Walking through a dark woods you come upon an abandoned house; upon entering the house you discover a closed thermos. You open the thermos and discover it contains still warm coffee; you conclude that some one else has been there recently. If you where a physicist you probably could even estimate how long ago the thermos was placed there.
That was the problem facing physicists, on the one side, and biologists and chemists, on the other side, at the beginning of the 20th century. Physicists, assuming the molten core of the earth was a remnant left over from the formation of the earth, could calculate the maximum age of the earth for the core not to have cooled to a solid. There answer was about 50,000 years, far less than that predicted by biologists and chemists. It was not until the discovery of radioactivity that physicists knew where the energy was coming from that kept the earth's core molten.
Interactive Activity Below is the basis of the radioactive decay chain starting with 238U in the upper right hand corner. The 'a' after the symbol U means it is alpha radioactive. You are to find out the next element in the chain. Since it looses two protons its Z will decrease by 2 and since it also looses 2 neutrons its A will decrease by 4. Try It! Click where you think the decay product should be and if you are correct its symbol will appear in the box. If the resulting isotope has an 'a' after its symbol, it is alpha radioactive, a 'b' would mean beta radioactive (A->A and Z->Z+1), and 'ab' means either event can occur (your choice). Starting with 238U follow the decay chain until you reach a stable nucleus! Above the table ( and, for your convenience,also below it ) is a window field which will contain information on each nucleus as you discover it. A\Z-> 81 82 83 84 85 86 87 88 89 90 91 92 238 237 236 235 234 233 232 231 230 229 228 227 226 225 224 223 222 221 220 219 218 217 216 215 214 213 212 211 210 209 208 207 206 A^/Z-> 81 82 83 84 85 86 87 88 89 90 91 92