Elements, Compounds, and MixturesA pure substance can be either an element or a compound. Elements are those pure substances that cannot be decomposed by ordinary chemical means such as heating, electrolysis, or reaction. Gold, silver, and oxygen are examples of elements. Compounds are pure substances formed by the combination of elements; they can be decomposed by ordinary chemical means. Baking soda is a compound; it contains the elements sodium, hydrogen, carbon, and oxygen, and it decomposes on heating.

Mercury(II) oxide is another compound; it contains the elements mercury and oxygen, and on heating it decomposes to those elements.

Mercury(II) Oxide

Compounds differ from mixtures in that the elements in a compound are held together by chemical bonds and cannot be separated by differences in their physical properties. The components of a mixture are not joined together by any chemical bonds, and they can be separated from one another by differences in their physical properties.Figure 3.1 reviews the relations between different kinds of matter. Notice that mixtures can be separated into their components by differences in physical properties. Compounds can be separated into their components only by chemical change.

FIGURE 3.1The differences between the various kinds of matter.

Atoms - The Atomic TheoryBy the end of the eighteenth century, experimenters had well established that each pure substance had its own characteristic set of properties such as density, specific heat, melting point, and boiling point. Also established was the fact that certain quantitative relationships, such as the Law of Conservation of Mass, governed all chemical changes. But there was still no understanding of the nature of matter itself. Was matter continuous, like a ribbon from which varying amounts could be snipped, or was it granular, like a string of beads from which only whole units or groups of units could be removed? Some scientists believed strongly in the continuity of matter, whereas others believed equally strongly in granular matter; both reasonings were based solely on speculation.In 1803, an English schoolmaster named John Dalton (1766-1844) summarized and extended the then-current theory of matter. The postulates of his theory, changed only slightly from their original statement, form the basis of modern atomic theory. Today, we express these four postulates as:1. Matter is made up of tiny particles called atoms. (A typical atom has a mass of approximately 10-23g and a radius of approximately 10-10m.)2. Over 100 different kinds of atoms are known; each kind is an element. All the atoms of a particular element are alike chemically but can vary slightly in mass and other physical properties. Atoms of different elements have different masses.3. Atoms of different elements combine in small, whole-number ratios to form compounds. For example, hydrogen and oxygen atoms combine in a ratio of 2:1 to form the compound water, H2O. Carbon and oxygen atoms combine in a ratio of 1:2 to form the compound carbon dioxide, CO2. Iron and oxygen atoms combine in a ratio of 2:3 to form the familiar substance rust, Fe2O3.4. The same atoms can combine in different whole-number ratios to form different compounds. As just noted, hydrogen and oxygen atoms combined in a 2:1 ratio form water; combined 1:1, they form hydrogen peroxide, H2O2(Figure 3.2). Carbon and oxygen atoms combined in a 1:2 ratio form carbon dioxide; combined in a 1:1 ratio, they form carbon monoxide, CO.FIGURE 3.2Atoms of the same elements combine in different ratios to form different compounds.

The ElementsElements are pure substances. The atoms of each element are chemically distinct and different from those of any other element. Approximately 110 elements are now known. By 1980, 106 of these had been unequivocally characterized and accepted by the International Union of Pure and Applied Chemistry (IUPAC). Since that time, elements 107 and 109 have been identified among the products of a nuclear reaction. The search for new elements continues in many laboratories around the world; new elements may be announced at any time.

