Discovering Isotopes of PenniumHASPI Medical Chemistry Lab 01

Background – Versions of Atoms make Visions of You You've been having headaches every day this week. When you go into the doctor they can't find anything in particular that would be causing it, but they want to scan your brain just to be sure there's nothing wrong.

The doctor sends you to get a PET scan (Positron Emission Tomography). When you get there they inject you with something... and they say it is RADIOACTIVE! Is it safe? Will you turn into Spiderman?

Although there are some minor risks when you are injected with a radiotracer, the risks are much lower than the other option... exploratory surgery. The radiotracer is an unstable isotope of an atom. The key to its safety as well as the ability to be imaged is the instability. By using a radioisotope with a short half-life, most of the radioactive atoms will decay within the next 24 hours, leaving you with a very short

period of time when you were exposed to radiation.

When the atom decays a small particle is emitted, and the gamma camera or PET scanner detects the emission. When all of these emissions are recorded you end up with a map of the body, and you can gain information about the anatomy as well as the physiology (functionality) of the organs. The element left after decay is not radioactive.

These kinds of scans are most often used in the fields of oncology, neurology and cardiology in order to help with diagnosis and choose the right treatment for each patient.

An isotope is just a version of an atom. We identify an element based on the number of protons it has, but sometimes the number of neutrons will vary. Due to the fact that each neutron weighs 1amu, this alters the mass of the atom. This doesn't change the way the atom interacts with other atoms, but it can change the stability of an atom.

For instance, there are a few versions, or isotopes, of carbon. Although only 3 occur naturally, there are a total of 15 that have been created. Carbon-12 and Carbon-13 are the only two isotopes which are stable, meaning that they won't break down over time. Carbon-14 has a VERY long half-life of 5, 730 years, so we actually can measure the amount of C-14 in anything that ever lived to find out how much C-14 is left and use that information to figure out how old it is... this is called Radiometric Dating. By using a synthetic (man-made) version of Carbon called Carbon-11 as a radiotracer, the half-life is only 20 minutes. This means that

after just 1 hour only 1/8 of the original injection remains.

The number following the name represents the mass of that isotope. If you subtract the atomic number you will find the number of neutrons in one atom of the isotope. The radiotracers usually used in a PET

scan include the following. Identify the number of neutrons, protons & electrons in each one as well as the mass.

Isotope Half-Life Isotopic Notation

Mass # of Protons # of neutrons

# of electrons

Carbon-11 20 min

Nitrogen-13 10 min

Oxygen - 15 2 min

Fluorine-18 110 min

A Decaying SupplyIn order to synthesize these elements in the lab scientists must use enriched Uranium in a nuclear reactor. As this can also be used for weapons, this type of lab is strictly regulated so that about 95% of the radioisotopes used world-wide come from just five aging reactors, none of which are in the US. This method also produces nuclear waste. A few years ago two of these labs went under renovations, leaving many hospitals with no radioisotopes for months, requiring doctors to use old techniques which are less effective.

A new technique of using a particle accelerator or a cyclotron to synthesize isotopes would make no nuclear waste and not require the dangerous use of enriched uranium. These technologies are new and will take time and cost money to implement in the beginning, but should result in a safer and more accessible way to synthesize medical radioisotopes.

Review Questions1. What is detected by the PET machine?

2. What fields of medicine most commonly use this technique?

3. What does the term synthetic mean?

4. Why are there so few labs making radioisotopes?

Discovering Isotopes of Pennium

Background:Before 1982, pennies were made of pure copper. Over the years, as inflation rose the cost of making a penny began to cost more than 1¢. This meant that people could take pennies out of their pocket, melt them down and sell the copper for more than the penny would have been worth. It also meant that the government would not be able to afford to make the amount of pennies needed, so they changed the penny so that the middle was made from Zinc, an inexpensive metal, and only the outside was plated with copper.

This means that a penny made before 1982 weighs a different amount than a penny made after 1982.

But what does this have to do with isotopes?:If you think about the fact that an isotope is an atom that acts in the same way but has a different mass because of what is in the nucleus, you can see that we can use pennies, which can be used the same way but have a different mass due to their center, in order to model the relationship between isotopes.

