Working through this chapter of the study guide will enable you to:

1. Understand the ways in which matter is classified by chemists.
2. Distinguish among types of solutions and tell how they form.
3. Identify the most common elements in our environment.
4. Use the periodic table to predict and explain the chemical properties of elements and explain why elements are collected into groups and periods as they are in the periodic table.
5. Name some of the basic inorganic compounds, and write their chemical formulas.

Discussion

People interested in the composition, structure, and transformation of matter have developed a specific science for this study; it is called chemistry. The five major divisions of chemistry specialize in certain areas of this study, and together they give us comprehensive understanding of the various elements themselves. They further show us the ways in which elements combine to form simple and complex compounds. It is also possible to combine elements in physical (non-chemical) ways to produce heterogeneous and homogeneous mixtures. One important class of mixtures is the aqueous solution, in which water serves as the solvent.

Specific names, symbols, and classification schemes have been developed to aid in the study of chemistry. Once we have learned these, it is possible to discuss the elements in an intelligent way and write down the forms and properties of the basic elements and the various compounds formed from their combinations. It is also helpful to classify elements as metals and nonmetals and to study how these classes of elements interact and combine. It is somewhat surprising that all of the diverse materials encountered in our everyday lives can be constructed using only about 100 atomic elements as the basic building blocks. It is even more of a wonder that most of these elements are found only in relatively small amounts, and that a dozen or so chemical elements actually make up well over 90% of all matter on Earth.

The periodic table is the most useful of the classification systems used in chemistry. In this table we can find the basic structure of the atoms of all of the known elements, and from their locations in specific groups and periods, make accurate predictions of their properties and reaction capabilities. This is due to the fact that the electrons surrounding the nucleus of a given atom arrange themselves in a definite shell structure, and the number of electrons in the outermost shell controls how each element can combine with other elements to form compounds. Once we understand this shell theory, we have a good basis for understanding the structure of matter throughout the entire universe. The periodic table thus provides us with a well-organized summary of chemical properties and reaction capabilities.


Section  11.1Classification of Matter

In chemical terms a sample of matter can be classified as either a pure substance or a mixture. Pure substances include elements and also chemical compounds, which have fixed proportions by mass. Compounds can be separated into individual elements only by chemical processes. Mixtures are made up of two or more substances that are not always combined in the same proportions. Mixtures are not made by chemical processes; instead, physical processes are necessary to assemble mixtures or to separate them into pure substances. Homogeneous mixtures are usually referred to as solutions.

A special kind of solution is produced when water is the solvent. Such solutions are called aqueous solutions (aq). It is possible to dissolve varying amounts of solute in water, or any other solvent, up to a maximum quantity, which is determined by the type of solute and the temperature of the solution. This maximum is referred to as the solubility of a particular solute. Values for the solubility of several common solutes can be found in Fig. 11.5 in the textbook. A solution in which more solute can be dissolved at a given temperature is said to be unsaturated, but if the maximum amount has been dissolved, the solution is said to be saturated. It is even possible under certain conditions to have more solute dissolved in a solution than the normal solubility allows, and such solutions are said to be supersaturated and are unstable. If only a few crystals of the solute are introduced into the supersaturated solution, the excess solute will precipitate and the solution will return to a saturated state.


Section  11.2Discovery of the Elements

The old Greek concept that all matter is composed of four basic "elements"—earth, air, fire, and water—served for several thousand years as the basis for explaining the structure of matter. With the importance of laboratory testing established by Robert Boyle in 1661, a new definition of element was initiated. Boyle's definition was that an element is any substance that cannot be separated into smaller components, regardless of the method employed. Further investigation showed that some of the substances already isolated by early scientists fit this definition. These included gold, iron, copper, sulfur, mercury, and seven others. Continued experimentation added many more elements to the list. Today a total of 114 individual elements are known, some of which have been synthesized by scientists and are not found at all in nature.

A system of symbols was developed to identify individual chemical elements. In general, this system uses the first one or two letters of the element's name. Early symbols used an element's Latin name, but most elements designated since 1850 use English names. These symbols are used not only in designating elements and writing chemical equations but also are indispensable in forming the periodic table of the elements. The names and symbols of the elements can be found in the periodic table in the inside front cover of the textbook.


