# Splitting an Atom Fire a slow neutron into a uranium\-235 nucleus and watch it stretch, split and start a chain reaction By PETE\! Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/YXahdhoXWBzz) ## Transcript ### 1. Splitting an Atom This is a uranium atom, drawn hugely out of scale\. The faint haze is its electrons\. Splitting an atom really means splitting the tiny nucleus at its centre, and that is what this lab will show you, slowed down a hundred billion billion times\. Drag the scene to look around\. A real nucleus is about 100,000 times smaller than its atom\. At this scale the electron haze would be kilometres wide\. ### 2. Inside the nucleus Uranium\-235 packs 92 protons \(red\) and 143 neutrons \(grey\-blue\) into one tiny drop\. The strong nuclear force holds them together, but only at very short range\. All those protons also push each other apart, so this big nucleus is only just holding on\. Real protons and neutrons have no colour\. The colours just tell them apart\. ### 3. Send in a slow neutron To split it, you need a neutron\. It has no electric charge, so the protons do not push it away\. A slow neutron works best, because it lingers near the nucleus long enough to be caught\. Fire one from the source and watch the nucleus swallow it\. A slow, or thermal, neutron has only about 0\.025 electronvolts of energy: it moves at roughly 2 kilometres per second\. ### 4. Wobble, stretch, snap The extra neutron makes uranium\-236, and the energy it brings sets the drop wobbling\. It stretches into a cigar, then a peanut with a thin neck\. Once the neck is thin enough, the short\-range glue cannot hold, and the nucleus snaps in two\. Lise Meitner and Otto Frisch explained fission this way in 1938, picturing the nucleus as a liquid drop\. ### 5. Two fragments, spare neutrons The two pieces are smaller nuclei, here barium\-141 and krypton\-92\. Both are full of protons, so they repel each other and fly apart at high speed\. Three spare neutrons and a flash of gamma rays escape at the same moment\. The split is uneven and changes every time\. Barium and krypton are just one common outcome; on average about 2\.4 neutrons come out\. ### 6. Where the energy comes from Weigh everything after the split and a little mass is missing, about a tenth of a percent\. Einstein's E = mc squared turns that into energy: about 200 MeV per fission\. Most of it is the speed of the fragments, which becomes heat as they crash into the atoms around them\. That is tens of millions of times more energy than burning one carbon atom releases\. ### 7. A chain reaction Each fission frees two or three neutrons, and each of those can split another uranium\-235 nucleus\. If, on average, one neutron from every split goes on to cause another, the reaction keeps itself going\. Send the spare neutrons on their way\. Reactors slow neutrons with water or graphite and soak up extras with control rods, keeping that average at exactly one\. ### 8. How it was discovered In December 1938 Otto Hahn and Fritz Strassmann fired neutrons at uranium and were baffled to find barium\. Lise Meitner and Otto Frisch worked out that the nucleus had split, and Frisch named it fission, after cells dividing\. Turn the scene and take another look\. Only 0\.7 percent of natural uranium is uranium\-235, so reactor fuel is enriched to about 3 to 5 percent\.