Average Atomic Mass:When you look at the periodic table you might notice that the masses are not whole numbers. If each proton and neutron weighs 1 amu (atomic mass unit), what might cause an element to have a listed mass that is not a whole number? The different masses of each isotope contribute to the average mass of a sample. If you consider carbon, which has an average atomic mass of 12.01, this is because almost all of the carbon atoms weigh 12, however there are some naturally occurring carbon atoms weighing 13 or 14, which increases the average mass to 12.01.

Introduction: Unless you’re a coin collector, you probably think all United States pennies are pretty much the same. To the casual observer, all the pennies in circulation do seem to be identical in size, thickness, and composition. But just as elements have one or more isotopes with different masses, the pennies in circulation have different masses. In this investigation, you are going to mass pennies of different dates to determine if there are any “isotopes” of an imaginary element called pennium, or Pe. Remember that chemical isotopes are atoms that have the same number of protons, but different numbers of neutrons. Thus, chemical isotopes have nearly identical chemical properties, but some different physical properties. In this investigation, you will determine the relative abundance of the isotopes of pennium and the masses of each isotope. You will then use this information to determine the atomic mass of pennium. Recall that the atomic mass of an element is the weighted average of the masses of the isotopes of the element. This average is based on both the mass and the relative abundance of each isotope as it occurs in nature.

Problem:

What are the masses and the relative abundances of pennium and what is the atomic mass of the element?

Materials:Laboratory balance 20 pennies

Procedure:1. Count out 20 pennies from the penny container.2. Find the mass of all 20 pennies. Enter this as the total mass of 20 pennium atoms in question 1 on

page 3.3. Find the mass of each penny separately. In the Data Table, record the year the penny was minted

and its mass to the nearest 0.1 g.4. Place the 20 pennies back in the container. Clean up your work area and wash your hands.5. Plot the mass of each penny as a function of its date on the graph paper provided. See Mrs. Gaines

for graph paper. 6. Answer the questions on page 3

By the end of this activity students will be able to● Explain the difference between two isotopes of the same atom● Calculate the average atomic mass of a model sample containing multiple isotopes of an atom

Calculate the percent abundance of isotopes in the modeled sample without opening the opaque sample bag of pennies

Penny No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Penny Date

Penny Mass (0.1g)

Sample Graph:

Questions: 1. What is the total mass of your 20 pennium atoms = _________ g2. Find the average atomic mass of the pennium (Pe) element using following equation:

Total mass of 20 pennieum atomsAverage atomic mass = ----------------------------------------------------

20

3. Count and record the number of each kind of penny: old and new. a. (isotope A) # of old pennies = _____ b. (isotope B) # of new pennies = _____

4. Calculate the percentage of occurrence of each isotope using the following equations:

# of old pennies% A = --------------------------------------- = x 100

Total # of pennies

# of new pennies% B = --------------------------------------- = x 100

Total # of pennies

5. Find the mass of one old penny and the mass of one new penny. a. Mass of isotope A = _____ gb. Mass of isotope B = _____ g

6. Calculate the average atomic mass of Pe using the following formula:

Average atomic mass = (mass of isotope A x %A) + (mass of isotope B x %B)

7. Most likely, the average atomic mass calculated in problem 2 doesn’t agree with the one calculated in problem 6. What is the reason for this difference?

8. Would you have this same problem with real atoms? Why or why not?9. What do the 20 pennies in this investigation represent? 10. What do the different masses of the pennies represent? 11. In what year(s) did the mass of Pe change? 12. How can you explain the fact that there are different “isotopes” of pennium? 13. Explain why these pennies are a good model for isotopes:14. Describe two shortcomings of the Pennium model for isotopes.15. Did your average mass of pennies match the mass of any one penny? Explain why or why not:16. Explain the relationship between the average mass you found for your pennies in the sample and

the average atomic mass of an element on the periodic table.

Practice: 1. New element Q has three isotopes Q-12 is 62.10% abundant, Q-13 is 2.43% abundant, and Q-14 is

35.47% abundant. Determine the average mass of element; Show all work!2. A new element was discovered on planet X. The element exists as two different isotopes, E-21 and

E-23. Which isotope is more abundant if the average mass is 21.7g. Explain.