After Little Boy exploded 600 m (2000 ft) above Hiroshima, people in the 1940’s said “USA has the atom bomb.”

To call Little Boy an “atom” bomb wasn’t wrong but it’s not precise. Here’s a simple model of the atom with the fewest number of parts, Hydrogen.

The defining characteristic of Hydrogen is the single proton in the nucleus. Hydrogen has other isotopes–the same number of protons, different number of neutrons.

A nucleon can be a proton or a neutron. The nucleon number doesn’t affect the chemical properties of an atom; only the proton number can change how an atom interacts with other atoms. Most of the time, there are the same number of electrons “orbiting” as there are protons in the nucleus. The number of electrons determines how an atom reacts with other atoms. Essentially, chemistry is the study of how the electrons of different atoms interact. These atom interactions are commonly called chemical reactions.

Most bombs create energy from interactions between the electrons–chemical reactions–and since electrons are part of the atom: All bombs, are atom bombs.

What made Little Boy different from all the other bombs detonated throughout history? First off all, Little Boy was the second atom bomb. Trinity Test was first.

Of course, atoms were involved in the energy release during Trinity and Hiroshima, but virtually none came from electron interaction. The energy source was the nucleus.

Little Boy used a much larger atom than Hydrogen: Uranium-235 (Trinity used Plutonium, another atoms with a fissionable nucleus). Uranium’s proton number is 92; the 235 stands for the number of nucleons. If you take the number of protons and subtract it from the number of nucleons, it will give you the neutron number. Here’s a model of U-235. (This isn’t what U-235 really looks like; no one can say for certain because it’s so small that we’ll never be able to see it in the way we see a spherical collection of small balls.)

It’s somewhat correct to say the atom bomb gets its energy by splitting the atom. First of all, we split the nucleus; it’s true that the electrons end up coming along in the process, but what’s taking place with the electrons is irrelevant once we start tinkering with the nucleus. We don’t really split the nucleus either. The goal isn’t to break the nucleus apart like a rack of billiard balls. What we need to do is destabilize the nucleus so it falls apart. We destabilize the nucleus by facilitating a neutron absorption to a U-235 nucleus.

Protons have positive charge and neutrons are changeless. Like charges repel more strongly the closer they are to each other. In a nucleus, the protons are extremely close; they should accelerate away from each other, not maintain a tight, orderly arrangement. This is how we know there must be some other, attractive force in play. It’s called strong nuclear force (SNF). As the name indicates, SNF is quite strong: more than 100 times stronger than the repulsive forces between protons. As of now, SNF is the strongest force we know.

Adding a neutron creates U-236, an unstable nucleus that falls apart quickly. U-236 is unstable because repulsion between protons briefly gains an advantage over the SNF binding all nucleons; this drives the nucleus apart and into smaller, stabler fragments. Energy is released because the new particles move away with relatively more kinetic energy.

The explanation for where the energy comes from is abstract. SNF is so strong, and peculiar in nature, that it can change mass into energy, or energy into mass depending on the arrangement of the nucleons and the type of nuclei they form. The fission of U-236 causes a nuclear rearrangement that releases energy.

The fission byproducts are often unstable, radioactive. The squiggly arrows with a greek letter gamma at the heads represent gamma decay–one of three types of radioactivity. It’s these byproducts that create the infamous radiation sickness long after the blast fades.

The fission of a single nucleus doesn’t produce much energy. But if each neutron flying away from a preceding fission finds another U-235, there can be a chain reaction.

There were over a billion-trillion U-235 nuclei in Little Boy. Although each nucleus was part of an atom, it’s more precise to call Little Boy a nuclear bomb because the energy arises almost exclusively from the rearrangement of nucleons and the creation of smaller nuclei.

If you liked this post, you might like this one, too: An Electron Story.