version 1 2012 Topic 2.1 The Atom Worksheet 1. Complete the following table Name of element Symbol Atomic Number (Z) Al Mass Number Number of Protons Number of Neutrons Number of Electrons 27 12 12 Potassium 20 16 15 16 56 26 59 27 59 27 2. here are four isoto pes of sulf ur with mass num bers 32, 33, 34, and 36. a. Write the symbol for each of these atoms. b. How are these isotopes alike? c. How are they different? d. Why is the atomic weight of sulfur not a whole number? ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ 295 Version 1 2012 T 3. Research radioisotopes and briefly state the uses for carbon, cobalt and iodine (in carbon dating and medicine) ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ 4. Briefly outline the some dangers of using radioisotopes __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ ______________________________________________ 296 Version 1 2012 Extension: Create a timeline for the history of the theories of the atom and its structure. Include the dates, people and what they theorized or discovered. 297 Version 1 2012 Topic 2.1 The Atom Extra Worksheet Complete the following table Name of element Symbol Atomic Number (Z) Mass Number Number of Protons Number of Neutrons Number of Electrons H Mg 8 11 27 14 Be2+ 26 24 Si Sr 17 18 1. Until 1897, chemists accepted the theory that the atom was the smallest particle of matter obtainable. Atoms were believed to be individual particles. Now we know that the atom is composed of at least three kinds of particles: _______________________, electrically negative; _______________________, electrically positive; and _______________________, electrically neutral. 298 Version 1 2012 2. According to the modern picture of an atom, it consists of a very dense central portion, called the _______________________, surrounded by _______________________ that make up the volume of the atom but add little to its _______________________ . 3. Every atom of the same element has the same number of _______________________ in its nucleus, although the number of _______________________ may vary. 4. All atoms of the same element have the same (atomic number, mass number) _______________________.. 5. The mass number of the atom of an atom is equal to the number of _______________________ plus the number of _______________________ in the nucleus of the atom. 6. If the mass number of the atom of a given element is known, the number of neutrons in its nucleus can be calculated by subtracting the _______________________ from the _______________________. For example, if an atom of the element sodium, atomic number 11, has a mass of 23, the atom has _______________________ neutrons. 7. Isotopes of a given element have a (different, the same) _______________________ atomic number. 8. Three isotopes of magnesium have mass numbers of 24, 25, and 26 have been discovered. The atomic number of magnesium is 12. In the spaces below, diagram only the nuclear structure of each isotope, as shown for magnesium-24. 9. In the space below make diagrams showing the nuclei of the three isotopes of uranium, atomic number 92, having mass numbers 234, 235, and 238. 299 Version 1 2012 Topic 2.2 The Mass Spectrometer 1. Draw a simple diagram of a mass spectrometer 2. Describe how in the mass spectrometer: a. The atoms are converted into ions. __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ b. The ions of different masses are separated. __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ 300 Version 1 2012 c. The ions are detected. ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ Study Hint The atomic mass for each element is reported on the periodic table. This number is a weighted average of the masses of each of the isotopes of an element. For example, the atomic mass of carbon is reported as 12.011 amu. Carbon is composed primarily of two isotopes: carbon-12 and carbon-13. The atomic mass is calculated from the relative abundance and the masses for these two isotopes. Using the equation below we can calculate the atomic mass for carbon. Carbon-12 makes up 98.93% of all of the carbon atoms, while carbon-13 is about 1.07% abundant. Since the carbon-12 isotope is more abundant, its mass is weighted more in the calculation of carbon’s atomic mass. An example calculation is done below. Example What is the atomic mass (the weighted average mass) for carbon? Isotope Carbon-12 Carbon-13 % Abundance 98.93% 1.07% Mass 12.000 amu 13.003 amu - substitute values in equation: (convert % to decimals) atomic mass = (0.9893)×(12.000amu) + (0.0107)×(13.003amu) - calculate: atomic mass = 12.01 amu Steps for Calculating Average Atomic Mass (When given percentages of each isotope and each isotopes mass) 1. Convert the percentages into decimals. (This percentage is known as its relative abundance or percent abundance). 