The study of chemistry is the study of matter. Matter is defined as any substance that has mass and takes up space. Therefore the study of chemistry includes any thing. One of the few examples of non-matter is energy, such as heat and light. However, most changes in matter include changes in energy, and chemists also study the energy that accompanies the changes in matter (Ch 6 Thermodynamics). Normally, the first steps in studying something are to describe and categorize it. Chemistry is no exception. Matter can be described as having either physical properties or chemical properties. Physical properties can be observed without changing the identity of the substance. Physical properties include such qualities as color, size, and luster. Chemical properties are observed when matter actually changes its identity. For example, a chemical property of water is that electricity can transform it into oxygen gas and hydrogen gas. Chemical and physical properties are used to categorize matter. One category of matter is as either a mixture or a pure substance. Mixtures can be separated physically, that is, the components can be separated without changing their identity. Physical separations can be as simple as sorting M&Ms by color, or they can be more complicated, such as the process of distillation, which requires turning one component into a gas while leaving another in its liquid state. One characteristic of mixtures that is useful to help identify mixtures, is that their composition is variable. For example, a mixture of sugar and water might be very sweet or only slightly sweet. The sugar and the water retain their identities whether they are mixed or not. If you heat the sugar water until all the water is boiled off, the sugar will remain. If you collect the gas as you boil, you can get the pure water. The process involves physical separation of the sugar and water. On the other hand, pure substances cannot be separated physically. If a pure substance is separated—and this is not always possible—it is separated chemically, yielding the same ratio of the components every time; these components have a different identity than when they were first combined. Mixtures can be divided into two categories. Homogeneous mixtures have a uniform composition. Sugar and water provide an example of this. Whatever portion of the mixture is sampled will have the same characteristics. The composition of heterogeneous mixtures depends on the location within the mixture. For example, variations in color across the surface of this page show that it is a heterogeneous mixture. Pure substances can also be divided into two categories, elements and compounds. Compounds can be separated chemically, divided into two or more components. However, these components will differ in identity from the compound and from each other. The identity and mass ratio of these components will be the same in a specific compound. For example, water can be chemically divided into hydrogen and oxygen, and the oxygen produced will always weigh eight times more than the resulting hydrogen. Elements cannot be chemically separated. All known elements are listed, usually by symbol, in the periodic table. Elements that are chemically combined create compounds. The smallest fraction of an element that still retains the identity of the element is called an atom. Atoms are made of even smaller particles called protons, neutrons and electrons. Protons are positively charged particles with a mass of about 1 atomic mass unit (amu). The identity of an element is defined as its number of protons. The number of protons is also called an element's atomic number (Z) and is listed with the element in the periodic table. Atoms are electrically neutral, so there is also a negatively charged particle called an electron. To maintain the zero charge, the number of electrons and protons must be the same. If protons and electrons are not equal, the atom will have a charge and be called an ion. Electrons are very lightweight, even compared with a proton. In fact, their mass is approximately 0 amu. Like the proton, the neutron has a mass of 1 amu. However, it does not have a charge. Therefore the sum of the protons and neutrons is called the mass number (A). It is possible for atoms to have the same number of protons but different numbers of neutrons; these atoms are called isotopes. Elements prefer some proton/neutron combinations above others. The percent of each isotope in an element is called its isotopic abundance. The average atomic mass takes into account not only all the naturally occurring isotopes, but also the relative abundance of each. The average atomic mass is also listed in the periodic table. To represent a type of atom, its symbol (an uppercase letter sometimes followed by a lowercase letter) is used. The symbol and the atomic number are two ways of expressing the same idea. To also include the number of neutrons, the mass number is included as a superscript before the symbol. The symbol together with the mass number is called isotopic notation. Radioactive elements will disintegrate. The rate at which they do this is normally expressed as half-life, which is the time required for half of the initial quantity to disintegrate. The periodic table not only lists all the elements, their symbols, atomic numbers, and average atomic mass, but does so in such a way as to group them by properties. Many periodic tables, including the one on the inside front cover of the text, have a staircase-like division that runs between aluminum (Al) and silicon (Si) and continues between germanium (Ge) and arsenic (As) to the bottom of the table. This "staircase" provides a useful way to categorize elements. Those to the left of the staircase are metals. Properties of metals include a metallic luster (they are shiny), malleability (it dents), and ductility (they can be pulled into a wire). Elements to the right of the staircase are nonmetals, which are neither malleable nor ductile. Elements that actually touch the staircase are metalloids. Metalloids tend to have the physical properties of metals, such as luster, and the chemical properties of nonmetals. Compounds can also be divided into categories. Molecular compounds are usually combinations of nonmetal atoms that are chemically bonded into a group, called a molecule. Ionic compounds are held together by oppositely charged atoms or groups of atoms called ions. Ions with a positive charge are called cations and are usually formed from metal atoms. Ions with a negative charge are usually formed from nonmetals and are called anions. An ionic compound will have anions and cations in a ratio that has a net charge of zero.