The energy of a nuclear bomb comes from inside the nucleus of the atom. Mass is converted into energy according to E = mc2. This energy is the binding energy of the nucleus, the glue that keeps the nucleus of the atom together.
Radiating particles
In some cases, the nuclear force is not able to keep a nucleus all together, and the nucleus loses some of its particles. French physicist Henri Becquerel accidentally discovered this effect in 1896. He'd been intrigued by the experiments with x-rays that Wilhelm Roentgen had been doing in Germany. Becquerel obtained a uranium salt to see if he could observe these x-rays.
In his laboratory at the Museum of Natural History in Paris (where his father and grandfather had also been physics professors), Becquerel started his experiments by exposing to the sun a photographic plate with the uranium salt sprinkled on it, thinking that sunlight would activate the x-rays. One cloudy day when he couldn't perform one of his experiments, he placed the photographic plate with the uranium salt in a drawer. A few days later, he went ahead and developed the plate anyway, thinking that he was going to get a faint image. But the image was very sharp, with high contrast. He soon realized that he'd discovered a new type of energetic radiation.
When Pierre and Marie Curie heard of Becquerel's experiment, they began to search for other elements that could emit similar rays. They found that thorium and uranium emit the same radiation. And in 1898, they discovered two new elements: polonium (named after Marie's native Poland) and radium. The Curies named the effect radioactivity.
In England, Ernest Rutherford designed experiments to investigate this new radioactivity phenomenon and was able to show that these rays come in two varieties, one more penetrating than the other. The less penetrating one, which he called alpha, has positive electric charge. The Curies in Paris discovered that the other one, called beta, is negatively charged.
Realizing limitations of the nuclear force
Why are these nuclei giving off particles? The nuclear force is supposed to be extremely strong. Why isn't it able to keep all these particles inside the nucleus?
The answer is that the nuclear force has a very short range of action. It's able to tie in particles that are close to each other. If the particles are too far apart, the force stops working. If the particles happen to be protons, which have positive charges, the electric force acting alone will push them apart.
When the nuclear particles are bundled up in a nucleus of an atom, each particle interacts only with its nearest neighbors. In a nucleus with more than 30 particles, a particle in the middle of the nucleus won't feel the nuclear force of a particle at the edges. Each of the nuclear particles in the cluster feels the nuclear attraction of the other particles in the cluster (its immediate neighbors). However, these particles don't feel the force of the particle near the edge.
Think of it this way: Imagine that you and a group of several friends are trying to stay together while swimming in rough waters. If you all decide to hold hands, each one of you will be holding on to the two nearest neighbors. The grip of a swimmer at one end of the large chain, no matter how strong it seems to his immediate neighbor, has no influence on a swimmer at the other end. If the water gets too rough, the whole group may break apart, creating small groups of two, three, or maybe four.
Like the rough waters that break apart your group, the electrical repulsion of the protons tries to break apart a large nucleus. However, in the nucleus, certain helpers try to keep the whole thing together: the neutrons. Neutrons don't have an electric charge, and the only force they feel is the nuclear attraction. They are the skilled swimmers who won't be pushed away by the rough waters. If you have enough of them in your group, it will stay together.
Studying alpha decay
Like the swimming group with the skilled swimmers, a nucleus with a balanced number of protons and neutrons is stable and stays together. But if a nucleus has too many protons, the total electric repulsion can overwhelm the attraction of the nuclear force, and a piece of the nucleus can fly apart.
The piece that leaves the nucleus is usually in the form of an alpha particle, a cluster of two protons and two neutrons. (This particle is also the nucleus of the helium atom.) It turns out that these four particles are held together very tightly by the nuclear force, so this cluster is a very stable configuration of nuclear particles. These are the particles that Rutherford identified as alpha radiation. Physicists call the effect of the alpha particles leaving the nucleus alpha decay.
Detecting beta decay
It seems as if having a lot of neutrons is good for a nucleus because neutrons don't feel the electrical repulsion but do feel the nuclear attraction. They are the skilled swimmers in rough waters. However, these skilled swimmers don't have a lot of stamina. A neutron on its own, away from the nucleus, lasts for only about 15 minutes. After these 15 minutes, it changes into a proton, an electron, and another small particle called the neutrino. This effect is called beta decay.
Inside the nucleus, surrounded by the other particles, neutrons last much longer. When there are enough protons around, a quantum physics effect prevents neutrons from creating more protons. Quantum physics describes it by giving each proton in the nucleus its own space or slot. When there are enough protons, all the slots are taken and no additional protons are allowed.
In a nucleus with too many neutrons, a neutron at the outer edges of the nucleus can decay into a proton because there will be empty slots for this new proton to stay in. Therefore,
A nucleus with too many neutrons is unstable and decays into a proton, an electron, and a neutrino.
The protons created by this decay stay in the nucleus. The electrons don't belong in the nucleus; there are no slots for them there. The same goes for the neutrinos. Therefore, the electrons and neutrinos are both ejected. Neutrinos are extremely difficult to detect. They can go through the entire Earth and come out at the other end without a single collision. But electrons are easy to detect. These breakaway electrons create the beta rays that the Curies and Rutherford saw.
In both cases, the alpha and beta decays, the radioactive nucleus changes into the nucleus of another element when it gives off the alpha or the beta particle.
A third type of radioactive decay exists in which the unstable nucleus gives off only very energetic radiation, but no particles are ejected. The radiation is electromagnetic and is called gamma rays. In this case, the nucleus simply gives back some energy that it gained previously, but it doesn't lose its identity.