C Isotopes Mass # Atomic # 12 6 Atoms of the same element with different mass numbers. Nuclear symbol: 12 6 C Mass # Each isotope has a different number of neutrons. Atomic # Hyphen notation: carbon-12 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Isotopes + + Carbon-12 Neutrons 6 Protons 6 Electrons 6 Carbon-14 Nucleus Neutron Proton + Electrons Nucleus Nucleus Neutron Proton + Carbon-12 Neutrons 6 Protons 6 Electrons 6 Electrons The chemistry of each element is determined by its number of protons and electrons. In a neutral atom, the number of electrons equals the number of protons. Symbols for elements are derived directly from the element’s name. Nuclei of atoms contain neutrons as well as protons. The number of neutrons is not fixed for most elements, unlike protons. Atoms that have the same number of protons, and hence the same atomic number, but different numbers of neutrons are called isotopes. Carbon-14 Neutrons 8 Protons 6 Electrons 6 Nucleus

6Li 7Li 3 p+ 3 n0 3 p+ 4 n0 2e– 1e– 2e– 1e– + + Lithium-6 Lithium-7 Nucleus Neutron Proton Nucleus Neutron Proton Electrons + Electrons + Nucleus Nucleus Lithium-6 Lithium-7 Neutrons 3 Protons 3 Electrons 3 Neutrons 4 Protons 3 Electrons 3

Cl Isotopes 37 17 Cl Chlorine-37 atomic #: mass #: # of protons: # of electrons: # of neutrons: 17 37 20 37 17 Cl Atoms that have the same number of protons, and hence the same atomic number, but different numbers of neutrons are called isotopes. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Relative Atomic Mass 12C atom = 1.992 × 10-23 g atomic mass unit (amu) 1 amu = 1/12 the mass of a 12C atom Neutron + 1 p = 1.007276 amu 1 n = 1.008665 amu 1 e- = 0.0005486 amu Electrons Atomic mass 1. The mass of any given atom is not simply the sum of the masses of its electrons, protons, and neutrons. 2. Atoms are too small to measure individually and do not have a charge. 3. The arbitrary standard that has been established for describing atomic mass is the atomic mass unit (amu), defined as one-twelfth of the mass of one atom of 12C. 4. Most elements exist as mixtures of several stable isotopes. The weighted average is of the masses of the isotopes is called the atomic mass. 5. Electrons added or removed from an atom produce a charged particle called an ion, whose charge is indicated by a superscript after the symbol for the element. Nucleus Proton Nucleus Carbon-12 Neutrons 6 Protons 6 Electrons 6

Average Atomic Mass Avg. (mass)(%) + (mass)(%) Atomic Mass 100 weighted average of all isotopes on the Periodic Table round to 2 decimal places Avg. Atomic Mass (mass)(%) + (mass)(%) = 100 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O. Avg. Atomic Mass (16)(99.76) + (17)(0.04) + (18)(0.20) 16.00 amu = = 100 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Average Atomic Mass (35)(8) + (37)(2) 10 Avg. Atomic = = Mass EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Avg. Atomic Mass (35)(8) + (37)(2) = = 35.40 amu 10 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Cl 35.4594 17 100 Mass spectrum of chlorine. Elemental chlorine (Cl2) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%) 90 80 Cl-35 70 AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447) 60 AAM = (26.412841 amu) + (9.046559 amu) AAM = 35.4594 amu 50 Abundance 40 30 Cl-37 Mass spectrum of chlorine. Elemental chlorine (Cl2) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%) 20 10 34 35 36 37 Mass

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

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 accomplishment 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 Copyright © 1997-2005 by Fred Senese

Mass Spectrometry - + Mass spectrum of mercury vapor 196 199 201 204 198 200 202 Mass spectrum of mercury vapor Photographic plate A gaseous sample is ionized by bombarding it with electrons in the lower part of the apparatus (not shown), producing positive ions. The ions pass through an electric field in which they are brought to a particular velocity. The ions then pass through a narrow slit into a curved chamber. A magnetic field is applied perpendicular to the beam of ions. All the ions with the same mass-to-charge ratio are deflected into the same circular path. (In most cases, the ionic charge is 1+ and the mass-to-charge ratio is the same as the mass.) Modern spectrophotometers use electronic detection devices (TOF = time of flight detectors) rather than photographic plates or film to establish mass-to-charge ratios and relative number of ions. - + Stream of positive ions Hill, Petrucci, General Chemistry An Integrated Approach 1999, page 320

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

Hg 80 The percent natural abundances for mercury isotopes are: 200.59 80 The percent natural abundances for mercury isotopes are: Hg-196 0.146% Hg-198 10.02% Hg-199 16.84% Hg-200 23.13% Hg-201 13.22% Hg-202 29.80% Hg-204 6.85% A B C D E F G (% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM (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

Separation of Isotopes U 238 92 Separation of Isotopes Natural uranium, atomic weight = 238.029 g/mol Density is 19 g/cm3. Melting point 1000oC. Two main isotopes: U 238 92 99.3% 0.7% (238 amu) x (0.993) + (235 amu) x (0.007) 236.334 amu + 1.645 amu U 235 92 237.979 amu Because isotopes are chemically identical (same electronic structure), they cannot be separated by chemistry. So Physics separates them by diffusion or centrifuge (mass spectrograph is too slow)…

