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What is an Atom?
Radiation and Radioactivity
Atoms are the extremely small particles of which we, and everything around us, are made. There are 92 naturally occurring elements and scientists have made another 17, bringing the total to 109. Atoms are the smallest unit of an element that chemically behaves the same way the element does.
When two chemicals react with each other, the reaction takes place between individual atoms--at the atomic level. The processes that cause materials to be radioactive--to emit particles and energy--also occur at the atomic level.
Atomic Structure
In the early 20th century, a New Zealand scientist working in England, Ernest Rutherford, and a Danish scientist, Niels Bohr, developed a way of thinking about the structure of an atom that described an atom as looking very much like our solar system. At the center of every atom was a nucleus, which is comparable to the sun in our solar system. Electrons moved around the nucleus in "orbits" similar to the way planets move around the sun.(While scientists now know that atomic structure is more complex, the Rutherford-Bohr model is still a useful approximation to begin understanding about atomic structure.)
nucleus
contains protons
and neutrons; together these are called "nucleons"
neutronshave no electrical charge, and like protons, are about 1800 times as heavy as an electron.
protonsare positively charged particles. All atoms of an element (radioactive and non-radioactive) have the same number of protons.
Protons and neutrons in the nucleus, and the forces among them, affect an atom's radioactive properties.
The particles that orbit the nucleus as a cloud are called electrons. They are negatively charged and balance the positive electrical charge of the protons in the nucleus.
Interactions with electrons in the outer orbits affect an atom's chemical properties
What holds the parts of an atom together?
Opposite electrical charges of the protons and electrons do the work of holding the electrons in orbit around the nucleus. Electrons closer to the nucleus are bound more tightly than the outer electrons because of their distance from the protons in the nucleus. The electrons in the outer orbits, or shells, are more loosely bound and affect an atom's chemical properties.
The nucleus is held together by the attractive strong nuclear force between nucleons: proton-to-proton, neutron-neutron, and proton-neutron. It is extremely powerful, but extends only a very short distance, about the diameter of a proton or neutron.
There are also electromagnetic forces, which tend to shove the positively-charged protons (and as a result the entire nucleus) apart. In contrast to the strong nuclear force, the electric field of a proton falls off slowly over distance extending way beyond the nucleus, binding electrons to it.
The balance between the strong nuclear force pulling the nucleus together and the positive charges of the protons pushing it apart is largely responsible for the properties of a particular kind of atom or nuclide. (a unique combination of protons, neutrons, and balance of energies.)
The delicate balance of forces among nuclear particles keeps the nucleus stable. Any change in the number, the arrangement, or energy of the nucleons can upset this balance and cause the nucleus to become unstable or radioactive. (Disruption of electrons close to the nucleus can also cause an atom to emit radiation.)
The amount of energy required to break up the nucleus into its parts is called the binding energy; it is often referred to as "cosmic glue."
