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3.1 The Element A. Abundances of Eleme B. Names and Symbols for the Elements 3.2 Atoms and Compounds A. Dalton's Atomic Theory B. Formulas of Compounds 3.3 Atomic Structure A. The Structure of the Atom • Thomson's Experiment • The Plum Pudding Model • Rutherford's Experiment B. Introduction to the Modern Concept of Atomic Structure C. Isotopes 3.4 Using the Periodic Table A. Introduction to the Periodic Table B. Natural States of the Elements 3.5 Ions and Their Compounds A. Ions • Cations and Anions • Ion Charges and the Periodic Table B. Compounds That Contain Ions T he chemical elements are essential to each of us in our daily lives. The most important element is carbon, which is found in virtually all of the molecules that make up the living cell. Although certain elements are present in our bodies in tiny amounts, these elements can have profound effects on our health and behavior. As we will see in this chapter, lithium can be a miracle treatment for someone with manicdepressive disease, and our cobalt levels can have a remarkable impact on behavior. Many elements in our bodies serve no useful purpose, but they are found in the food we eat, the water we drink, and the air we breathe. As a result, the human body typically contains significant amounts of elements such as aluminum, barium, strontium, uranium, and gold, which are usually deposited in the bones or liver. Many elements have great economic significance. For example, about 1 billion tons of iron and 60 million tons of aluminum are produced in the world each year and are used mainly in structural materials. Iron is also important in another way that we seldom think about. The earth's core, 4000 miles in diameter, consists mostly of molten iron. The center of the earth's core, with a diameter of about 1500 miles, may consist of solid iron. As we will see as we progress in our study of chemistry, most of the chemical elements have an important impact on our lives. Prereading Questions 1. What elements do you think are most abundant in the human body? 2. Do you know any chemical formulas for compounds? For example, 3. what is the chemical formula for water? What is the chemical formula for table salt? Do you know any other formulas? Have you heard the terms electrons, protons, neutrons, or nucleus! What do you know about these? What does the term periodic mean? How do you think this applies to the term periodic table? Name at least one element that normally exists as a solid and at least one element that normally exists as a gas. The Elements Key Term Element symbols Objectives I To learn about the relative abundances of the elements > To learn the names of some elements 1 To learn the symbols of some elements The Greeks were the first to try to explain why chemical changes occur. By about 400 B.C. they had proposed that all matter was composed of four fundamental substances: fire, earth, water, and air. The next 2000 years of chemical history were dominated by alchemy. Some alchemists were mystics and fakes who were obsessed with the idea of turning cheap metals into gold. However, many alchemists were sincere scientists, and this period saw important events: the elements mercury, sulfur, and antimony were discovered, and alchemists learned how to prepare acids. As we saw in Chapter 2, all of the earth's materials (and those of other parts of the universe) can be broken down chemically into about 100 different elements. At first it might seem amazing that the millions of known substances are composed of so few fundamental elements. Fortunately for those trying to understand and systematize it, nature often uses a relatively small number of fundamental units to assemble even extremely complex materials. For example, hemistry Explorers Robert Boyle (1627-1691) The first scientist to recognize the importance of careful measurements was Ireland's Robert Boyle. He is best known for his pioneering work on the properties of gases, but Boyle's most important contribution to science was probably his insistence that science should be firmly grounded in experiments. For example, Boyle held no preconceived notions about how many elements there might be. His definition of the term element was based on experiments: A substance was an element unless it could be broken down into two or more simpler substances. For example, air could not be an element, as the Greeks believed, because it could be broken down into many substances. As Boyle's experimental definition of an element became generally accepted, the list of known elements grew and the Greek system of four elements died. proteins, a group of substances that serve the human body in almost uncountable ways, are all made by linking together a few fundamental units to form huge molecules. A nonchemical example is the English language, in which hundreds of thousands of words are constructed from only 26 letters. Compounds are made by combining atoms of the various elements, just as words are constructed from the 26 letters of the alphabet. Words bed Compounds chief CO2 a, b,>,c, c, d, e, f, h, i ' H, O, N, C 'f > fad NH, ""bag H2O Did You Know? N2O Just as you had to learn the letters of the alphabet before you could learn to read and write, you need to learn the names and symbols of the chemical elements before you can read and write chemistry. The number of elements changes regularly as new elements are made in particle accelerators. A. Abundances of Elements At present, about 115 different elements are known, 88 of which occur naturally. (The rest have been made in laboratories.) The elements vary greatly in abundance. In fact, only nine elements account for most of the compounds found in the earth's crust. In Table 3.1, the elements are listed in order of abundance (mass percent) in the earth's crust, oceans, and atmosphere. Table 3.1 Distribution (Mass Percent) of the 18 Most Abundant Elements in the Earth's Crust, Oceans, and Atmosphere Element Mass Percent Element Mass Percent oxygen 49.2 titanium 0.58 silicon 25.7 chlorine 0.19 aluminum 7.50 phosphorus 0.11 iron 4.71 manganese 0.09 calcium 3.39 carbon 0.08 sodium 2.63 sulfur 0.06 potassium 2.40 barium 0.04 magnesium 1.93 nitrogen 0.03 hydrogen 0.87 fluorine 0.03 all others 0.49 •ft. Active Reading Question 1. What element is nearly half of the earth's crust, oceans, and atmosphere? 