and Radioactivity

Look at the periodic table in the inside cover of your textbook. For each element, it provides the name, the chemical symbol, and two numbers. Look at the element iron. What does the number 26 indicate? We will deal with the other number ( = 55.847) in Part C. What is the name of the element with 6 fewer protons than iron has? With 3 more protons than iron? As a review, list the names and chemical symbols of all the elements starting with the letter c and with atomic number less than 57.

With your lab partner, visit WebElements. Browse for several minutes in order to check out its features. This is an award-winning web site. With your partner, list three of its strong points. As you soon will see, this periodic table has a short-coming; namely, that it does not list naturally occurring radioisotopes that occur in trace amounts. For example, tritium (hydrogen-3) is not listed as a naturally occurring isotope of hydrogen. Find another short-coming, perhaps relating to its ease of navigation or about the content itself.

Using the periodic table in your textbook, construct a chart that contains the atomic number and the "other number" for each of these 5 elements. Iron is done for you. Element Atomic number (Z) "Other number" iron 26 55.847 hydrogen     carbon     potassium     uranium     Here is a chart with the naturally occurring isotopes of iron listed by their mass numbers (A). None of these are radioactive. To what do the numbers 54, 56, 57 and 58 refer?   Isotope     Radioactive?     Naturally occurring?   Natural abundance 54Fe   No     Yes   5.845% 56Fe   No     Yes   91.754% 57Fe   No     Yes   2.119% 58Fe   No     Yes   0.282% Although you could use WebElements to get this type of information for other elements, other web periodic tables are faster and better. Use the Table of the Isotopes at the Lawrence Berkeley Lab to draw up similar charts for hydrogen, carbon, potassium and uranium. This "other number" on the periodic table goes by several names.  Usually it is called the atomic mass (or atomic weight). But a longer and more correct name is the average atomic mass (or average atomic weight). Examine the data in the table above for iron and explain where the number 55.847 comes from. If you only had looked at iron, you might have concluded that stable isotopes are naturally occurring (and radioactive ones aren't). This is not the case, as you saw with the other elements. List three elements that have naturally occurring radioisotopes. For each element you select, write down one of its naturally occurring radioisotope. D. Elements and Their Radioisotopes (naturally occurring or otherwise) Iron, hydrogen, carbon, potassium and uranium also have radioactive isotopes, most of which do not occur naturally on our planet. Here are some of the radioactive isotopes of iron together with their half-lives and their mode of radioactive decay.    Isotope     Radioactive?    Naturally  occurring?   Half-life Mode of decay 53Fe   Yes     No   8.51 m beta-plus or EC 55Fe   Yes     No   2.73 y beta-plus or EC 59Fe   Yes     No   44.51 d beta-minus 60Fe   Yes     No   1.5 x 106y beta-minus 61Fe   Yes     No   6.0 m beta-minus 62Fe   Yes     No   68 s beta-minus Note: EC stands for Electron Capture, a process that is similar to beta-plus emission. For now, we will not worry about either of these two types of decay other than to observe that they occur. Which isotope of iron is most abundant on our planet?  Is it radioactive or stable? Are any naturally-occurring isotopes of iron radioactive? Compare these two isotopes of iron:  56Fe and 60Fe. How do they compare in terms of the stability of their nuclei? Their chemical behavior? (a) Would you expect a nail made from 60Fe to rust (react with oxygen, 4Fe + 3O2 --> 2Fe2O3) the same way that nails in our world made from 56Fe do? (b) Hemoglobin, a protein in your body that carries oxygen (O2), contains iron. If you ingested some iron-60 over a period of time, would you expect to find some radioactive hemoglobin in your body?  Answer this question both based on the half-life of 60Fe and on the ability of your body to tell 60Fe from 56Fe. Write the nuclear reaction for the beta-minus decay of 62Fe to form a new element. 61Fe undergoes  the same type of radioactive decay as 62Fe. (a) Does it decay to form the same new element? If so, give it.  If  not, give the different element. (b) Does it decay to form the same  isotope of this element? If so, give it.  If  not, give the different isotope. The periodic table you received the first day of class starred the 11 elements that make up 99% of your body. (a)  List them by chemical symbol. (b)  Of these elements, list three that have naturally occurring radioisotopes. Hint: use your answers from part C. (c)  Which of these three radioisotopes has the longest half-life?  (d) Which of these three radioisotopes is present in our world (and your body) in the greatest percent?

For this part, you and your partner each will need to pick another element from the periodic table. Select from:

    He   N   O   Na   Mg   Al   S   Cl   Co   Zn   Sr   Cd   I   Cs   Au   Hg   Pb   Po   Ra   Th   Np   Pu   Cf

Cross off your choice from the list of those selected on the chalk board, so that each person in the lab has a different element. For your element, look up both the stable and radioactive isotopes. For each isotope, calculate the number of neutrons.

Analysis:  For isotopes that decay by beta emission (beta plus or beta minus emission), look at the mass numbers.  Compare this mass number to the nearest stable isotope.  Is the beta minus emitter heavier or lighter than the stable isotope?  The beta plus?

In the chart, write "heavier" or "lighter" next to the type of decay.

When you are finished, join with the other students at your lab table to answer the questions below. Then pool your information with everybody in the section. First, each person should briefly present information about his/her element. Then answer the following questions, using the elements you just looked up as well as the ones from Part D.

1. Which elements tend to have more isotopes - the heavier or the lighter ones? Give examples to support your answer.  
2. After a certain Z, all isotopes are radioactive. At which atomic number does this occur? Look up other elements as needed to pin this down exactly.
3. Which elements are more likely to decay by alpha emission?
4. (a) What type of decay typically occurs for isotopes with "too many neutrons," that is, too many in comparison to the stable isotope?  You labeled these as "heavier."
   (b) What type(s) of decay occur for isotopes with "too few neutrons"? You labeled these as "lighter."

Now that you have used two periodic tables on the web, see if you and your partner can find others. Feel free to look for ones in other languages or in different formats. To search for information on the web, use a search engine such as Google.  Search for periodic tables on the web. Your search should bring up both WebElements, the Table of the Isotopes, as well as others. If you have time, experiment by searching with different key words.

Select one and give the URL, title of the site, and provide a brief review.

Turn in the worksheet at the end of lab