A. Names and Symbols of the ElementsEach element has a name. Many of these names are already familiar to you - gold, silver, copper, chlorine, platinum, carbon, oxygen, and nitrogen. The names themselves are interesting. Many refer to a property of the element. The Latin name for gold isaurum,meaning "shining dawn." The Latin name for mercury,hydrargyrum,means "liquid silver."The practice of naming an element after one of its properties continues. Cesium was discovered in 1860 by the German chemist Bunsen (the inventor of the Bunsen burner). Because this element imparts a blue color to a flame, Bunsen named it cesium from the Latin wordcaesius,meaning "sky blue."Other elements are named for people. Curium is named for Marie Curie (1867-1934), a pioneer in the study of radioactivity. Marie Curie, a French scientist of Polish birth, was awarded the Nobel Prize in Physics in 1903 for her studies of radioactivity. She was also awarded the Nobel Prize in Chemistry in 1911 for her discovery of the elements polonium (named after Poland) and radium (Latin,radius,"ray").Some elements are named for places. The small town of Ytterby in Sweden has four elements named for it: terbium, yttrium, erbium, and ytterbium. Californium is another example of an element named for the place where it was first observed. This element does not occur in nature. It was first produced in 1950 in the Radiation Laboratory at the University of California, Berkeley, by a team of scientists headed by Glenn Seaborg. Seaborg was also the first to identify curium at the metallurgical laboratory at the University of Chicago (now Argonne National Laboratory) in 1944. Seaborg himself was named a Nobel laureate in 1951 in honor of his pioneering work in the preparation of other unknown elements.Each element has a symbol, one or two letters that represent the element much as your initials represent you. The symbol of an element represents one atom of that element. For 14 of the elements, the symbol consists of one letter. With the possible exceptions of yttrium (Y) and vanadium (V), you are probably familiar with the names of all elements having one-letter symbols. These elements are listed in Table 3.1. For 12 of these elements, the symbol is the first letter of the name.Potassium was discovered in 1807 and named for potash, the substance from which potassium was first isolated. Potassium's symbol, K, comes fromkalium, the Latin word for potash. Tungsten, discovered in 1783, has the symbol W, for wolframite, the mineral from which tungsten was first isolated.TABLE 3.1Elements with one-letter symbols

SymbolElement

Bboron

Ccarbon

Ffluorine

Hhydrogen

Iiodine

Nnitrogen

Ooxygen

SymbolElement

Pphosphorus

Kpotassium

Ssulfur

Wtungsten

Uuranium

Vvanadium

Yyttrium

Most other elements have two-letter symbols. In these two-letter symbols, the first letter is always capitalized and the second is always lowercased. Eleven elements have names (and symbols) beginning with the letter C. One of these, carbon, has a one-letter symbol, C. The other ten have two-letter symbols (see Table 3.2).TABLE 3.2Elements whose name begins with the letter C

SymbolElement

Cdcadmium

Cacalcium

Cfcalifornium

Ccarbon

Cecerium

Cscesium

SymbolElement

Clchlorine

Crchromium

Cocobalt

Cucopper

Cmcurium

B. Lists of the ElementsWhile you study chemistry, you will often need a list of the elements.To see a list of the elements click here.The list includes the symbol, the atomic number, and the atomic weight of the element. The significance of atomic numbers and weights will be discussed in Chapter 4. For now it is sufficient to know that each element has a number between 1 and 110 called itsatomic number.This number is as unique to the element as its name or symbol.The second list, called the periodic table, arranges the elements in order of increasing atomic number in rows of varying length. The significance of the length of the row and the relation among elements in the same row or column will be discussed in Chapter 5. The periodic table appears by clicking on the inside of the front cover of this text. Throughout the text we will refer to the periodic table, because it contains an amazing amount of information. For now you need only be aware that elements in the same column have similar properties and that the heavy stair-step line that crosses the table diagonally from boron (B) to astatine (At) separates the metallic elements from the nonmetallic elements. The periodic table is also shown in Figure 3.3. The screened areas mark the elements you will encounter most often in this text.

