Anything that occupies space and has mass is called matter. Matter can exist in any one of three physical states: solid, liquid, or gaseous.

Matter undergoes changes that may be either chemical changes or physical changes. When water is transformed into steam it still retains all the chemical properties of water; that is, only its physical state is changed. Upon cooling, it returns to its original liquid state. However, when a piece of wood is burned, the original substance disappears, and new substances are formed. This is a chemical change, the kind of change with which chemistry is primarily concerned.

All matter is made up of fundamental substances called elements. An element cannot be broken down or decomposed by ordinary chemical means. The smallest part of an element that can take part in chemical reactions is the atom, and the chemical properties of an element are really the properties of its atoms. Chemists study the structure of atoms of different elements to gain a better understanding of their chemical behavior.

An atom has a nucleus made up of protons, which are positively charged, and neutrons, which are uncharged. Outside the nucleus are one or more electrons, which are negatively charged and may be thought of as revolving about the nucleus. Chemical reactions between atoms involve only the arrangement of the electrons in the atom; the arrangement of the protons and neutrons in the nucleus is not affected. All the atoms of one element have the same chemical properties, but two or more atoms of an element may differ slightly in weight. These atoms, called isotopes of the element, have different numbers of neutrons.

The elements are classified into two general categories: metals and nonmetals. However, since some elements have properties of both classes, there is no sharp dividing line between the two groups. The metals are characterized by such physical properties as luster; high conductivity of heat and electricity; and high tensile strength, malleability, and ductility (that is, they may be hammered or rolled into sheets or drawn into wire). All the nonmetals except one are either brittle solids or gases at ordinary temperatures. The exception is bromine, a liquid. The solid nonmetals have lower tensile strength and are poorer conductors of heat and electricity than metals.

There are two distinctive properties that determine the chemical characteristics of the atoms of an element: (1) atomic number, and (2) atomic weight.

The positive charge on the nucleus is its atomic number. This is the number of protons in the nucleus and, since the whole atom is neutral, it also is the number of electrons an atom holds outside the nucleus. Isotopes of an element have the same atomic number. The range of atomic numbers is from 1 (for hydrogen) to more than 100.

Atomic weight values are determined by comparing the average weight of an element's atoms with the weight of the most abundant isotope of carbon, C-12, which is arbitrarily given the value 12. Since naturally occurring carbon is about 99 per cent C-12 and slightly more than 1 per cent C-13, the atomic weight of carbon is 12.011. A gram-atomic weight of an element is the amount whose weight in grams is numerically equal to the atomic weight. A gram-atomic weight of any element always contains the same number of atoms-6.02 x 1023 atoms. This number is called Avogadro's number.

A compound is a substance composed of two or more elements that are chemically combined. The elements may be present either as electrically charged atoms or groups of atoms called ions, or as atoms that are joined to form molecules.

Each molecule of a pure compound contains the same relative number of atoms. Thus, the water molecule is made up of 2 atoms of hydrogen combined with 1 atom of oxygen and is written symbolically H2O. Hydrogen and oxygen are gases; water is a liquid.

Just as atoms can be assigned a relative atomic weight, so molecules have a molecular weight. The molecular weight is obtained by adding together the atomic weights of the atoms present in the molecule. In the example above, the molecular weight of a molecule of water is 18 (16 for oxygen plus 1 for each of the two hydrogen atoms). A gram-molecular weight of a substance is the amount whose weight in grams is numerically equal to the molecular weight. A gram-molecular weight of any substance contains 6.02 x 1023 molecules.

A mixture is a material composed of two or more substances that are not chemically joined. These substances retain their individual properties and characteristics even though they have been mixed together. The proportions of a mixture, unlike those of a compound, can be varied. Furthermore, its constituents can be separated from each other by nonchemical means. For example, a mixture of sand and salt will have some of the characteristic taste of the salt and the gritty feel of the sand. It can be separated by dissolving out the salt in water to leave the sand. The water then can be evaporated away from the salt solution to reclaim the salt.

For the sake of convenience, chemists have developed a kind of shorthand system by which they can record chemical reactions. This system consists of: (1) symbols, which represent the atoms of the elements; (2) formulas, which show how these atoms are combined into molecules; and (3) equations, which express chemical changes between molecules and atoms.

Every element-except for new elements that have not yet been given official names-is represented by a definite symbol universally used in chemical literature and the same in all languages. The symbol stands for one atom of an element.

Every pure substance, either an element or a compound, can be chemically identified by a formula. The formula indicates what elements are present and how many atoms of each are combined in the molecule of the substance. Thus in the example above, H2O is the formula for the water molecule. Some formulas are very complicated. The formula for vitamin B12, for example, is C63H90CoN14O14P. There is only one atom each of phosphorus and cobalt but larger numbers of the carbon, hydrogen, oxygen, and nitrogen atoms in the molecule.

Sometimes elements exist as molecules that are combinations of like atoms. Hydrogen gas and oxygen gas are diatomic, H2 and O2. Some can be polyatomic, for instance, S8 or P4.

A chemical equation is a way of describing chemical changes. An equation indicates: (1) what substances enter into the reaction; (2) what substances result from the reaction; and (3) the number of molecules of each substance that enter into and result from the reaction. For example, when methane is mixed with oxygen, they combine, forming water and carbon dioxide. This reaction is expressed by the following equation:

CH4 + 2O2 2H2O + CO2

The equation indicates that one molecule of methane (CH4) combines with two molecules of oxygen (as indicated by the coefficient 2 in front of the formula for free oxygen, (O2) to form two molecules of water (H2O) and one molecule of carbon dioxide (CO2). Since the equation is balanced (has the same number of atoms of each element on both sides of the arrow), the reaction is complete-all the methane and all the oxygen combine into water and carbon dioxide.

