The nucleus, which is composed on protons and neutrons, is the "core" of the atom in which 99.9% of the mass of the atom is located. Because the mass of subatomic particles is so small, scientists use a special unit to measure them. They call the unit an atomic mass unit, or amu. To get an idea as to how small an amu is, it would take 600,000,000,000,000,000,000,000 protons to make one gram.However, the nucleus does not give an atom its size. 
Whirling outside the nucleus are particles about 1/2000th the mass of either a proton or neutron--these particles are called electrons. Electrons have an atomic mass (amu) of 0.0006. Based upon an amu, you can obviously decipher that an electron is much smaller than either a proton or a neutron. Electrons do not move in fixed paths around the nucleus, their paths are constantly changing. The electrons orbit billions of times per second in their energy levels (orbits).
If you think about it, the nucleus shouldn't even exist. Where are protons found? In the nucleus, of course. But I thought like charges were suppose to repel each other. If so, then how does the nucleus stay together. There are four forces that hold an atom together. The first is the electromagnetic force. It is the force that allows like charges to repel and opposite charges to attract. The electromagnetic force of attraction between the positively charged nucleus and the negatively charged electrons holds the electrons in orbit. The second force is the strong force. The strong force is the force that overcomes the force of repulsion between like charges. It is the force that holds the protons together in the nucleus. It is the most powerful force in nature. What do you think happens to the strength of the strong force as you add more and more protons to the nucleus? Would the strong force be strong enough to hold the like charges together? The answer is no. The nucleus becomes unstable and the third force, known as the the weak force pushes parts of the nucleus out in an attempt to become stable. It is the force that allows for alpha, beta, and gamma to exit the nucleus during the decay of certain radioactive elements. The final force is gravity. Gravity is the force of attraction between all objects in nature, and its role in the atom is not clearly understood.
Radioactive decay, or
radioactivity, is a natural process that was discovered by Henri Becquerel whose picture appears to the right. While working with the element Uranium, Becquerel was able to prove that Uranium gives off an inivisible form of energy that behaves much like light. Today we know this invisible energy as nuclear radiation. But exactly what is nuclear radiation. Nuclear radiation or radioactivity is the natural release of energy in the form of particles and rays from a radioactive element. A radioactive element has a nucleus that is unstable. In order to stabilize itself, the nucleus will release particles and bits of energy. Nuclear radiation consists largely of alpha and beta particles and gamma rays. An alpha particle, which is made up of two protons and two neutrons, is identical with a helium nucleus. When the alpha particle is released, the number of protons and neutrons is reduced by two. As atomic number goes down, the identity of the element changes. For example, Uranium-238 has 92 protons in its nucleus, if two protons are released from the nucleus, 90 are left. The element with 90 protons in the nucleus is Thorium. Uranium's identity has changed. The process of one element changing into another element is known as transmutation. Take a look at the equation below:
Can transmutation take place in the laboratory? The answer is yes. Every element after Uranium (atomic number 92) was manufactured in the laboratory (synthetically). How? If Uranium has 92 protons in it's nucleus, the only way to manufacture a new element is to get the nucleus to accept another proton: 92protons + 1proton = 93protons (a new element). After a great deal of experimentation, the elements Neptunium and Plutonium were created. They were the first transuranium elements - elements with atomic numbers greater than 92. Getting the nucleus to accept another proton is extremely difficult. Do you remember what like charges do to one another? They repel. In order to ge the nucleus to accept the positive proton, enough force has to be generated. Long tunnels with devices known as particle accelerators (seen to the above right) speed up charged particles to almost the speed of light and bombard the target nucleus. With enough energy the particles can enter the nucleus and create a new element.
During the chain reaction there is a loss in mass, and according to Einstein, "when matter is lost, energy is created." How much energy is created? Einstein equation e=mc2 answers that question. Energy (e) is equal to (=) mass (m) times the speed of light squared (c2). Using this equation it is safe to say that a small amount of mass (the nucleus of an atom), multiplied by the speed of light squared (186,000 X 186,000) will produce a tremendous amount of energy! When numerous atomic nuclei are split in a chain reaction, huge quantities of energy are released. 
A significant problem can result from a nuclear chain reaction if the fission process is not controlled - the ATOMIC BOMB - is created when a uncontrolled nuclear chain reaction occurs. All of the energy is released simultaneously.
Take a look at the image to the right, in an instant the energy of the atom is released. The atomic bomb has been used twice, at Hiroshima and Nagasaki, Japan. Early in the morning of August 6th, 1945 the first atomic bomb ever used against humanity was dropped. Nicknamed "Little Boy," the bomb instantly killed almost 70,000 people. Three days later the second bomb, "Fat Man," was dropped over Nagasaki with equal devastation. *NOTE: Physicists who have studied these two atomic explosions estimate that the bombs utilized only 1/10th of 1 percent of their respective explosive capabilities.
Radioactivity cannot be seen or felt, so how would we be able to detect the radiation produced by radioactive elements.
Becquerel used photographic film, which is a method still used today by scientist. Other ways have also been created. In 1928, Hans Geiger designed an instrument that detects and measures radioactivity. Named the Geiger Counter in his honor, this instrument produces an electric current in the presence of a radioactive substance. The electric current produces flashes and clicks to indicate the strength of the radiation.
A cloud chamber contains a gas cooled to a temperature below its usual condensation point. When a radioactive substance is put inside the chamber, droplets of the gas condense around the radioactive particles creating a cloud-like appearance. The bubble chamber is similar in some ways to the cloud chamber, but uses a superheated liquid instead. When radioactive particles pass through the chamber, they cause the liquid to boil. The bubbles created by the boiling liquid indicates radioactive particles are present. Normally hydrogen is the most commonly used substance in a bubble chamber. Radioactive isotopes have many practical uses. Because they can be detected so easily, they can be used to follow a substance through an organism, or through an industrial process. Such radioactive isotopes are called tracers. When a small quantity of radioisotopes are mixed with naturally occurring substances, all the molecules go through the same reactions together. This characteristic is extremely important in the medical field. Such isotopes can be used to find problems in the internal body systems.