An atom, in the simplest model, consists of a nucleus and orbiting electrons.

Atomic Structure and Radioactivity

Before we can get to the bombs, we have to start small, atomically small. An atom, you'll remember, is made up of three subatomic particles -- protons, neutrons and electrons. The center of an atom, called the nucleus, is composed of protons and neutrons. Protons are positively charged, neutrons have no charge at all and electrons are negatively charged. The proton-to-electron ratio is always one to one, so the atom as a whole has a neutral charge. For example, a carbon atom has six protons and six electrons.

It's not that simple though. An atom's properties can change considerably based on how many of each particle it has. If you change the number of protons, you wind up with a different element altogether. If you alter the number of neutrons in an atom, you wind up with an isotope. For example, carbon has three isotopes: 1) carbon-12 (six protons + six neutrons), a stable and commonly occurring form of the element, 2) carbon-13 (six protons + seven neutrons), which is stable but rare and 3) carbon-14 (six protons + eight neutrons), which is rare and unstable (or radioactive) to boot.

As we see with carbon, most atomic nuclei are stable, but a few aren't stable at all. These nuclei spontaneously emit particles that scientists refer to as radiation. A nucleus that emits radiation is, of course, radioactive, and the act of emitting particles is known as radioactive decay. If you're particularly curious about radioactive decay, you'll want to peruse How Nuclear Radiation Works. For now, we'll go over the three types of radioactive decay:

  1. Alpha decay: A nucleus ejects two protons and two neutrons bound together, known as an alpha particle.
  2. Beta decay: A neutron becomes a proton, an electron and an antineutrino. The ejected electron is a beta particle.
  3. Spontaneous fission: A nucleus splits into two pieces. In the process, it can eject neutrons, which can become neutron rays. The nucleus can also emit a burst of electromagnetic energy known as a gamma ray. Gamma rays are the only type of nuclear radiation that comes from energy instead of fast-moving particles.

Remember that fission part especially. It's going to keep coming up as we discuss the inner workings of nuclear bombs.