ISOTOPES & ATOMIC MASS

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Morris Horton
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1 1 Please check the list posted in the front of the room for your clicker box number ISOTOPES & ATOMIC MASS Chem 105, Mon Clicker information Written exercise Mass spectrometer and isotopes Define mole More on isotopes, including OWL problem Periodic Table of the Elements

2 2 Which of the following contains THREE significant figures? x x If your clicker was registered before 7 PM Sunday, your box should change color when you click. Then click another number 1-3, and the box should change color again. Finally, toggle it back by going to your final answer.

3 3 Which of the following contains THREE significant figures? x (If there were a decimal point here, this would have 3 sig figs. As it stands, it has 2.)

4 4 Announcements 1. JK Office hours are: Mon 3:30-4:30; Wed 2:15-3: Ask when your TA s office hours are (2 hr/week). 3. Use extra time after the lab proper to ask questions about lecture material or OWL. 4. Lab this week is Safety training - an OSHA requirement. 5. Clicker recording begins today. Written exercise due in class WED Mass spectrometer and isotopes Define mole More on isotopes, including OWL problem Periodic Table of the Elements

5 Written exercise 5

6 6 Working with numbers of atoms or molecules In chemistry, we want to measure out numbersof atoms and molecules. But we have no atommeter or molecule-meter! So, we have to rely on mass measurements. If we know the average mass of the atom or molecule, then we measure out a certain number using a lab balance.

7 7 Measuring Atomic Mass Mass proton ~ mass neutron >> mass electron. However, atom masses cannot be calculated simply by adding up the masses of constituent protons, neutrons, and electrons. Because small amounts of mass are converted into the Nuclear Binding Energy(by E = mc 2 ). Therefore atomic masses must be measured experimentally using a mass spectrometer.

8 8 Mass spectrometer measures atomic and molecular masses Atoms or molecules (gas phase) in here. Electromagnet (variable field) Detector Vacuum tube Beam of Cations (A + or M + ) Accelerating voltage Electron gun shoots high energy electrons which knocks off electrons and makes cations. (A + e - A + + 2e - )

9 An interactive simulator available in OWL as Active Figure 2.3b 9

10 10 Ne Relative peak heights tell us % of each isotope.

11 11 Ne Exact spacing tells the relative masses of isotopes atomic mass unit. (This figure does not reflect how accurately this can be done in a real mass spectrometer.)

12 Which chlorine isotope makes up the indicated ion beam? Cl Cl 3. Could be either one Cl 37Cl Could be eithe...

Which chlorine isotope makes up the indicated ion beam? 13 1. 35 Cl 2. 37 Cl 3.

13 Which chlorine isotope makes up the indicated ion beam? Cl Cl 3. Could be either one (2) The heavier atom s path is BENT LESS by the magnetic field.

14 14 Measuring Atomic Mass We assume that the major isotope of carbon, 12 C, has a mass of exactly atomic mass units (amu). All other isotope masses are then measured relative to this one using a mass spectrometer (like Neon above). For example, the minor stable isotope of carbon, 13 C, has an exact mass of amu.

15 15 Now we define the MOLE= the number of 12 C atoms that have a mass of g. This is a very large number, which is called Avogadro s Number = x atoms. (Given on exams no need to memorize.) The mass of a mole of atoms of an isotope (in grams) = the mass of one atom (in amu). How is Avogadro s Number determined?the best wayis from the dimensions and density of a crystal: if you know the exact spacing, you can calculate how many atoms total are in the crystal.

16 16 If you spread Avogadro's number of unpopped popcorn kernels across the USA, the entire country would be covered in popcorn to a depth of over 9 miles.

17 17 Elements Occur Naturally as Mixtures of Isotopes Natural Carbon (on earth) contains 98.89% C-12 (mass amu) and 1. 11% C-13 (mass amu) mole of earth carbon has a mass of ( molx g/mol) + (0.0111mol x g/mol) = g This is the Atomic Weight that is recorded on the Periodic Table.

18 Comets originating far out in our galaxy have different isotopic contents than Earthdue to slight differences in chemistry occuring in far-away stars. (Otherwise, the chemistry across the galaxy is similar to Earth s.) 18 Carbon Isotope Abundances in Comets. Susan Wyckoff et al, Astrophysical J., 535:991-9(2000).

19 19 Land plants and algae incorporate 12 C and 13 C slightly differently. So the food consumed by whales in the open ocean vs along the North Slope, contains slight differences in 13 C/ 12 C ratio. Baleen growth over 9 years

20 20 Atomic mass is the average mass of a collection of atoms: Fraction M 1 x (M 1 ) + Fraction M 2 x (M 2 ) + = Atomic mass This equation can be worked forwards or backwards...

21 Check out OWL. 21

22 22 The element lithium has an atomic weight of 6.94 and consists of two stable isotopes lithium-7 and lithium-6. The isotope lithium-7 has an atomic mass of 7.02 amu and a percent natural abundance of 92.6%. The isotope lithium-6 has a percent natural abundance of 7.42%. What is the atomic mass of lithium-6? amu

23 23 The element lithium has an atomic weight of 6.94 and consists of two stable isotopes lithium-7 and lithium-6. The isotope lithium-7 has an atomic mass of 7.02 amu and a percent natural abundance of 92.6%. The isotope lithium-6 has a percent natural abundance of 7.42%. What is the atomic mass of lithium-6? amu A mole of lithium atoms from nature has a mass of 6.94 g

24 Fraction 7 Li = 0.926 moles 7 Li/mole natli Fraction 6 Li = 0.

24 24 Fraction 7 Li = moles 7 Li/mole natli Fraction 6 Li = moles 6 Li/mole natli Mass of 7 Li/ mol natli + mass 6 Li/ mol natli = 6.94 g/ mol natli mol Li 7.02 g mol Li x 6.94 g + = 7 6 mol nat Li mol Li mol nat Li mol Li mol nat Li

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26 26 American system of numbering groups Non-metals (incl H) 1A 2A 3B 4B 5B 6B 7B ----8B B 2B 3A 4A 5A 6A 7A 8A metalloids metals Inert gases See wall periodic tables

27 27 1A 2A 3B 4B 5B 6B 7B ----8B B 2B 3A 4A 5A 6A 7A 8A Please know names and formulas of the elements inside the red box. FAQ: In Chem 105, do I have to memorize the names and formulas of the elements? Yes, some of them as shown below. Exams will contain a periodic table, but no names. Why? Cause that s how the American Chemical Society standard final exam is done: you are expected to know that potassium is K, and not Po!

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29 29 Main Group elements 1A 2A 3B 4B 5B 6B 7B ----8B B 2B 3A 4A 5A 6A 7A 8A

30 30 1A 2A 3B 4B 5B 6B 7B ----8B B 2B 3A 4A 5A 6A 7A 8A Transition metals

31 31 1A 2A 3B 4B 5B 6B 7B ----8B B 2B 3A 4A 5A 6A 7A 8A Lanthanides & Actinides

ISOTOPES & ATOMIC MASS

1 If you missed it last Friday, the list in the front or back of the room shows your clicker box number. ISOTOPES & ATOMIC MASS A little clicker test Lab pre-test results Mass spectrometer and isotopes