The atom is the smallest unit of matter. It is like a miniature solar system with a nucleus in the center consisting of protons and neutrons. (The exception is the hydrogen atom which has only one proton and no neutron.)
The proton has a positive charge and the number of protons in the nucleus identifies the atom. For instance, hydrogen has one proton, carbon has six and tungsten has 74. The number of protons is called the atomic number and is represented by the symbol, Z.
Neutrons have approximately the same weight as protons, but they are electrically neutral. The combined number of protons and neutrons is called the atomic mass number (symbol is A). Atoms of the same element may have varying numbers of neutrons. These different forms of an element are called isotopes (see Joyce Bryant’s unit).
Spinning around the central nucleus with the speed of light are the tiny electrons. They weigh only l/l800th as much as protons. There is always an equal number of protons and electrons, and since electrons have a negative charge, that means that atoms are electrically neutral.
The electrons move in definite orbits or shells around the nucleus. These shells are designated by letters starting with the K shell nearest the nucleus, followed by the L, M, N, 0, P and Q shells.
The maximum number of electrons that can occupy each electron shell can be figured by using the formula 2n
where n = the shell number. For instance, the maximum number of electrons in the M shell (third shell from the nucleus) would be 18 (2 x 3
What is important to know in radiology is that each shell has a characteristic binding energy; in fact, the shells are also called energy shells. This binding energy is a measure of the force of attraction that the nucleus exerts on the electrons; or to express it differently, how much energy it would take to remove an electron from that particular energy shell.
The electrons in the innermost K shell are bound much more tightly to the nucleus than the electrons in the outermost shells. For instance, in an atom of barium, the characteristic binding energy of the K shell is approximately 37 kilo-electron volts (keV), whereas the binding energy of the 0 shell (the fifth shell) is only 0.039 keV. (The electrons in the outermost shells are called free electrons, indicating that it takes very little energy to remove them.)
Heavier atoms such as tungsten have a much greater characteristic binding energy of their shells than do light atoms such as carbon. The binding energy of the K shell of tungsten is approximately 69 keV, whereas the K shell energy of carbon is only 0.28 keV. (See Table I, which compares the K shell energy of various atoms).
Electrons are able to jump from shell to shell. If they jump from a higher shell to a lower shell, they will give off energy equivalent to the difference in energies between the two shells. If this energy is high enough, it will be released in the form of an x-ray photon.
Problem: How much energy in keV would be given off if an L shell electron (4.9 keV) in an iodine atom dropped to the K shell (33.2 keV). Answer: 28.3 keV = EK EL.
One more fact should be emphasized about the atom. It consists mostly of empty space. If the nucleus of an atom were enlarged to the size of a grape seed, the radius of the atom would stretch the length of a football field, about 300 feet. Or, to put it another way, if the hydrogen nucleus were a ball three inches in diameter, then the electron would be a ball 1/4 inch in diameter spinning 1.5 miles away from the nucleus
. Another useful way to explain to your students the fact that atoms are mostly empty space is this quote from Christensen:.
If all the electrons in the atoms of the world could be removed and the nuclei packed together (a condition that exists in the white dwarf stars), the diameter of the earth would be reduced to about one-tenth mile