1. Metals and nonmetalsMetals appear below and to the left of the heavy diagonal line in the periodic table. The characteristic properties of a metal are:1. It is shiny and lustrous.2. It conducts heat and electricity.3. It is ductile and malleable; that is, it can be drawn into a wire and can be hammered into a thin sheet.4. It is a solid at 20C. Mercury is the only exception to this rule; it is a liquid at room temperature. Two other metals, gallium and cesium, have melting points close to room temperature (19.8C and 28.4C).Nonmetals vary more in their properties than do metals; some may even have one or more of the metallic properties listed. Some nonmetals are gaseous; chlorine and nitrogen are gaseous nonmetals. At 20C one nonmetal, bromine, is a liquid, and others are solids - for example, carbon, sulfur, and phosphorus.Bromine

Carbon

Sulfur

Red Phosphorus

C. Distribution of the ElementsThe known elements are not equally distributed throughout the world. Only 91 are found in either the Earth's crust, oceans, or atmosphere; the others have been produced in laboratories. Traces of some but not all of these elements have been found on Earth or in the stars. The search for the others continues. You might read of its success or of the isolation of new elements as you study this text.TABLE 3.3Distribution of elements in the Earth's crust, oceans, and atmosphere

ElementPercent oftotal mass

oxygen49.2

silicon25.7

aluminum7.50

iron4.71

calcium3.39

sodium2.63

potassium2.40

magnesium1.93

hydrogen0.87

titanium0.58

ElementPercent oftotal mass

chlorine0.19

phosphorous0.11

manganese0.09

carbon0.08

sulfur0.06

barium0.04

nitrogen0.04

fluorine0.03

all others0.49

Table 3.3 lists the 18 elements that are most abundant in the Earth's crust, oceans, and atmosphere, along with their relative percentages of the Earth's total mass. One of the most striking points about this list is the remarkably uneven distribution of the elements (see Figure 3.4). Oxygen is by far the most abundant element. It makes up 21% of the volume of the atmosphere and 89% of the mass of water. Oxygen in air, water, and elsewhere constitutes 49.2% of the mass of the Earth's crust, oceans, and atmosphere. Silicon is the Earth's second most abundant element (25.7% by mass). Silicon is not found free in nature but occurs in combination with oxygen, mostly as silicon dioxide (SiO2), in sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal, as well as in various silicate minerals such as granite, asbestos, clay, and mica. Aluminum is the most abundant metal in the Earth's crust (7.5%). It is always found combined in nature. Most of the aluminum used today is obtained by processing bauxite, an ore that is rich in aluminum oxide. These three elements (oxygen, silicon, and aluminum) plus iron, calcium, sodium, potassium, and magnesium make up more than 97% of the mass of the Earth's crust, oceans, and atmosphere. Another surprising feature of the distribution of elements is that several of the metals that are most important to our civilization are among the rarest; these metals include lead, tin, copper, gold, mercury, silver, and zinc.FIGURE 3.4Relative percentages by mass of elements in the Earth's crust, oceans, and atmosphere.

The distribution of elements in the cosmos is quite different from that on Earth. According to present knowledge, hydrogen is by far the most abundant element in the universe, accounting for as much as 75% of its mass. Helium and hydrogen together make up almost 100% of the mass of the universe.Table 3.4 lists the biologically important elements - those found in a normal, healthy body. The first four of these elements - oxygen, carbon, hydrogen, and nitrogen--make up about 96% of total body weight (see Figure 3.5). The other elements listed, although present in much smaller amounts, are nonetheless necessary for good health.TABLE 3.4Biologically important elements (amounts given per 70-kg body weight)

MajorelementsApproximateamount (kg)

oxygen45.5

carbon12.6

hydrogen7.0

nitrogen2.1

calcium1.0

phosphorous0.70

magnesium0.35

potassium0.24

sulfur0.18

sodium0.10

chlorine0.10

iron0.003

zinc0.002

Elements present in lessthan 1-mg amounts(listed alphabetically)

arsenic

chromium

cobalt

copper

fluorine

iodine

manganese

molybdenum

nickel

selenium

silicon

vanadium

FIGURE 3.5The distribution of elements (by mass) in the human body.

