Matter is the substance of which a physical object is composed, whether it be a piece of iron, wood, or cloth, or whether it is a gas, liquid, or solid. Matter is composed of fundamental substances called elements. There are 110 elements that have been found in the universe. Elements, in turn, are composed of atoms. An atom is the smallest particle of an element that can exist alone or in combination. All matter is made up of atoms or a combination of atoms, and all atoms are electrical in structure.
Suppose a piece of chalk is broken in half and one piece discarded. Then the remaining piece is broken in half and one piece discarded. If this procedure is continued, eventually the piece of chalk will be broken into such a small piece that by breaking it once more there will no longer be a piece of chalk but only a molecule of chalk. A molecule is the smallest particle of a substance that has the properties of that substance. If a molecule of chalk is broken down into smaller segments, only individual atoms will exist, and they will no longer have the properties of chalk. The atom is the basic building block of all matter. The atom is the smallest particle that can combine with other atoms to form molecules.
Although the atom is a very small particle, it is also composed of several parts. The central part is called the nucleus. Other parts, called electrons, orbit around the nucleus. Each electron is a relatively small, negatively charged particle. The electrons orbit the nucleus in much the same way that the planets orbit the sun.
The nucleus, the center section of an atom, is composed of protons and neutrons. The proton is a heavy, positively charged particle. The proton has an electric charge that is opposite but equal to that of the electron. All atoms contain a like number of protons and electrons. The neutron is a neutral particle, which means that it is neither positively nor negatively charged. The neutrons tend to hold the protons together in the nucleus.
The simplest atom that exists is the hydrogen atom, which consists of one proton that is orbited by one electron, as shown in Figure 1(a). Not all atoms are as simple as the hydrogen atom. Other atoms have more particles. The difference in each different atom is the number of electrons, neutrons, and protons that the atom contains. The hydrogen atom has one proton and one electron. The oxygen atom has eight protons, eight neutrons, and eight electrons, as shown in Figure 1(b). The silver atom contains 47 protons, 61 neutrons, and 47 electrons. The more particles an atom has, the heavier the atom is. Since there are 110 elements, but millions of different types of substances, there must be some way of combining atoms and elements to form these substances.
When elements (and atoms) are combined, they form a chemical union that results in a new substance, called a compound. For example, when two hydrogen atoms combine with one oxygen atom, the compound water is formed. The atomic structure of one molecule of water is shown in Figure 1(c).
The chemical symbol for a compound denotes the atoms that make up that compound. Refrigerant 22 (R-22) is a substance commonly used in refrigeration systems. A refrigerant is a fluid that absorbs heat inside the conditioned area and releases heat outside the conditioned area. The chemical symbol for one molecule of R-22 is CHC1F2. One molecule of the refrigerant contains one atom of carbon, one atom of hydrogen, two atoms of fluorine, and one atom of chlorine. The chemical name for R-22 is monochlorodifluoromethane. All materials can be identified according to their chemical makeup, that is, the atoms that form their molecules.