2. Atoms and Study of the Past

3. I. What is an atom? An atom (element) is the basic unit of matter. The Periodic Table displays the 115 known atoms.

4. I. What is an atom? This is how the Helium (He) atom is abbreviated on the Periodic Table. The mass number indicates the average number of particles (protons and neutrons) in its center (nucleus) The atomic number indicates the number of protons in its nucleus This is a diagram of the Helium atom. Notice the protons and neutrons in the nucleus and the electrons revolving around.

5. II. Where do atoms come from? 12 to 15 Billion years ago the universe exploded in the Big Bang. The universe began with a cataclysm that generated space and time, as well as all the matter and energy the universe will ever hold.

6. II. Where do atoms come from? Within seconds after the Big Bang, the universe was made up of particles which smashed together to form protons and neutrons. Protons and neutrons came together to form the nuclei of simple elements: hydrogen, helium and lithium. These basic atoms eventually combined to form all the atoms of the universe. Later (4.6 bya) the earth formed. Still later (3.8 bya), life began on early earth.

7. Molecules are formed when atoms are attracted and bond to other atoms. This attraction is electrical (protons are + and electrons are - ). III. What do atoms do?

8. Atoms have varying levels of stability. Unstable atoms tend to break down into more stable forms. This break down is known as radioactive decay. III. What do atoms do?

10. Why absolute-age dating works: Radioactive isotopes were incorporated into the Earth when the Solar System formed. All rocks and minerals contain tiny amounts of these radioactive atoms. Radioactive substances emit nuclear particles at a constant rate. As parent atoms decrease the number of daughter atoms increases by the same amount. This indicates the increasing age of an object. IV. How can atoms be used to study the past?

11. How absolute age dating works:  Radioactive dating is used to determine the ratio of the original parent atoms (less stable) to daughter atoms (more stable) within a given sample of a rock or fossil.  It takes too long to measure complete decay, so the time it takes for one-half of the original amount to decay is used. This is called the half-life of the radioactive isotope.  Each half life, half of a radioactive element decays. IV. How can atoms be used to study the past?

12. Radioactive decay occurs at a constant rate, specific to each radioactive atom. Since the 1950s, geologists have used radioactive atoms as natural "clocks" for determining numerical ages of certain types of rocks. Radiometric clocks are "set" when each rock forms. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off. It's this resetting process that gives us the ability to date rocks that formed at different times in earth history. IV. How can atoms be used to study the past? How absolute age dating works:

13. A commonly used radiometric dating technique relies on the breakdown of potassium ( 40 K) to argon ( 40 Ar). Precise measurements of the amount of 40 K relative to 40 Ar in an rock can tell us the amount of time that has passed since the rock formed. IV. How can atoms be used to study the past? Example 1

14. Half-life simulation IV. How can atoms be used to study the past? Example 2 Example 3 Half-life Quiz

15. 100 % U 0 years 50 % U (50 % lead) 4.5 billion years 25 % U (75 % lead) 9 billion years U left time Graph of U decay IV. How can atoms be used to study the past? Uranium decays into lead and other particles. Its half life is 4.5 billion years. Example 4