Early scientists felt that all matter must be built from some basic unit, just as a wall may be constructed from a basic unit, the brick. In trying to find this basic unit, they separated matter by all the methods (chemical and physical) available to them until they could not separate it any further. They felt this separation must result in the building block of matter, which they called the atom (from the Greek word for indivisible). They also observed that the basic units or atoms for various elements differed in their properties, as iron was certainly different from carbon. This led them to try to find the structure of the atom. The difficulty of this problem can be seen when you consider that one cubic centimeter of gold contains as many as 59,000,000,000,000,000,000,000 atoms. The atom is so small that it defies conception. Through ingenious methods, particularly in the last 100 years, we have discovered many facts about this tiny particle, which enables us to understand many of the changes that occur around us.
Atomic Model. In order for us to picture what an atom looks like, we can use a description with which most people are familiar--the solar system model. In this model, the atom is thought of as a tiny solar system in which there is a central core (like the sun) with other particles traveling in circular paths or orbits (like the planets). While more complex and exact models have been developed, this is the best approximation for general use.
The Nucleus. The central core from the solar system model is called the nucleus (which is derived from the Latin word nucis meaning nut or kernel). The nucleus contains two types of particles, the proton and the neutron.
(1) The proton. The proton is a particle that has a mass (or weight) of one amu (atomic mass unit) and a positive one (+1) electrical charge. The symbol for the proton is p, p+ or H+.
(2) The neutron. The neutron has a mass of one amu (atomic mass unit) but has no electrical charge; that is, it is a neutral particle. In an atom that has more than one proton, the positive charges tend to repel each other. The neutrons serve to bind the protons so that this electrical repulsion does not cause them to fly off into space. The symbol for the neutron is n.
(3) Atomic number and atomic weight. Two important figures commonly used when discussing an atom are its atomic number and its atomic weight.
- Atomic number. The atomic number of an atom is equal to the number of protons in the nucleus of the atom. For example, a carbon atom has six protons in its nucleus; therefore, the atomic number of carbon is six.
- Atomic weight. The atomic weight of an atom is equal to the number of protons in the nucleus of the atom (one amu each) plus the number of neutrons in the nucleus of the atom (one amu each). Therefore, a carbon atom with six protons and six neutrons has an atomic weight of 12.
The Outer Structure. The particles that orbit the nucleus (as the planets orbit the sun) are called electrons. These particles have an electrical charge of negative one (-1), but their mass is so small that it is considered to be zero. Actually, the mass of the electron is 1/1 837 of the mass of a proton, but the mass, which contributes to the mass - atom is so small that it is not important. The symbol for the electrons is e or .
(1) Electron configuration. Since we may have many electrons going around the nucleus, It might appear that there could be a collision of electrons. Collisions do not occur because the electrons are located in orbits, which are different distances from the nucleus and because of the repulsion between like charges. The number of electrons and their locations are called the electron configuration. This electron configuration is different for each element.
(2 Electron shell. The term electron shell (or energy level) describes where electrons are located (i.e., a specific region around the nucleus). Since electrons can be forced to leave their atoms, the term energy level indicated the amount of energy required to remove the electrons from the various levels or shells. A nucleus can have seven shells, but more chemicals of medicinal importance contain electrons in the first four, which are labeled the K, L, N, and N shells. The K shell is the closest to the nucleus and the N shell is the farthest from the nucleus (figure 1-1). These shells contain different numbers of electrons. The maximum number each shell can hold is equal to 2N2, where N is the number of the shell (K=1, L=2, M=3, and so forth.). Thus the maximum number of electrons that each of the first four shells can hold Is:
K = 2(12) = 2
L = 2(22) = 8
N = 2(32) = 18
N = 2(42) = 32
Since, for example, the M shell can contain as many as 18 electrons, the possibility for collision might still appear to exist. The reason collisions do not occur is that a shell is subdivided into smaller energy levels, called subshells and orbitals, which we will not need to consider.
(3) Number of electrons. What determines the number of electrons an atom will contain? For an atom to exist freely in nature, it must be electrically neutral (without a charge). There are two particles in an atom that have charges--the proton, which is positive, and the electron, which is negative. For electrical neutrality, the sum of the charges must be zero. In other words, the number of electrons (negative charges) must equal the number of protons (positive charges).
Figure 1-1. First four electron shells.
Atomic Structure of Elements. As previously stated, each element consists of a single type of atom. Since all atoms consist of the three basic particles we have just discussed (except hydrogen, which usually has no neutrons), the only ways in which elements can differ are atomic number (the number of protons) and atomic weight, (the number of protons and neutrons). There are over 106 different elements which scientists know to have a different atomic number and atomic weight. These elements have a large assortment of properties. Two elements are liquids at room temperature, eleven are gases, and all others are solids.
Periodic Law. While investigating the properties of the elements, scientists discovered an interesting fact that is now called the periodic law. This law states that the properties of the elements are periodic functions of the atomic number. As the atomic number increases, the properties of the elements repeat themselves at regular Intervals.
Periodic Table. The periodic law allowed the scientists to group together the elements that had similar properties and form a systematic table of the elements. This table is the periodic table (Table 1-2). The vertical columns are called groups, and the horizontal rows are called periods. This table contains a lot of information that we will not generally use; however, we are concerned with the basic information we can obtain about the elements. Figure 1-2 includes four blocks for elements from the periodic table showing the information, which can be obtained from it. You should note that the number of neutrons is not given in the periodic table. This can be determined by subtracting the atomic number from the atomic weight.
Table 1-2. Periodic table of the elements.
Figure 1-3. Identifying the components of the periodic table.
Isotopes. All the atoms of a particular element are not identical. Slight variations in the number of neutrons are found to occur naturally. Variations can also be produced in reactors. Atoms that have the same number of protons but a different number of neutrons (same atomic number, but different atomic weights) are called isotopes. Sometimes isotopes are referred to by their mass numbers, H2, H3, U239, and so forth. All of the isotopes of a particular element have identical electronic configurations; and since electronic configurations determine chemical properties, isotopes of an element exhibit identical chemical behavior. Induced nuclear reactions can produce both stable and radioactive nuclei. If the nucleus of the atom is unbalanced during the bombardment reaction the atom is called a radioisotope. Radioisotopes, such as cobalt66 for treatment of cancer and iodine131 for diagnosing of thyroid tumors, are of vital importance in the medical field. The presence of isotopes helps to explain why many atomic weights in the periodic table are not whole numbers since all of the isotopes must be considered when computing the average atomic weight of the element.