2. Multiply the percentage of each isotope by its respective mass. 3. Add the numbers from step two together. 301 Version 1 2012 Steps for Calculating Percent Abundance (When given molar masses of each isotope) 1. Because percent abundances will always add up to 100% (1), assign one isotope to have a percentage of “x” and the other isotope to have a percentageof “1-x.” 2. Look up the average atomic mass of the atom on the periodic table. 3. Set up your problem so it looks like the setup below: X•(Mass of Isotope A) + (1-X)•(Mass of Isotope B) = Average Atomic Mass 4. Solve for “X.” Multiply this number by 100 to turn it into a percentage. This is the percent abundance of isotope A. 5. The percent abundance of isotope B is 100% minus the percent abundance of isotope A. 3. The element nitrogen has two isotopes with masses of 14 (99.5%) and 15 (0.5%). Calculate the average atomic mass of nitrogen. (show your work) 4. The element chlorine has two isotopes with masses of 35 (75.5%) and 37 (24.5%). Calculate the average atomic mass of chlorine. (show your work) 5. Calculate the atomic mass of an element which has two isotopes with masses of 10 (20%) and 11 (80%). Show your work and name the element. 302 Version 1 2012 6. Calculate the atomic mass of an element which has two isotopes with masses of 12 (98.9%) and 13 (1.1%). Show your work and name the element. 7. Magnesium consists of three isotopes with masses of 24 (78.5%), 5 (10%), and 26 (11.5%).What is the atomic mass of magnesium? (show your work) 8. Naturally occurring rubidium is 72.17% 85Rb (at.mass = 84.912 amu). The remaining atoms are 87Rb (at. mass = 86.909 amu). Calculate the atomic mass of Rb. 85.47 amu 9. The mass spectrum of an element shows that 78.99% of the atoms have a mass of 23.985 amu, 10.00% have a mass of 24.986 amu and the remaining 11.01% have a mass of 25.982 amu. a) Calculate the atomic mass of this element. 24.30 amu 303 Version 1 2012 b) Give the symbol for each of the isotopes present. 10. Antimony occurs naturally as two isotopes, one with a mass of 120.904 amu and the other with a mass of 122.904 amu. a) Give the symbol that identifies each of these isotopes. b) Use the atomic mass of antimony from the periodic table to calculate 121 the natural abundance of each of these isotopes. Sb 57.5% 123 Sb 42.5% 11. Argon has three naturally occurring isotopes: argon-36, argon-38, and argon-40. Based on argon’s reported atomic mass, which isotope do you think is the most abundant in nature? Explain. 304 Version 1 2012 12. Copper is made of two isotopes. Copper-63 is 69.17% abundant and it has a mass of 62.9296 amu. Copper-65 is 30.83% abundant and it has a mass of 64.9278 amu. What is the weighted average mass of these two isotopes? 13. Calculate the atomic mass of silicon. The three silicon isotopes have atomic masses and relative abundances of 27.9769 amu (92.2297%), 28.9765 amu (4.6832%) and 29.9738 amu (3.0872%). 14. Calculate the atomic mass of silicon. The three silicon isotopes have atomic masses and relative abundances of 27.9769 amu (92.2297%), 28.9765 amu (4.6832%) and 29.9738 amu (3.0872%). 15. Gallium has two naturally occurring isotopes. The mass of gallium-69 is 68.9256 amu and it is 60.108% abundant. The mass of gallium-71 is 70.9247 amu and it is 39.892% abundant. Find the atomic mass of gallium. 305 Version 1 2012 16. Bromine has two naturally occurring isotopes. Bromine-79 has a mass of 78.918 amu and is 50.69% abundant. Using the atomic mass reported on the periodic table, determine the mass of bromine-81, the other isotope of bromine. 17. . Calculate the atomic mass of lead. The four lead isotopes have atomic masses and relative abundances of 203.973 amu (1.4%), 205.974 amu (24.1%), 206.976 amu (22.1%) and 207.977 amu (52.4%). 18. Antimony has two naturally occurring isotopes. The mass of antimony121 is 120.904 amu and the mass of antimony-123 is 122.904 amu. Using the average mass from the periodic table, find the abundance of each isotope. (Remember that the sum of the two abundances must be 100). 