D.How Elements Occur in NatureElements occur as single atoms or as groups of atoms chemically bonded together. The nature of these chemical bonds will be discussed in Chapter 7. Groups of atoms bonded together chemically are called molecules or formula units.Molecules may contain atoms of a single element, or they may contain atoms of different elements (in which case the molecule is of a compound.) Just as an atom is the smallest unit of an element, a molecule is the smallest unit of a compound - that is, the smallest unit having the chemical identity of that compound.Let us consider how the elements might be categorized by the way they are found in the universe.1. The noble gasesOnly a few elements are found as single, uncombined atoms; Table 3.5 lists these elements. Under normal conditions, all of these elements are gases; collectively, they are known as the noble gases. They are also called monatomic gases, meaning that they exist, uncombined, as single atoms (monomeans "one"). The formula for each of the noble gases is simply its symbol. When the formula of helium is required, the symbol He is used. The subscript 1 is understood.TABLE 3.5The noble gases

SymbolElement

Hehelium

Neneon

Arargon

Krkrypton

Xexenon

Rnradon

2. MetalsPure metals are treated as though they existed as single, uncombined atoms even though a sample of pure metal is an aggregate of billions of atoms. Thus, when the formula of copper is required, its symbol, Cu, is used to mean one atom of copper.

Copper Metal

3. NonmetalsSome nonmetals exist, under normal conditions of temperature and pressure, as molecules containing two, four, or eight atoms. Those nonmetals that occur as diatomic (two-atom)molecules are listed in Table 3.6. Thus, we use O2as the formula for oxygen, N2for nitrogen, and so on. Among the nonmetals, sulfur exists as S8and phosphorus is found as P4. For other nonmetals (those not listed in Table 3.5 or 3.6) a monatomic formula is used - for example, As for arsenic and Se for selenium.TABLE 3.6Diatomic elements

FormulaNameNormal state

H2hydrogencolorless gas

N2nitrogencolorless gas

O2oxygencolorless gas

F2fluorinepale yellow gas

Cl2chlorinegreenish yellow gas

Br2brominedark red liquid

I2iodineviolet black solid

4. CompoundsAlthough many elements can occur in the uncombined state, all elements except some of the noble gases are also found combined with other elements in compounds. In Section 3.1 we defined a compound as a substance that can be decomposed by ordinary chemical means. A compound can also be defined as a pure substance that contains two or more elements. The composition of a compound is expressed by a formula that uses the symbols of all the elements in the compound. Each symbol is followed by a subscript, a number that shows how many atoms of the element occur in one molecule (the simplest unit) of the compound; the subscript 1 is not shown. Water is a compound with the formula H2O, meaning that one molecule (or formula unit) of water contains two hydrogen atoms and one oxygen atom. The compound sodium hydrogen carbonate has the formula NaHCO3, meaning that a single formula unit of this compound contains one atom of sodium, one atom of hydrogen, one atom of carbon, and three atoms of oxygen. Notice that the symbols of the metals in sodium hydrogen carbonate are written first, followed by the nonmetals, and that, of the nonmetals, oxygen is written last. This order is customary.Sometimes a formula will contain a group of symbols enclosed in parentheses as, for example, Cu(NO3)2. The parentheses imply that the group of atoms they enclose act as a single unit. The subscript following the parenthesis means that the group is taken two times for each copper atom.

Copper Nitrate

The properties of a compound are quite unlike those of the elements from which it is formed. This fact is apparent if we compare the properties of carbon dioxide, CO2(a colorless gas used in fire extinguishers), with those of carbon (a black, combustible solid) and oxygen (a colorless gas necessary for combustion). The properties of compounds are discussed in greater detail in Chapter 6.