Cl Assume you have only two atoms of chlorine. 35.453 17 Assume you have only two atoms of chlorine. One atom has a mass of 35 amu (Cl-35) The other atom has a mass of 36 amu (Cl-36) What is the average mass of these two isotopes? 35.5 amu Looking at the average atomic mass printed on the periodic table...approximately what percentage is Cl-35 and Cl-36? 55% Cl-35 and 45% Cl-36 is a good approximation

Cl Using our estimated % abundance data 55% Cl-35 and 45% Cl-36 35.453 17 Using our estimated % abundance data 55% Cl-35 and 45% Cl-36 calculate an average atomic mass for chlorine. Average Atomic Mass = (% abundance of isotope "A")(mass "A") + (% "B")(mass "B") AAM = (% abundance of isotope Cl-35)(mass Cl-35) + (% abundance of Cl-36)(mass Cl-36) AAM = (0.55)(35 amu) + (0.45)(36 amu) AAM = (19.25 amu) + (16.2 amu) AAM = 35.45 amu

Isotopes Dalton was wrong. Atoms of the same element can have different numbers of neutrons different mass numbers called isotopes The word isotope comes from the Greek words isos, meaning "equal," and topos, meaning "place." C-12 vs. C-14 California WEB

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

Using a periodic table and what you know about atomic number, mass, isotopes, and electrons, fill in the chart: Element Symbol Atomic Number Mass # of protons # of neutron # of electron charge 8 Potassium 39 +1 Br 45 -1 30 35 Atomic Number = Number of Protons Number of Protons + Number of Neutrons = Atomic Mass Atom (no charge) : Protons = Electrons Ion (cation) : Protons > Electrons Ion (anion) : Electrons > Protons

Using a periodic table and what you know about atomic number, mass, isotopes, and electrons, fill in the chart: A N S W E R K E Y Element Symbol Atomic Number Mass # of protons # of neutron # of electron charge 8 Potassium 39 +1 Br 45 -1 30 35 Oxygen O 8 16 K 19 19 20 18 Bromine 35 80 35 36 Zinc Zn 30 65 Atomic Number = Number of Protons Number of Protons + Number of Neutrons = Atomic Mass Atom (no charge) : Protons = Electrons Ion (cation) : Protons > Electrons Ion (anion) : Electrons > Protons

Atomic Mass How heavy is an atom of oxygen? There are different kinds of oxygen atoms. More concerned with average atomic mass. Based on abundance of each element in nature. Don’t use grams because the numbers would be too small California Web http://www.unitednuclear.com/scale6000.jpg

Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass we need the average from percent abundance. (1 amu) (1 amu) carbon atom (12 amu) The Mole - abbreviated mol Defined as the amount that contains the number of carbon atoms in exactly 12 g of isotopically pure carbon-12. The mass of carbon-12 contains 6.022 x 1023 atoms (Avogadro’s number)

Mass spectrums reflect the abundance of naturally occurring isotopes. Natural Abundance of Common Elements Hydrogen 1H = 99.985% 2H = 0.015% Carbon 12C = 98.90% 13C = 1.10% Nitrogen 14N = 99.63% 15N = 0.37% Oxygen 16O = 99.762% 17O = 0.038% 18O = 0.200% Sulfur 32S = 95.02% 33S = 0.75% 34S = 4.21% 36S = 0.02% Chlorine 35Cl = 75.77% 37Cl = 24.23% Bromine 79Br = 50.69% 81Br = 49.31%

For example….Methane For carbon 1 in approximately 90 atoms are carbon-13 The rest are carbon-12 the isotope that is 98.9% abundant. So, for approximately 90 methane molecules…1 carbon is carbon-13

C-13 Where’s Waldo?

Calculating averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = (4 x 50) + (1 x 60) = 260 g Average mass = (4 x 50) + (1 x 60) = 260 g 5 5 Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 California WEB

Calculating averages Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 80% of the rocks were 50 grams 20% of the rocks were 60 grams Average = % as decimal x mass + % as decimal x mass + % as decimal x mass + California WEB

Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = ATOMIC WEIGHT Boron is 20% B-10 and 80% B-11. That is, B-11 is 80 percent abundant on earth. For boron atomic weight = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

Periodic Table Dmitri Mendeleev developed the modern periodic table. Argued that element properties are periodic functions of their atomic weights. We now know that element properties are periodic functions of their ATOMIC NUMBERS.

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. Isotope Percent Abundance Mass Mg-24 78.99 23.9850 Mg-25 10.00 24.9585 Mg-26 25.9826 18.94575 2.49585 Atomic mass is not a whole number because it is an average. This is why their are the decimal numbers on the periodic table. http://www.trustedreviews.com/images/article/inline/4066-Flashbulb.jpg 11.01 2.86068 24.304 amu California WEB

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. Isotope Percent Abundance Mass Cu-63 69.1 62.93 Cu-65 64.93 43.48463 30.9 20.06337 63.548 http://upload.wikimedia.org/wikipedia/commons/e/e2/United_States_penny,_obverse,_2002.jpg Cu 29 63.548

Given the average atomic mass of an element is 118.21 amu and it has Protons Neutrons Electrons Mass number Cu-65 A B 29 C Argon D E F 40 Ba2+ 56 G H I 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?

Given the average atomic mass of an element is 118.21 amu and it has Protons Neutrons Electrons Mass number Cu-65 A = 29 B = 36 29 C = 65 Argon D = 18 E = 22 F = 18 40 Ba2+ 56 G = 81 H = 54 I = 137 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. 119.7932 amu Extra Credit: What is a cation? A positively charged atom. An atom that has lost a(n) electron(s).

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.