An atoms behaviour is not always down to its electrons! For example: carbon has three isotopes:
Isotopes are atoms with the same proton number but different number of neutrons.
The three isotopes of carbon as illustrated above are named as follows:
Carbon-12: C12- just less than 99% of carbon atoms
Carbon-13: C13 - just greater than 1% of carbon atoms
Carbon-14: C14 - very small percentage.
The Carbon-14 atom is the most interesting! It has an unstable nucleus because of the extra neutrons.
As a simple rule, atoms prefer having the same number of neutrons as protons. Otherwise the imbalance can cause the nucleus to be unstable. The result of this unstable nucleus is that the atom 'throws' out particles, in order, to become more stable. In the case of the carbon-14 atom it throws out a particle to become a nitrogen atom. This process of losing particles is called decay.
Carbon-14 is said to be radioactive. It is called a radioisotope or radionuclide. When it decays it gives out radiation.
It is not only carbon that has radioactive isotopes. Atoms belonging to elements at the bottom of the Periodic Table tend to have several radioisotopes because they have lots of neutrons as compared to protons.
All radioisotopes eventually turn into stable atoms by giving out radiation.
Decay is a Random Process
You can't control the decay of a nucleus because it is a nuclear reaction not a chemical one. Hence, heat or catalysts have no effect. This lack of control is why decay is random.
Take the example above, it could take seconds or hundreds of years to turn an atom of polonium-216 into an atom of lead-208!
Even though, decay is random, scientists have come up with a way of estimating the time taken for decay to occur for a specific radioisotope. This is called the Half-Life.
The half-life is the time taken for half the radioisotope in a sample to decay.
Radiation consists of three types of particle:
1. Alpha: An alpha particle is made up of 2 protons and 2 neutrons, so it has a proton number of 2, and a mass of 4 units. This is identical to the nucleus of a helium atom.
In equations or diagrams like the one below, the nucleus of a helium atom is often used to show the emission (giving out) of alpha radiation. An alpha particle moves quickly out of the radioisotope but soon slows down in air. Its penetrating power is poor as even paper and skin can stop it in its tracks!
2. Beta: A beta particle is much smaller than an alpha particle. It has almost no mass which means, it moves very fast. It can travel as far as 30cm in air, through skin and thin pieces of metal.
Beta particles are formed when a neutron turns into a proton and electron. The new electron is 'thrown' out of the atom and the proton remains. So the mass of the isotope is unchanged, but the atomic number (number of protons) increases by 1.
3. Gamma Rays: Gamma rays usually are emitted at the same time as beta and alpha particles. They are high energy electromagnetic rays that travel at high speed (the speed of light) with a great penetrating power. They can travel through several metres of air, through thick sheets of metal, and deep into the human body. Only thick sheets of lead or concrete will stop them.