306 Version 1 2012 2,8,6 Sulphur 307 Version 1 2012 What do you notice about the number of outer shell electron in the last column? What do you notice about the number of outer shell electrons and the column? What do you notice about the number of shells and the row? Questions Use ‘x’ to represent electrons on the shells. example (include the name and the numerical representation). Fill in the electron shells for the first 20 elements as shown in the Electron Configuration Electromagnetic spectrum questions. Read the information. Then type your answers in the spaces provided and then print out the finished sheet to give to your teacher. Electromagnetic waves are transverse waves. Although we cannot see their vibrations theses are at right angles to the direction of wave travel. They also travel very fast. In empty space there is nothing to slow them down and they all travel at the same speed. Their speed is 300 million metres per second! Gamma rays X-rays ultraviolet Visible light Short wavelength, high frequency infrared microwaves radio waves long wavelength, low frequency 1. Which colour of light has the longest wavelength? 2. Which type of electromagnetic wave has the longest wavelength? 3. What can be used to split visible light into the colours of the spectrum? 4. How fast to radio waves travel in empty space? 5. If a spacecraft sent some radio waves and some microwaves through space to Earth, which would get there first? 308 Version 1 2012 6. Which electromagnetic waves have a shorter wavelength than that of light? 7. Which type of electromagnetic wave has the shortest wavelength? 8. Which colour of light is next to infrared waves in the electromagnetic spectrum? 9. Which type of wave is next to violet light in the electromagnetic spectrum? 309 Version 1 2012 Line Spectra 1) Below is a depiction of the Bohr model of the hydrogen atom. a) b) c) d) e) Label G as an electron in the ground state. Label L the largest absorption of energy possible by an electron for this atom. Label S the lowest energy gap. Label J, the jump that will produce the highest frequency energy. Why would an emission line spectrum for this atom show closer and closer together lines? 2) Below is a typical line spectrum. 310 Version 1 2012 a) Briefly, how is a line spectrum produced? b) Assuming that this spectrum results in the hydrogen atom finishing in the ground state, dra w a sketch showing how the second lowest frequency line was produced. c) How does the speed of the produced radiation change from high to low frequency? d) Give 2 methods to excite substances enough that they will emit light. 311 Version 1 2012 312 Version 1 2012 Spectral Lines, The Bohr Model and Quantum Theory Introduction You've just completed the four-part experiment on Spectroscopy where you've seen the visible light spectra of different light sources, seen the emission spectra of different elements and even calculated the wavelength of the major colors in the spectra. Did you wonder where these spectra come from? Asked yourself how are they created and why are they all different? These are excellent questions and ones that some rather famous people asked themselves too. The Investigation Start at: http://www.colorado.edu/physics/2000/quantumzone/index.html Then go to: Table of Contents ! Science Trek ! Quantum Atom (Spectral Lines) As you progress through the readings, answer these questions: 1. What can be done with sunlight? 2. What did scientists see when they looked at bright light coming off an element like hydrogen? 2. Pick three elements and draw their spectra. Describe how they are the same and different. 313 Version 1 2012 3. What puzzle did Niels Bohr come up with the solution to? 4. What was his solution? 5. Describe what happens when you click on different orbits in the model of the atom. 6. Why was Bohr's model so radical? 7. What is this kind of Physics called? 314 Version 1 2012 8. Where are electrons located? 10. Bigger ____________10a = ______________ _____________10b. 10. Describe the detour to the Schrödinger Atom. 11. Why does thinking about energy levels make sense? 12. If the energy goes ___________13a, the extra energy appears as a ___________ 13b. And for the electron to get ___________13c energy, it needs to _____________13d a photon. 13. Explain how the atomic spectrum of an element is created. 315 Version 1 2012 15. What are two uses of knowing about Einstein's Legacy? The Conclusion Summarize what you've learned about an atom and how the atomic spectrum of an element is produced. 