a. Man has long wondered about the structure of matter. As far back as 500 BC, there were many imaginative ideas concerning the true nature of matter. These ideas were in many ways little more than philosophical notions. The Greek, Empedocles, believed that all matter was composed of four basic substances or elements--earth, air, fire, and water. Other Greeks, the atomists, such as Epicurus and Democritus, thought that all matter consisted of elemental, indivisible units called atoms.

b. With the advent of scientific methods, man discovered the basic substances of which all matter is composed--the natural elements. Water was separated into hydrogen and oxygen. Air was found to be basically a mixture of oxygen and nitrogen. They and the other elements that were discovered could not be further divided into simpler substances; thus, the name element, a basic substance. There are 92 natural elements. These include, for example, iron, sulfur, aluminum, carbon, sodium, and chlorine. Several others have been produced artificially, for instance, plutonium.

c. Scientific analysis has shown that the Greek atomists were, in a sense, correct in that elements are composed of basic units or atoms. Each element has its own characteristics and its own characteristic atoms. An element is a substance, which cannot be separated into simpler substances by ordinary chemical means. An atom is the smallest unit of an element that possesses all the characteristics of the element.

d. The Greek atomists were, however, in one sense incorrect. Atoms have been found to be divisible; atoms have been divided into more fundamental particles called electrons, protons, and neutrons. The electron was first discovered as the basic unit of electricity. It is a very tiny, negatively-charged particle considerably lighter than an atom. The proton is a positively-charged particle having exactly the same magnitude of charge as the electron; however, it is much larger than the electron in mass, being approximately 1,840 times the electron mass. A gram of protons contains roughly 6 x 1023 protons. After the discovery of the electron and the proton, the neutron was predicted as the particle that would be formed if a proton and electron were closely combined. It would thus be neutral in charge. In 1932, the neutron was actually discovered as predicted.

e. There was still the problem of determining how these basic particles are arranged to make up an atom. It has been learned that atoms are composed of a positively charged central mass called the nucleus (which contains protons and neutrons) and electrons which move in orbits or shells around, but very far from, this nucleus. It is reasonable to assume the electrons experience an attractive force due to the positive nucleus. However, they move rapidly enough so that the centrifugal force (tending to throw the electrons out of orbit) balances the attractive force. Most objects in the world have no charge on them; therefore, we can reasonably suppose that the atoms of which matter is composed are electrically neutral, that is, they have no net charge. Thus, atoms normally contain exactly as many electrons moving in shells around the nucleus, as there are protons in the nucleus. There are no electrons as such in the nucleus, since neutrons are distinct particles different from either protons or electrons. Thus, because the electrons are very small in mass and since they move around the nucleus at distances relatively far from the nucleus, the atom is primarily empty space with the major portion of its mass concentrated in the nucleus. An atom is about 10-8 centimeters in diameter; this essentially refers to the diameter of the electron orbits, the nucleus being about 10-12 centimeters in diameter (Figure 1-1).

Figure 1-1. Typical simple atom.
f. Neutrons, being neutral in charge, do not affect the chemical nature of the atom and it is the number of protons in the nucleus that determines the element to which the atom belongs. For example, an atom of the lightest element, hydrogen, has one proton in the nucleus and one electron traveling around the nucleus. An atomcontaining two protons in the nucleus and two electrons traveling around the nucleus belongs to the element helium. Similarly, the 92d natural element, uranium, is composed of atoms containing 92 protons in the nucleus and 92 electrons traveling in shells around the nucleus. The number of neutrons in the nucleus of atoms of any particular element varies, but there are usually more neutrons than protons in the nucleus of an atom. Two atoms of the same element with different numbers of neutrons are called isotopes.

g. Electrons are not distributed at random about the nucleus, but they exist in arrangements that follow definite laws. The model of the atom proposed by Niels Bohr in 1913 pictures the electrons as moving in circular orbits about the nucleus. Although we now know this model is not strictly correct, its features give a good explanation of the simple phenomena, which we wish to consider. Figure 1-2 shows a picture of the hydrogen atom.

Figure 1-2. Hydrogen atom.

h. The next heaviest element after hydrogen is helium. This contains two protons in the nucleus and, consequently, two electrons revolving about the nucleus (Figure 1-3). Both these electrons may be thought of as roughly the same distance from the nucleus. It turns out that no more than two electrons will ever be found at this distance from the nucleus in helium. Of course, this is not unique with helium, but is also true of elements with more than two protons in the nucleus. Both these electrons are said to lie in a shell, which is actually an orbit around the nucleus. No more than two electrons may be present in the shell, no matter what atom is under consideration. Normal hydrogen has only one electron in the shell, so we call the shell incomplete.

i. Atoms which have more than two protons in the nucleus have an equal number of electrons; in this case, the electrons in excess of two will lie in shells past the first one. The shells are normally designated by capital letters, the first shell being denoted by “K," the second by "L," the third by "M," etc. It is possible that one of the electrons revolving about a helium nucleus might be in the K-shell and the other in the L-shell. This puts only one electron in the K-shell and the K-shell can hold two. Such a

Figure 1-3. Helium atom.

condition is referred to as an “excited” atom. It is caused by subjecting the atom to an external stimulus, such as a source of radiation or collision with another atomic particle (Figure 1-4). The electron in the L-shell will jump back into the K-shell and in the process will give off a bit of electromagnetic energy. For this particular case, there will be a light of a frequency in the visible range. This bit of electromagnetic energy given off is called a photon. Therefore, the electronic structure has a great deal to do with the emission of radiation from matter. One way in which these electron jumps can be arranged to occur in large numbers is to build a neon lamp. The electric current passing through the neon gas knocks some of the electrons into higher shells and, in jumping back, light is given off. Sometimes the jump is referred to as a jump from one energy level to another.

Figure 1-4. Excited atom.

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