2. What percent of the mass of the crust, ocean, and atmosphere do the top nine elements make? 3.1 The Elements 53 Oxygen, in addition to accounting for about 20 percent of the earth's atmosphere (where it occurs as O2 molecules), is also found in virtually all rocks, sand, and soil on the earth's crust. In these materials, oxygen is not present as O2 molecules but exists in compounds that usually contain silicon and aluminum atoms. The list of elements found in living matter is very different from that for the earth's crust, as shown in Table 3.2. Oxygen, carbon, hydrogen, and nitrogen form the basis for all biologically important molecules. Some elements found in the body (called trace elements) are crucial for life, even though they are present in relatively small amounts (Table 3.3). For example, chromium helps the body use sugars to provide energy. Table 3.2 Top Ten Elements in the Human Body Table 3.3 Trace Elements in the Human Body Element Mass Percent oxygen 65.0 arsenic carbon 18.0 chromium hydrogen 10.0 cobalt nitrogen 3.0 copper calcium 1.4 fluorine phosphorus 1.0 iodine magnesium 0.50 manganese potassium 0.34 molybdenum sulfur 0.26 nickel sodium 0.14 selenium Trace Elements silicon vanadium One more general comment is important at this point. As we have seen, elements are fundamental to understanding chemistry. However, students may be confused by the many different ways that chemists use the term element illustrated below. 54 Word Meaning Element Microscopic form Single atom of that element Element Macroscopic form Sample of that element large enough to weigh on a balance Element Generic form When we say the human body contains the element sodium or lithium, we do not mean that free elemental sodium or lithium is present. Rather we mean that atoms of these elements are present in some form. Chapter 3 Chemical Foundations: Elements, Atoms, and Ions B. Names and Symbols for the Elements The names of the chemical elements have come from many sources. Sometimes the names come from descriptions of the element's properties, sometimes the name reflects the place where the element was discovered, and sometimes the name honors a famous scientist. Sources of Names of Chemical Elements Greek, Latin, or German words describing a property of the element Place where the element was discovered Latin Greek gold aurum (shining down) lead plumbum (heavy) americium —» America germanium —» californium California polonium —» Poland chlorine color iodine color *—'^ i Germany bromine stench _Jj^t^^^^ ^itiBpB» francium —> France Famous scientists Albert Einstein einsteinium Alfred Nobel nobdium Marie Curie curium We often use abbreviations to simplify the written word. For example, it is much easier to put MA on an envelope than to write out Massachusetts, and we often write USA instead of United States of America. Likewise, chemists have invented a set of abbreviations or element symbols for the chemical elements. These symbols usually consist of the first letter or the first two letters of the element names. The first letter is always capitalized, and the second is not. For example, fluorine oxygen carbon F 0 C neon silicon Ne Si Sometimes, however, the two letters used are not the first two letters in the name. For example, zinc chlorine Zn Cl cadmium platinum Cd Pi Information In the symbol for an element, only the first letter is capitalized. 3.1 The Elements 55 Chemistry in Your World Trace Elements: Small but Crucial We all know that certain chemical elements, such as calcium, carbon, nitrogen, and iron, are essential for humans to live. Many other elements, present in tiny amounts, are also essential for life. proper calcium levels in the bones, and copper is involved in the production of red blood cells. In the Body In Our Personal Environment Exposure to trace elements in water, food, and air also affects our health. Exposure to aluminum in baked goods and cheese, as well as from cookware and utensils, is of concern to scientists. In Behavior Chromium assists in the metabolism of sugars, cobalt is present in vitamin Bi 2 , iodine is necessary for the proper functioning of the thyroid gland, manganese plays a role in maintaining Lithium has helped some people with bipolar I disorder, a disease in which a person's behavior | varies between inappropriate highs and the o blackest of depressions. e Studies on inmates at a prison in Illinois have shown that trace elements in the hair of prisoners are related to behaviors. There is an inverse relationship between the levels of cobalt in the hair and the degree of violence in the prisoners. The symbols for some other elements are based on the original Latin or Greek name. Current Name Original Name Symbol gold aurum Au lead plumbum Pb natrium sodium Na Fe iron ferrum A list of the most common elements and their symbols is given in Table 3.4. You can also see the elements represented on a table in the inside back cover of this text. We will explain the form of this table (which is called the periodic table) in later chapters. Active Reading Question 1. Give three examples of elements whose symbols are the first two letters of the name of the element. 