316 Version 1 2012 317 Version 1 2012 318 Version 1 2012 Exam Questions 1. How many protons, neutrons and electrons are present in each atom of 31P? Protons Neutrons Electrons A. 16 15 16 B. 15 16 15 C. 15 31 15 D. 16 31 16 (Total 1 mark) 2. Which is correct for the following regions of the electromagnetic spectrum? Ultraviolet (UV) Infrared (IR) A. high energy short wavelength low energy low frequency B. high energy low frequency low energy long wavelength C. high frequency short wavelength high energy long wavelength D. high frequency long wavelength low frequency low energy (Total 1 mark) 3. What is the atomic number of a neutral atom which has 51 neutrons and 40 electrons? A. 40 B. 51 C. 91 D. 131 (Total 1 mark) 319 Version 1 2012 4. What is the relative atomic mass of an element with the following mass spectrum? A. 24 B. 25 C. 26 D. 27 (Total 1 mark) 5. Which gives the correct order of these processes in a mass spectrometer? A. ionization deflection acceleration B. ionization acceleration deflection C. acceleration ionization deflection D. deflection acceleration ionization (Total 1 mark) 320 Version 1 2012 6. Which species have the same number of electrons? I. S2– II. Cl– III. Ne A. I and II only B. I and III only C. II and III only D. I, II and III (Total 1 mark) 7. The table below shows the number of protons, neutrons and electrons present in five species. Species Number of protons Number of neutrons Number of electrons X 6 8 6 Y 7 7 7 Z 7 7 8 W 8 8 8 Q 8 10 8 Which two species are isotopes of the same element? A. X and W B. Y and Z C. Z and W D. W and Q (Total 1 mark) 321 Version 1 2012 8. Describe the emission spectrum of hydrogen. Outline how this spectrum is related to the energy levels in the hydrogen atom. ................................................................................................................................................ ................................................................................................................................................ ................................................................................................................................................ ................................................................................................................................................ ................................................................................................................................................ ................................................................................................................................................ (Total 3 marks) 9. (i) Describe and explain the operation of a mass spectrometer. (5) (ii) State three factors that affect the degree of deflection of ions in a mass spectrometer. (3) 322 Version 1 2012 (iii) Strontium exists as four naturally-occurring isotopes. Calculate the relative atomic mass of strontium to two decimal places from the following data. Isotope Percentage abundance Sr-84 0.56 Sr-86 9.90 Sr-87 7.00 Sr-88 82.54 (2) (Total 10 marks) 323 Version 1 2012 Exam Question Markscheme 1. B [1] 2. A [1] 3. A [1] 4. A [1] 5. B [1] 6. A [1 7. D [1] 8. series of lines/lines; electron transfer/transition between higher energy level to lower energy level / electron transitions into first energy level causes UV series / transition into second energy level causes visible series / transition into third energy level causes infrared series; convergence at higher frequency/energy/short wavelength; Allow any of the above points to be shown on a diagram. 3 [3] 324 Version 1 2012 9. (i) (ii) (iii) a vaporized sample must be used; bombarded with (high energy) electrons to form positive ions; accelerated by passing through an electric field; deflected by passing through a magnetic field; detected by producing a current; Award [2 max] if just the words vaporization, ionization, acceleration, deflection and detection are used with no explanation. (size of the positive) charge (on the ion); mass (of the ion); strength of the magnetic field; velocity/speed (of the ions) / strength of electric field; m/z scores the first two marking points. [(0.56 × 84) + (9.90 × 86) + (7.00 × 87) + (82.54 × 88)] ; 100 = 87.71; Award [1 max] if answer not given to two decimal places. Award [2] for correct final answer. Apply –1(U) if answer quoted in g or g mol–1. 5 3 max Ar = 2 [10] 325 Version 1 2012