Then, count the number of pieces of candy left with the M facing down. These are the parent isotope that did not change during the first half life. The teacher should have each team report how many pieces of parent isotope remain, and the first row of the decay table Figure 2 should be filled in and the average number calculated.

The same procedure of shaking, counting the "survivors", and filling in the next row on the decay table should be done seven or eight more times. Each time represents a half life. Each team should plot on a graph Figure 3 the number of pieces of candy remaining after each of their "shakes" and connect each successive point on the graph with a light line. AND, on the same graph, each group should plot points where, after each "shake" the starting number is divided by exactly two and connect these points by a differently colored line.

## List of human evolution fossils

After the graphs are plotted, the teacher should guide the class into thinking about: Is it the single group's results, or is it the line based on the class average? U is found in most igneous rocks.

Unless the rock is heated to a very high temperature, both the U and its daughter Pb remain in the rock. A geologist can compare the proportion of U atoms to Pb produced from it and determine the age of the rock.

### Magnetic flips

The next part of this exercise shows how this is done. Each team is given a piece of paper marked TIME, on which is written either 2, 4, 6, 8, or 10 minutes. The team should place each marked piece so that "U" is showing. This represents Uranium, which emits a series of particles from the nucleus as it decays to Lead Pb- When each team is ready with the pieces all showing "U", a timed two-minute interval should start.

### MATERIALS REQUIRED FOR EACH GROUP

During that time each team turns over half of the U pieces so that they now show Pb This represents one "half-life" of U, which is the time for half the nuclei to change from the parent U to the daughter Pb A new two-minute interval begins. Continue through a total of 4 to 5 timed intervals. That is, each team should stop according to their TIME paper at the end of the first timed interval 2 minutes , or at the end of the second timed interval 4 minutes , and so on. After all the timed intervals have occurred, teams should exchange places with one another as instructed by the teacher.

The task now for each team is to determine how many timed intervals that is, how many half-lives the set of pieces they are looking at has experienced.

## Dating ancient fossils

The half life of U is million years. Both the team that turned over a set of pieces and the second team that examined the set should determine how many million years are represented by the proportion of U and Pb present, compare notes, and haggle about any differences that they got. Right, each team must determine the number of millions of years represented by the set that they themselves turned over, PLUS the number of millions of years represented by the set that another team turned over. Pb atoms in the pegmatite is 1: Using the same reasoning about proportions as in Part 2b above, students can determine how old the pegmatite and the granite are.

They should write the ages of the pegmatite and granite beside the names of the rocks in the list below the block diagram Figure 1. Because of its short half-life, the number of C isotopes in a sample is negligible after about 50, years, making it impossible to use for dating older samples. C is used often in dating artifacts from humans.

## Dating Fossils – How Are Fossils Dated?

Corina Fiore is a writer and photographer living in suburban Philadelphia. She earned a B. Fiore taught high school science for 7 years and offered several teacher workshops to regarding education techniques.

She worked as a staff writer for science texts and has been published in Praxis review materials for beginning teachers. How to Calculate Radioactivity. How to Calculate Subatomic Particles.

By Corina Fiore; Updated April 24, Paul Grand at creativecommons. Depending on which text editor you're pasting into, you might have to add the italics to the site name. Which Elements Are Isotopes? What Is Chronometric Dating?

But while the difficulties of single life may be intractable, the challenge of determining the age of prehistoric artifacts and fossils is greatly aided by measuring certain radioactive isotopes. Until this century, relative dating was the only technique for identifying the age of a truly ancient object.

By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site. Though still heavily used, relative dating is now augmented by several modern dating techniques.