Section  11.3Occurrence of the Elements

Although about 90 elements occur naturally, the majority of matter around us is made up of only a few of them. The following list shows some of the most abundant and where they are found. Notice that although our own environment has relatively high percentages of the heavier elements, the universe as a whole is still composed almost entirely of the hydrogen and helium that were probably formed in the Big Bang when the universe was first created.

Earth's crust	O (47%) and Si (27%)
Earth's core	Fe (85%), Ni (15%)
Earth's atmosphere	N (78%), O (21%), Ar (1%)
The Human Body	O (65%), C (18%)
Entire universe	H (75%), He (24%)

Molecules are formed from two or more atoms that have been combined chemically. Some elements can exist as individual atoms, but other are so chemically active that even in their pure form they combine into two-atom molecules, which are therefore referred to as diatomic. For this reason, free hydrogen is always written as H₂ instead of just H. When the atoms of two different elements unite into a molecule, they form a compound, and their chemical symbols are combined to provide a molecular formula for that compound. An example is the combination of carbon and oxygen into CO₂ (carbon dioxide). Note that subscripts are used to designate the number of atoms of each element that are present in the molecule. In this example, one carbon atom (the subscript 1 is never written, but is simply assumed) and two oxygen atoms form the molecule.

Some elements can combine in two or more ways by using different bonding structures. If these multiple combinations occur in the same physical phase, they are called allotropes. These are not multi-element compounds but are made up entirely of atoms of the same element. Examples of allotropes are graphite, diamond, and buckminsterfullerene, which are three forms of carbon that are all solids, and oxygen, which can exist in two gaseous forms—common diatomic oxygen (O₂) and highly reactive ozone (O₃).


Section  11.4The Periodic Table

The periodic table is an orderly arrangement of the chemical elements into horizontal rows called periods and vertical columns called groups. The elements are placed in the table in order of increasing atomic number and in such a way that those having similar chemical properties go into the same column. This is the basis for the famous periodic law of chemical properties. Periodic ordering comes from the shell electron configuration for each element, because atoms with the same number of electrons in their outer electron shells will have similar chemical properties. This outer electron shell is called the valence shell, and the electrons therein are termed valence electrons. The periodic table also allows us to classify elements into three subgroups, called the representative elements, the transition elements, and the inner transition elements.

It should be noted that other properties of atoms, such as their sizes and ionization energies, are also represented in a systematic fashion by the periodic table. In general, elements are classified as metals or nonmetals depending on whether they tend to lose their valence electrons or gain (sometimes share) electrons during chemical reactions. Metals are found on the left-hand side of the staircase line of the periodic table, whereas nonmetals are located to the right. A few of the elements bordering the staircase line are known as semimetals because they tend to display properties of both metals and nonmetals. Table 11.2 in the textbook lists the chemical and physical properties of the metals and nonmetals, and you should study these chemical and physical characteristics carefully.


Section  11.5Naming Compounds

When compounds are formed from atoms, their compositions can be described by the use of chemical formulas. Chemical formulas are quite easy to understand once it is realized that the chemical symbol for each element present is shown along with a subscript to indicate the number of each type of atom in that specific compound. (Remember, the subscript 1 is not written.) The names given to chemical compounds are somewhat more complicated. Some are special names that have come down through history and are still in use today. An example is H₂O, which is called water. Most compounds, however, are named using rules involving their makeup with regard to the presence of metal, nonmetal, and polyatomic ion components. These rules are discussed in the textbook and should be studied now so that you will be familiar with them as you progress through the remainder of the chapters on chemistry.


Section  11.6Groups of Elements

Elements are often catalogued by their group position (column) in the periodic table. Several groups of the representative elements are listed below, and you should be familiar with their members and general properties.

Group 8A	Noble gases	Complete valence shells; usually chemically inert
Group 1A	Alkali metals	One single valence electron; react readily with other elements, and so are called active metals
Group 7A	Halogens	One valence electron short of noble gas configuration; very active nonmetals
Group 2A	Alkaline earth metals	Two valence electrons; active metals but not as active as Group 1A

Hydrogen is a unique element because it sometimes reacts like an alkali metal and sometimes like a halogen. That is because it has only one electron in its valence shell and can easily lose it; but it also has to gain only one additional electron in its outer shell to reach a stable noble gas configuration. Hydrogen is a common component of complex molecules such as proteins, nucleic acids, and carbohydrates.

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