By Jeanne Bishop, Westlake High School, Westlake, OH
In this activity students will model the time after the Big Bang when the first nuclei of hydrogen and helium were created. The students will move and display cards that show the elements that are formed. The teacher should emphasize the need to be quiet and follow directions for this activity. Use a large area--an outside location, a large classroom with seats moved back, or a gym.
Materials Needed: Small cards or stiff paper (index cards or other) with p or PROTON printed on one side, and n or NEUTRON printed on the other side, in large letters, one for each student. Recall that for every 7 protons there is one neutron, although the number of neutrons may have to be increased if the class size is small. Also give each student cards individually marked DEUTERIUM (D = 2H), HELIUM 3 (3He), and HELIUM 4 (4He). These can be color-coded, for easiest identification. Two different, larger signs with 10 BILLION DEGREES K (or 10,000,000,000 billion kelvins or 1010 kelvins) on one and 1 BILLION DEGREES K (or 1 billion kelvins or 109 kelvins) on the other. One ping pong ball, held by the teacher. Make two cards, held by the teacher for BERILLIUM 7 (7Be) and LITHIUM7 (7Li).
Sequence:
1. To begin, students arrange themselves in a tight central group representing the matter that emerges in the first second, a soup of elementary particles. Students in the model represent parts of forces that will explode apart to become protons (p's or normal H nuclei) and neutrons (n's). Give one student the two temperature cards. Students have sets of cards with names or formulas: p and n, 2H, 3H, 3He and 4He.
Students hide all cards, since there are no elements before the Big Bang. If indoors, turn out the lights, representing the absence of electromagnetic energy before the Big Bang
2. When the lights are turned on, the teacher or a student calls "Big Bang." With this cue, the student inside the tight group with the temperature sign (10 BILLION DEGRES K) holds it up. Most students hold up PROTON card and a few hold up NEUTRON. Students start moving out from the dense center.
3. Some students should encounter (that is, safely bump) other students. For a while, the "particles" should not stick together.
4. The teacher gives the cue call of "100 seconds." The student with the temperature signs pulls away the 10 BILLION DEGREES K and sticks up 1 BILLION DEGREES K. Students hold the PROTON signs and continue to move outward. When two PROTONS meet, the two students decide if they want to bounce away or stick by holding hands (or locking arms) to form a deuterium nucleus. Then these students hide their PROTON cards and one displays a DEUTERIUM (2H) card. Couples representing deuterium continue bending and moving outward to encounter other PROTONS. Couples representing deuterium cannot stick to other single or couples of students until the next time cue is given. The reaction is: p + p -> D + positron + neutrino. Another way to get Deuterium is p + n -> D
5. If a DEUTERIUM bumps into a neutron and they join, HYDROGEN 3 is formed. The three students should hold hands (or lock arms) and exchange their cards for a 3H. The reaction is: D + n -> 3H
6. The teacher gives the cue call of "1000 seconds." The DEUTERIUM couples do not have to join to other single or couples of students when they bump. However, now students can join if they wish. For a DEUTERIUM sticking to a free PROTON, the three students should all hold hands, hide the DEUTERIUM and NEUTRON cards, and show a HELIUM (3He) card. The reaction is: D + p -> 3He
7. Following formation of 3He, if an 3He joins another 3He, only four of the six students should hold hands, hide the DEUTERIUM cards, and show a HELIUM 4 (4He) card. Two of the students who were part of the 3He's should leave the HELIUM 4 group and hold up their original PROTON cards. (Note: Students may want to stick more often that what correctly models the early universe.) The reaction is 3He + 3He -> 4He + p + p Other possible reactions to produce 4He are: 3He + D -> 4He + p; 3He + D -> 4He + n + positron; 3He + n ->4He
8. The teacher directs one of the remaining HELIUM 3 (3He) join with an HELIUM 4 (4He) and give them the BERILLIUM 7 (7Be) card and a ping-pong ball representing an electron. First this group should hold up the BERILLIUM 7 (7Be) card, then throw out the ping-pong ball. Then they should hide the BERILLIUM 7 card and hold up the LITHIUM 7 (7Li) card.
9. The teacher gives the cue call of "End". Students freeze in position with signs up so that all can see what has occurred. Ask students if what the model shows is the correct ratio of nuclei from element formation in the Big Bang (90 percent total H and 10 percent total He with almost no free neutrons). Probably the outcome will be wrong. Ask students what can be done to make the H to He ratio correct. Students probably will need to split some nuclei. Discuss who should split and then do it. Student with the temperature card pulls it down, indicating a further cooling of the universe.
Follow-up questions (while still in model positions or later)
1. What is wrong with "freezing" the motion at the end of the activity?
2. How are the numbers of particles and atoms we have formed different from what really occurred?
3. Why can't the groups keep combining like they did in this activity to form all the elements heavier than hydrogen, helium, and lithium?
4. Since the atoms are moving, what caused the atoms to clump together to form stars, quasars, and galaxies
An extended version of this activity, with student worksheets, is available at
http://imagine.gsfc.nasa.gov/docs/teachers/elements/.