1. 1.4 Isotopes, Radioisotopes, and Atomic Mass B3.1 explain the relationship between the atomic number and the mass number of an element, and the difference between isotopes and radioisotopes of an element B3.2 explain the relationship between isotopic abundance of an element’s isotopes and the relative atomic mass of the element

2. Atomic Mass If a proton has a mass of 1, and a neutron has a mass of 1, how come the elements on the periodic table have masses that are decimals? What unit does the periodic table use to measure mass?

3. Mass of an Atom In the head of a pin there are approximately 8.0 x 10 19 atoms of iron. Because atoms are so small, standard units of measurement are not practical. Scientists use a smaller unit to describe the mass of atoms, the Atomic Mass Unit (u or amu).

4. Relative Atomic Mass Mass 12 C atom = 1.992 × 10 -23 g 1 p= 1.007276 amu 1 n = 1.008665 amu 1 e - = 0.0005486 amu 1 amu= 1 / 12 the mass of a 12 C atom 1.992 × 10 -23 g / 12 = 1.66 x 10 -24 g = 1 amu + + + + + + Nucleus Electrons Nucleus Neutron Proton Carbon-12 Neutrons 6 Protons 6 Electrons6

5. Isotopes The number of protons inside the nucleus at the centre of an atom decides what element it is. Dif­ferent atoms with the same number of protons and a dif­ferent number of neutrons are known as isotopes. For example, there are three nat­ur­ally occur­ring isotopes of carbon: carbon-12, carbon-13 and carbon-14. Most (98.9%) of the natural carbon is carbon-12 and the remaining 1.1% is made up of stable carbon-13 and radio­active carbon-14.

6. Isotopes + + + + + + Nucleus Electrons Nucleus Neutron Proton Carbon-12 Neutrons 6 Protons 6 Electrons6 Nucleus Electrons Carbon-14 Neutrons 8 Protons 6 Electrons6 + + + + + + Nucleus Neutron Proton

7. 3 p + 3 n 0 2e – 1e – 3 p + 4 n 0 2e – 1e – 6 Li 7 Li + + + Nucleus Electrons Nucleus Neutron Proton Lithium-6 Neutrons 3 Protons 3 Electrons3 Nucleus Electrons Nucleus Neutron Proton Lithium-7 Neutrons 4 Protons 3 Electrons3 + + +

8. Naming Isotopes Put the mass number after the name of the element carbon- 12 carbon -14 uranium-235 California WEB

9. Isotopic Abundance

10. Isotopic Abundance Why is the average mass not the same as the mass on the periodic table? The abundance of each isotope has to be taken into consideration. A regular average calculation treats each isotope the same

11. Isotopic Abundance A weighted average takes into account the abundance of each isotope

12. Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an AVERAGE value. Average Atomic Mass = weighted average of the masses of the isotopes of an element

13. Average Atomic Mass Weighted average of all isotopes The mass indicated on the Periodic Table Usually rounded to 2 decimal places Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Avg. Atomic Mass = (mass)(%) + (mass)(%) … 100

14. Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Avg. Atomic Mass = (16)(99.76) + (17)(0.04) + (18)(0.20) 100 = 16 amu

15. Average Atomic Mass EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Avg. Atomic Mass = (35)(8) + (37)(2) 10 = 35 amu

16. . mass spectrometry is used to experimentally determine isotopic masses and abundances interpreting mass spectra average atomic weights - computed from isotopic masses and abundances - significant figures of tabulated atomic weights gives some idea of natural variation in isotopic abundances Weighing atoms gas sample enters here filament current ionizes the gas ions accelerate towards charged slit magnetic field deflects lightest ions most ions separated by mass expose film The first mass spectrograph was built in 1919 by F. W. Aston, who received the 1922 Nobel Prize for this accomplishmentF. W. Aston1922 Nobel Prize Copyright © 1997-2005 by Fred SeneseFred Senese

17. Negative grid Magnet Detector To vacuum pump Mass numbers Evacuated glass tube Heated filament Electron beam Neon gas inlet (+) (-) Image Copyrighted by Houghton Mifflin Company Mass Spectrophotometer

18. 100 90 80 70 60 50 40 30 20 10 0 34 35 3637 Abundance Mass Mass spectrum of chlorine. Elemental chlorine (Cl 2 ) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%) AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447) AAM = (26.412841 amu) + (9.046559 amu) AAM = 35.4594 amu Cl-35 Cl-37 Cl 35.4594 17

19. Mass Spectrophotometer electron beam magnetic field gas stream of ions of different masses lightest ions heaviest ions Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138

20. Mass Spectrum for Mercury 196 197 198 199 200 201 202 203 204 Mass number Relative number of atoms 30 25 20 15 10 5 196 199 201 204 198 200 202 Mass spectrum of mercury vapor The percent natural abundances for mercury isotopes are: Hg-196 0.146% Hg-196 0.146% Hg-198 10.02% Hg-198 10.02% Hg-199 16.84% Hg-199 16.84% Hg-200 23.13% Hg-200 23.13% Hg-201 13.22% Hg-201 13.22% Hg-202 29.80% Hg-202 29.80% Hg-204 6.85% Hg-204 6.85% (The photographic record has been converted to a scale of relative number of atoms)

21. Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. Cu 29 63.548 IsotopePercentAbundanceMass Cu-6369.162.93 Cu-65 64.93 43.48463 20.0633730.9 63.548

22. ProtonsNeutronsElectronsMass number Cu-65AB29C ArgonDEF40 Ba 2+ 56GHI A. B. C. D. E. F. G. H. I. Given the average atomic mass of an element is 118.21 amu and it has three isotopes (“A”, “B”, and “C”): isotope “A” has a mass of 117.93 amu and is 87.14% abundant isotope “B” has a mass of 120.12 amu and is 12.36% abundant Find the mass of isotope “C”. Show work for credit. Extra Credit: What is a cation?

23. Given the average atomic mass of an element is 118.21 amu and it has three isotopes (“A”, “B”, and “C”): isotope “A” has a mass of 117.93 amu and is 87.14% abundant isotope “B” has a mass of 120.12 amu and is 12.36% abundant Find the mass of isotope “C”. Show work for credit.

24. The percent natural abundances for mercury isotopes are: Hg-196 0.146% Hg-196 0.146% Hg-198 10.02% Hg-198 10.02% Hg-199 16.84% Hg-199 16.84% Hg-200 23.13% Hg-200 23.13% Hg-201 13.22% Hg-201 13.22% Hg-202 29.80% Hg-202 29.80% Hg-204 6.85% Hg-204 6.85% (0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x 0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x x = 200.63956 amu Hg 200.59 80 (% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM ABCDEFGABCDEFG

25. Using a periodic table and what you know about atomic number, mass, isotopes, and electrons, fill in the chart: ElementSymbolAtomic Number Atomic Mass # of protons # of neutron # of electron charge 888 Potassium 39+1 Br45 303530 Atomic Number = Number of Protons Number of Protons + Number of Neutrons = Atomic Mass Atom (no charge) : Protons = Electrons Ion (cation) : Protons > ElectronsIon (anion) : Electrons > Protons

26. Atomic Mass Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 26 with a mass of 25.9826 amu. What is the atomic mass of magnesium? If not told otherwise, the mass of the isotope is the mass number in amu. California WEB IsotopePercentAbundanceMass Mg-2478.9923.9850 Mg-2510.0024.9585 Mg-26 25.9826 24.304 amu 18.94575 2.49585 2.86068 11.01