You have probably read in history books about the atomic bombs used in World War II. You may also have seen fictional movies where nuclear weapons were launched or detonated (Fail Safe, Dr. Strangelove, The Day After, Testament, Fat Man and Little Boy, The Peacemaker, just to name a few). They're on TV, too -- Jack Bauer struggles to stop a nuclear bomb detonation on the TV show "24." In the news, while many countries have been negotiating to disarm their arsenals of nuclear weapons, other countries have been developing nuclear weapons programs.

Nuclear Explosion Image Gallery

Photo courtesy NARA Atomic Cannon Test, 1953. See more nuclear bomb pictures.

We have seen that these devices have incredible destructive power, but how do they work? In this article, you will learn about the physics that makes a nuclear bomb so powerful, how nuclear bombs are designed and what happens after a nuclear explosion.

Nuclear bombs involve the forces, strong and weak, that hold the nucleus of an atom together, especially atoms with unstable nuclei. There are two basic ways that nuclear energy can be released from an atom:

• Nuclear fission - You can split the nucleus of an atom into two smaller fragments with a neutron. This method usually involves isotopes of uranium (uranium-235, uranium-233) or plutonium-239.
• Nuclear fusion -You can bring two smaller atoms, usually hydrogen or hydrogen isotopes (deuterium, tritium), together to form a larger one (helium or helium isotopes); this is how the sun produces energy

In either process, fission or fusion, large amounts of heat energy and radiation are given off.

To build an atomic bomb, you need:

• A source of fissionable or fusionable fuel
• A triggering device
• A way to allow the majority of fuel to fission or fuse before the explosion occurs (otherwise the bomb will fizzle out)

Atomic Structure
Before we talk about the physics of atomic bombs, it's a good idea to go over the basic properties of atoms.

Atoms are incredibly small -- the smallest is about 10^-8­ cm in diameter. For an idea of how small this really is, think of a baseball. The diameter of a baseball is about 7 cm. If an atom were the size of a baseball, an actual baseball would be about 3044 miles high.

An atom 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.

An atom's properties can change considerably based on how many of each particle it has:

• The number of protons in an atom determines the type of element. Elements are classified by their atomic number, which is simply the number of protons in an atom's nucleus. Some common elements on Earth are oxygen, carbon and hydrogen. You can see the elements on the periodic table here.­
• There are different types of atoms called isotopes. These isotopes look and act the same in nature -- the only difference is the number of neutrons in the nucleus.
• You can calculate the “mass” of an atom by counting the number of protons and neutrons inside the nucleus. This number is called the ­atomic mass. Carbon has three isotopes, for example -- carbon-12 (six protons + six neutrons), carbon-13 (six protons + seven neutrons) and carbon-14 (six protons + eight neutrons).

If atoms are so small, then how can they release the kind of energy that creates an atomic bomb? Find out on the next page. ­