The Reactions of Elements:Simple EquationsA study of chemistry involves the study of chemical changes or, as they are more commonly called, chemical reactions. Examples of chemical reactions are: the combination of elements to form compounds, the decomposition of compounds (such as sodium hydrogen carbonate or mercury(II) oxide), and reactions between compounds, such as the reaction of vinegar (a solution of acetic acid) with baking soda (sodium hydrogen carbonate). Reactions are usually described using chemical equations. Equations may be expressed in words: Mercury(II) oxide decomposes to mercury and oxygen. Using formulas, we state this reaction as:2 HgO2 Hg + O2A chemical equation has several parts: The reactants are those substances with which we start (here mercury(II) oxide, HgO, is the reactant). The arrow () means "reacts to form" or "yields." The products are those substances formed by the reaction (here mercury and oxygen are the products). The numbers preceding the formulas are called coefficients. Sometimes the physical state of the reaction component is shown; we use a lowercase, italic letter in parentheses following the substance to show its state. For example, if the equation for the decomposition of mercury(II) oxide were written as:2 HgO(s)2 Hg(l) + O2(g)we would know that the mercury(II) oxide was a solid, the mercury was a liquid, and the oxygen was a gas when the reaction was carried out. The same equation is repeated below with all the parts labeled:

Table 3.7 lists the parts of an equation and the notations commonly used.TABLE 3.7Parts of an equation

ReactantsThe starting substances, which combine in the reaction. (Formulas must be correct.)

ProductsThe substances that are formed by the reaction. (Formulas must be correct.)

Arrows

Found between reactants and products, means "reacts to form."

Used between reactants and products to show that the equation is not yet balanced.

Placed after the formula of a product that is a gas.

Placed after the formula of a product that is an insoluble solid, also called a precipitate.

Physical stateIndicates the physical state of the substance whose formula it follows.

(g) Indicates that the substance is a gas

(l) Indicates that the substance is a liquid

(s) Indicates that the substance is a solid

(aq) Means that the substance is in aqueous (water) solution

CoefficientsThe numbers placed in front of the formulas to balance the equation.

ConditionsWords or symbols placed over or under the horizontal arrow to indicate conditions used to cause the reaction.

Heat is added

hvLight is added

elecElectrical energy is added

A. Writing Chemical EquationsA correctly written equation obeys certain rules.1. The formulas of all reactants and products must be correct.Correct formulas must be used. An incorrect formula would represent a different substance and therefore completely change the meaning of the equation. For example, the equation2 H2O22 H2O + O2describes the decomposition of hydrogen peroxide. This reaction is quite different from the decomposition of water, which is described by the equation2 H2O2 H2+ O2When an uncombined element occurs in an equation, the guidelines in Section 3.3D (parts 1, 2 and 3) should be used to determine its formula.

2. An equation must be balanced by mass.An equation is balanced by mass when the number of atoms of each element in the reactants equals the number of atoms of that element in the products. For example, the equation shown for the decomposition of water has four atoms of hydrogen in the two molecules of water on the reactant side and four atoms of hydrogen in the two molecules of hydrogen gas on the product side; therefore, hydrogen is balanced. It has two atoms of oxygen in the two reacting molecules of water and two atoms of oxygen in the single molecule of oxygen produced; therefore, oxygen is also balanced.2 H2O2 H2+ O2four (2 X 2) H atoms on the left = four (2 X 2) H atoms of the righttwo (2 X 1) O atoms on the left = two (1 X 2) O atoms on the rightWhen the atoms are balanced, the mass is balanced and the equation obeys the Law of Conservation of Mass.

You can write and balance equations in three steps:1. Write the correct formulas of all the reactants. Use a plus sign (+) between the reactants and follow the final reactant with an arrow. After the arrow, write the correct formulas of the products, separating them with plus signs.2. Count the number of atoms of each element on each side of the equation. Remember that all elements present must appear on both sides of the equation.3. Change the coefficients as necessary so that the number of atoms of each element on the left side of the equation is the same as that on the right side. Only the coefficients may be changed to balance an equation; the subscripts in a formula must never be changed.