2. Give three examples of elements with symbols in which the first letter is not the first letter of the name of the element. 56 Chapter 3 Chemical Foundations: Elements, Atoms, and Ions Table 3.4 The Names and Symbols of the Most Common Elements* Element Symbol Element Symbol aluminum Al lithium Li arsenic As mercury (hydrargyrum) Hg barium Ba neon Ne boron B nitrogen N bromine Br oxygen O calcium Ca platinum Pt carbon C potassium (kalium) K chromium Cr silicon Si cobalt Co silver (argentium) Ag copper (cuprum) Cu sodium (natrium) Na gold (aurum) Au sulfur S Pb zinc Zn lead (plumbum) *Where appropriate, the original name is shown in parentheses so that you can see the sources of some of the symbols. Section 3. leview Questions 1. Give four different chemical meanings for the word element and an example of each. 2. How many of the top ten elements in the human body are also top ten elements in the earth's crust, ocean, and atmosphere? What are these elements? 3. Make a list of elements that you think were named after a place. 4. Give the symbol for each of the given elements. a. iron b. zinc c. hydrogen d. lithium e. copper f. neon 5. Give the name for each of the given symbols. a. Ba b. Cl c. Si d. F e. N f. Br 6. The graph below shows the mass percentages of the five most abundant elements in the human body. Use this graph to answer the following questions. 1.6% Oxygen Carbon Hydrogen Nitrogen Calcium Phosphorus Others a. What is the fourth most abundant element by mass in the human body? b. Which element is almost twice as abundant by mass as hydrogen in the human body? c. What percentage of the human body is not made up of oxygen, carbon, or hydrogen? 3.1 The Elements 3.2 Key Term Law of constant composition Dalton's atomic theory Atoms and Compounds Objectives I To learn about Dalton's theory of atoms I To understand and illustrate the law of constant composition } To learn how a formula describes a compound's composition Atoms Compound Chemical formula A. Dalton's Atomic Theory As scientists of the eighteenth century studied the nature of materials, several things became clear: 1. Most natural materials are mixtures of pure substances. 2. Pure substances are either elements or combinations of elements called compounds. 3. A given compound always contains the same proportions (by mass) of the elements. For example, water always contains 8 g of oxygen for every 1 g of hydrogen, and carbon dioxide always contains 2.7 g of oxygen for every 1 g of carbon. This principle became known as the law of constant composition. It means that a given compound always has the same composition, regardless of where it comes from. John Dalton, an English scientist and teacher, was aware of these observations. In the early 1800s, he offered an explanation for them that became known as Dalton's atomic theory. The main ideas of this theory (model) can be stated as follows: Dalton's Atomic Theory 1. Elements are made of tiny particles called atoms. 2. All atoms of a given element are identical. 3. The atoms of a given element are different from those of any other element. 4. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms. 5. Atoms are indivisible in chemical processes. That is, atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together. Dalton's model successfully explained important observations such as the law of constant composition. This law makes sense because if a compound always contains the same relative numbers of atoms, it will always contain the same proportions by mass of the various elements. 58 Chapter 3 Chemical Foundations: Elements, Atoms, and Ions mistry Explorers John Dalton (1766-1844) Dalton was an English scientist who made his living as a teacher in Manchester. Although Dalton is best known for his atomic theory, he made contributions in many other areas, including meteorology (he recorded daily weather conditions for 46 years, producing a total of 200,000 data entries). A rather shy man, Dalton was color-blind to red (a special handicap for a chemist) and suffered from lead poisoning contracted from drinking substances that had been drawn through lead pipes. Like most new ideas, Dalton's model was not accepted immediately. However, Dalton was convinced that he was right and used his model to predict how a given pair of elements might combine to form more than one compound. Dalton pictured compounds as collections of atoms. For example, nitrogen and oxygen might form a compound containing one atom of nitrogen and one atom of oxygen (written NO), a compound containing two atoms of nitrogen and one atom of oxygen (written N 2 O), or a compound containing one atom of nitrogen and two atoms of oxygen (written NO 2 ). NO NO, N,O When the existence of these substances was verified, it was a triumph for Dalton's model. Because Dalton was able to predict correctly the formation of multiple compounds between two elements, his atomic theory became widely accepted. j==^_-i^L Active Reading Question How does Dalton's model explain the law of constant composition? B. Formulas of Compounds A compound is a distinct substance that is composed of the atoms of two or more elements and always contains exactly the same relative masses of those elements. In light of Dalton's atomic theory, this statement simply means that a compound always contains the same relative numbers of atoms of each element. For example, water always contains two hydrogen atoms for each oxygen atom. Reading Tip Here, relative refers to ratios. 3.2 Atoms and Compounds 59