blocks and cannot be broken apart by normal chemical means. The truth is that they are also mostly empty space, a dense nucleus of protons and neutrons surrounded by a cloud of electrons. An atom is almost unimaginably small. It would take about 50 million atoms in a row to measure one centimeter. Nevertheless, everything that you touch is made up of atoms. As insubstantial as they may seem, atoms form our familiar physical world. Elements are the fundamental building materials of nature. There are fewer than 100 elements that occur naturally. Each element is comprised of only one kind of atom, and each element has distinctive characteristics. Carbon, hydrogen, and oxygen are examples of elements. Most matter consists of molecules, which are groups of atoms bonded together. A molecule is the smallest identifiable unit into which a substance can be divided and still retain its composition and chemical properties. A few molecules are composed of the atoms of a single element. For instance, oxygen normally occurs as a molecule of two oxygen atoms (its chemical formula is O2). Most molecules are compounds, that is they contain the atoms of more than one element. Compounds exhibit distinctive characteristics, which are different from the characteristics of any of their constituent elements. The ratio of elemental atoms in a compound is fixed. Every molecule of water (H2O) contains two atoms of hydrogen and one atom of oxygen. Carbon dioxide molecules (CO2) contain one atom of carbon and two atoms of oxygen, while a molecule of carbon monoxide (CO)–a completely different substance– contains one atom each of carbon and oxygen. Glucose sugar (C6H12O6) is also a molecule composed of carbon, hydrogen, and oxygen, but it is nothing like either water or carbon dioxide. Some compounds, such as salt (sodium chloride, NaCl), are made up of electrically charged ions rather than actual bonded molecules. The Three States of Matter The three fundamental states of matter are solid, liquid, and gas. A solid has a definite shape and volume. The atoms of a solid are relatively ordered and structured. A liquid has a definite volume but no definite shape. The molecules of a liquid flow, and a liquid Chapter 3- Diving Physics will take the shape of its container. A gas has neither definite volume nor definite shape. A gas will expand to fill any closed container and it can be compressed into a smaller container such as a scuba cylinder. Gases and liquids are collectively referred to as fluids because they will flow. It is possible for most substances to exist in all three states, depending on temperature and pressure. Solid water is ice; liquid water is water; and gaseous water is steam or water vapor. GASES IN DIVING Air is the gas that is most familiar to us. It is the mixture of gases comprising our atmosphere, and recreational scuba divers usually breathe compressed air. Air is primarily composed of nitrogen and oxygen, which, along with some other gases, are present in nearly constant concentrations (figure 3-2). Some gases in the air, such as water vapor and carbon dioxide, are present in varying concentrations depending on place and time. The primary component gases of air are: Component Gas Percent by Volume Nitrogen (N2) 78.084 Oxygen (O2) 20.946 Argon (Ar) 0.934 Carbon Dioxide (CO2) 0.033 (average) The concentration of carbon dioxide actually varies depending on place and season, and much of the concern about global warming is due to the increasing Diving Physics 69 Nitrogen (N2) 78.084% Oxygen (O ) 20.946% Argon (Inert) 0.934% Carbon Dioxide (CO ) 0.033% and Rare Gases 0.003% 2 2 FIGURE 3-2. COMPONENTS OF DRY AIR
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