ISOTOPES & ATOMIC MASS

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Elisabeth Cathleen Byrd
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1 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 Define mole More on isotopes, including an OWL problem Periodic Table of the Elements Start Web, OWL.

2 2 Which one of the following contains THREE significant figures? x x

3 3 Which of the following contains THREE significant figures? x (These zeros ARE significant, total = 5.) (This one definitely has 3 sig figs..the power of 10 does not count in sig figs, because that only really determines which metric units are used. It s not part of the measurement.) (If there is no decimal point, this zero is NOT significant.)

4 4 Announcements 1. Lab this week: Safety training - an OSHA requirement. 2. Ask your TA when his or her office hours are (2 hr/week). 3. Use extra time at the end of lab to ask questions about lecture material or OWL. 4. JK office hours are: Mon, Wed 2:15-3: Clicker recording begins today. Mass spectrometer and isotopes Define mole More on isotopes, including OWL problem Periodic Table of the Elements

5 5 Pre-test Results are posted on LAB Blackboard A low score by itself may not be bad IF you study hard this semester. However, if you got a low score AND several of the other danger signs for Chem 105 (described in the syllabus) apply, perhaps you should discuss your situation with your advisor.

6 6 Working with numbers of atoms or molecules In chemistry, we want to measure out numbers of 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 atoms or molecules, then we measure out a certain number of them 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. This is 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 mass 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 knock off electrons and makes cations. (A + e - A + + 2e - )

9 An interactive simulator available in OWL as 2.3 VIS - Mass Spectrometer GO THERE. 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 the high accuracy this can be obtained in a real mass spectrometer.)

12 Chlorine exists as TWO isotopes. Which one is the OUTER beam? Cl Cl 3. Could be either one 35Cl 37Cl Could be eithe...

13 Which chlorine isotope makes up the indicated ion beam? Cl Cl 3. Could be either one (2) The heavier ion 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 the mass spectrometer. 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 total mass of g. This is a very large number, which is called Avogadro s Number. This has been determined experimentally to be x atoms. (This number is 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). Trying to imagine the size of 602,210,000,000,000,000,000,000

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. 16

17 17 How is Avogadro s Number determined? One way uses the dimensions and density of a crystal: x ray diffraction is used to determine the exact atom spacing. Then you can calculate how many atoms total are in the crystal.

18 Density of Al = 2.70 g/cm 3 Atomic mass = 26.

1 Å 3 8 4 atoms 10 A 66.1A cm 3 3 1cm 26.98g Al 2.

18 18 Density of Al = 2.70 g/cm 3 Atomic mass = g/mol X ray data 4 Al atoms in a cube of 66.1 Å atoms 10 A 66.1A cm 3 3 1cm 26.98g Al 2.70g Al mol x10 atoms mol This data does not give exactly 6.022

19 19 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 ( mol x g/mol) + (0.0111mol x g/mol) = g The atomic weight recorded on the periodic table = weighted average of all natural occurring isotopes.

20 20 Small variations in 12 C/ 13 C ratios are meaningful, and are used in biochemistry, geology, astrobiology, and ecology research. For example, we ll look briefly at isotope ratios in comets and whale baleen.

21 In Astrophysics, isotopic ratios are used to identify objects from outside the Solar System. Comets originating from elsewhere in our galaxy have different 12 C/ 13 C ratios compared to those from within the Solar System. 21 Carbon Isotope Abundances in Comets. Susan Wyckoff et al, Astrophysical J., 535:991 9(2000).

22 22 Whale tissues reflect the 13 C/ 12 C ratio in the food they eat. Land plants and marine algae incorporate 12 CO 2 and 13 CO 2 slightly differently. So tissues formed while in the open ocean in the winter differ in 13 C/ 12 C ratio compared to those formed along northern Alaska in the summer. This shows that the whales summer food is rich in terrestrial nutrients washed off the North Slope. Baleen growth over 9 years

23 23 Go to OWL 2.4b MAS - Average Atomic Weight

24 24 Atomic weight is the average atomic weights of a collection of atoms: Fraction M 1 x (M 1 ) + Fraction M 2 x (M 2 ) + = Atomic weight This equation can be worked forwards or backwards...

25 25 That is, a mole of natural lithium atoms from nature has a mass of 6.94 g. 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

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

26 26 Fraction 7 Li = moles 7 Li/mole natli Fraction 6 Li = moles 6 Li/mole natli mol Li 7.02g mol Li x 6.94 g + = 7 6 mol nat Li mol Li mol nat Li mol Li mol nat Li

27 Periodic Table 27

28 28

29 29 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

30 1A 2A 3B 4B 5B 6B 7B ----8B----- 1B 2B 3A 4A 5A 6A 7A 8A Please know names and symbols of the elements

Yes, some of them as shown above. Exams will contain a periodic table, but no names. Why?

30 30 1A 2A 3B 4B 5B 6B 7B ----8B B 2B 3A 4A 5A 6A 7A 8A Please know names and symbols of the elements inside the red box. FAQ: In Chem 105, do I need to memorize the names and formulas of the elements? Yes, some of them as shown above. 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!

31 The End 35

ISOTOPES & ATOMIC MASS

1 Please check the list posted in the front of the room for your clicker box number ISOTOPES & ATOMIC MASS Chem 105, Mon 9-13-10 Clicker information Written exercise Mass spectrometer and isotopes Define