Chapter 17 · Radioactivity and Nuclear Chemistry
17.7The Discovery of Fission and the Atomic Bomb
8 min · two checks
Predict
What is fission, and why can it run away?
The idea
- Define fission and a chain reaction.
- Contrast an uncontrolled burst with a steady one.
In the late 1930s, physicists found that uranium-235, struck by a neutron, can split into two mid-sized nuclei, releasing more neutrons and a great deal of energy. If each fission causes more than one additional fission, the number grows exponentially: a chain reaction. Below that threshold the reaction dies out. Right at one neutron causing one new fission, the rate is steady.
A bomb is engineered so that a supercritical mass comes together suddenly and the multiplication factor is well above one. The energy release is a nuclear process, not a chemical explosion, which is why the energy per gram dwarfs TNT. The historical weapons used this fact. The chemistry course’s job is the nuclear accounting: mass number and charge still balance, and the missing mass appears as energy through E = mc², a conversion ordinary chemical reactions do not show at a measurable scale.
Keep these
- Fission splits a heavy nucleus and emits neutrons.
- A chain reaction multiplies when neutrons are not lost.
- A bomb is an uncontrolled chain reaction. That is not a reactor.