Matter can be defined as anything that has mass (weight) and occupies space. Thus, matter is everything that exists. It may exist in the form of solids, liquids, or gases. The smallest particle of matter in any state or form, that still possesses its identity, is called a molecule.

Substances composed of only one type of atom are called elements. But most substances occur in nature as compounds, that is, combinations of two or more types of atoms. Water, for example is a compound of two atoms of hydrogen and one atom of oxygen. A molecule of water is illustrated in figure 8-1. It would no longer retain the characteristics of water if it was compounded of one atom of hydrogen and two atoms of oxygen.

The Atom 

The atom is considered the basic building block of all matter. It is the smallest possible particle that an element can be divided into and still retain its chemical properties. In its simplest form, it consists of one or more electrons orbiting at a high rate of speed around a center, or nucleus, made up of one or more protons, and, in most atoms, one or more neutrons as well. Since an atom is so small that some 200,000 could be placed side by side in a line 1 inch long, it cannot be seen, of course. Nevertheless, a great deal is known about its behavior from various tests and experiments. 

The simplest atom is that of hydrogen, which is one electron orbiting around one proton, as shown in figure 8-2. A more complex atom is that of oxygen (see figure 8-3), which consists of eight electrons rotating in two different orbits around a nucleus made up of eight protons and eight neutrons. 

An electron is the basic negative charge of electricity and cannot be divided further. Some electrons are more tightly bound to the nucleus of their atom than others and rotate in an imaginary shell or sphere closer to the nucleus, while others are more loosely bound and orbit at a greater distance from the nucleus. These latter electrons are called "free" electrons because they can be freed easily from the positive attraction of the protons in the nucleus to make up the flow of electrons in a practical electrical circuit.

The neutrons in a nucleus have no electrical charge. They are neither positive nor negative but are equal in size and weight to the proton. Since a proton weighs approximately 1,845 times as much as an electron, the overall weight of an atom is determined by the number of protons and neutrons in its nucleus. The weight of an electron is not considered in determining the weight of an atom. Indeed, the nature of electricity cannot be defined clearly because it is not certain whether the electron is a negative charge with no mass (weight) or a particle of matter with a negative charge. Electricity is best understood in terms of its behavior, which is based in part on the charge an atom carries. When the total positive charge of the protons in the nucleus equals the total negative charge of the electrons in orbit around the nucleus, the atom is said to have a neutral charge. If an atom has a shortage of electrons, or negative charges, it is positively charged and is called a positive ion. If it possesses an excess of electrons, it is said to be negatively charged and is called a negative ion.

Electron Movement

In a state of neutral charge, an atom has one electron for each proton in the nucleus. Thus, the number of electrons held by the atoms making up the various elements will vary from one, in the case of hydrogen, to 92 for uranium.

The electrons revolving around a nucleus travel in orbits, sometimes called shells or layers. Each shell can contain a certain maximum number of electrons, and if this number is exceeded, the extra electrons will be forced into the next higher, or outer, shell.

The shell nearest the nucleus can contain no more than two electrons. In an atom containing more than two electrons, the excess electrons will be located in the outer shells. The second shell can have a maximum of eight electrons. The third shell can hold up to 18 electrons, the fourth 32, etc. It should be noted, however, that in some large complex atoms electrons may be arranged in outer shells before some inner shells are filled.