Because gram-molecular weights of all substances have the same number of molecules, the equation also indicates that one gram-molecular weight of methane (16 grams, approximately) will combine with two gram-molecular weights of oxygen (64 grams) to form two gram-molecular weights of water (36 grams) and one gram-molecular weight of carbon dioxide (44 grams).

The capacity of an atom to combine with other atoms is referred to as its valence. This property of an atom results in its ability to combine with different numbers of atoms of different elements. Hydrogen is assigned a valence of +1; oxygen, of -2. The + and - designations are convenient to help visualize the properly constructed molecular formula as being neutral. Thus, H2O implies 2 x (+1) is balanced by 1 x (-2). In the case of lime, calcium oxide, the formula CaO implies that the valence of the calcium atom is +2, balancing oxygen's valence of -2. If the correct formula for hydrogen chloride is HCl, then chlorine must have a valence of -1. The prediction then can be made that the compound of calcium and chlorine must be CaCl2. Two atoms of chlorine, each -1, are needed to satisfy the +2 valence of calcium.

Modern chemical theory interprets valence in terms of the number and arrangement of atoms' electrons. A useful way of picturing the arrangement of the electrons is in terms of the shell model. According to this concept, an atom's electrons are arranged in groups (called shells) whose members are equidistant from the atom's center. The way in which an element combines with another generally depends on the number of electrons that occupy the outermost shell of an atom of the element. The outermost shell is thus called the valence shell and the electrons in it are called valence electrons.

In forming compounds, atoms tend to lose their valence electrons or to gain extra electrons to form a complete valence shell. In general, valence shells may contain up to eight electrons. An important exception is the valence shell of hydrogen; it can contain no more than two electrons. Most metals have only a small number of valence electrons and tend to lose them. In losing electrons, an atom becomes positively charged, forming a positive ion. Other elements, notably active nonmetals, have a tendency to capture extra electrons to complete the valence shell. In gaining electrons, an atom becomes a negative ion. Positive and negative ions may then be held together by electrical attraction to form a chemical compound. Common salt, NaCl is an example of this ionic, or electrovalent, type of bonding. Under other circumstances, atoms are thought of as being arranged into molecules in such a way that electrons are shared between them. The bonds that occur in this manner are called covalent bonds.

Modern chemistry has many laws. These laws are basic principles that appear to govern the chemical behavior of atoms and molecules. Although chemists generally have no means of watching or measuring individual atoms, their theories account very satisfactorily for the behavior of large numbers of atoms. Consequently they often speak in terms of atoms when describing chemical phenomena. The most important chemical laws are as follows:

• Law of Conservation of Mass. The atom is indestructible in chemical reactions. According to this principle, a chemical reaction leaves the total mass of the substances involved unchanged. Strictly speaking, the mass is the same only if the energy gained or lost in the reaction is taken into account. However, in chemical reactions, the energies are relatively small and the changes in mass are too small to be detected.
• The Law of Constant Composition. In making any given compound, the atoms of the elements involved always combine in exactly the same way. In other words, a compound can have only one formula. Thus the proportion of the atomic weights of the elements in a compound will always be the same. For example, the compound hydrogen peroxide, H2O2, must be different from H2O, water.
• Law of Multiple Proportions. Atoms combine with each other only in whole numbers; no molecule will hold only a fraction of an atom. A formula such as HO1/2 is not possible.

In a chemical change the substances taking part in the reaction lose their original identity. The substances formed as a result of the reaction have new physical and chemical properties. There are vast numbers of different chemical reactions. It is possible to classify some of the simplest under the following headings:

In this type of reaction, an element or compound combines with another element or compound. For example, copper can react with chlorine to form copper chloride:

Cu + Cl2 CuCl2

Combination often results in the release of heat.

In this type of reaction, a compound breaks down into simpler substances. Decomposition can be considered the reverse of combination. For example, water can be broken down by electricity to form the gases hydrogen and oxygen:

2H2O + electricity 2H2 + O2

Some forms of decomposition result in the release of energy. Explosions are violent decomposition reactions that release large amounts of energy.

In this type of reaction, one element or radical replaces another element or radical in a compound. A radical is a group of atoms that behaves chemically as a single atom. For example, zinc can replace the hydrogen in hydrochloric acid to form zinc chloride and hydrogen gas:

Zn + 2HCl ZnCl2 + H2

In this type of reaction, an element or radical of one compound changes places with an element or radical of another compound. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water:

HCl + NaOH NaCl + H2O

Every substance has chemical energy because of its chemical composition. This energy is latent, or inactive, when there is no chemical activity. However, when a chemical change takes place, the energy becomes kinetic, or active. For example, a fuel such as coal has latent energy because of its high combustibility. When the coal is ignited the energy becomes kinetic and is released in the form of heat.

Chemical energy is measured in the form of heat, the unit of which is the calorie. One calorie, also called the small calorie, is the quantity of heat required to raise the temperature of one gram of pure water one degree Celsius. In the laboratory, a more convenient unit is the kilocalorie (kcal.), which is equal to 1000 calories. Chemical engineers often use the British thermal unit (Btu), which is defined as the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. One Btu equals 252 calories.

In the 18th century scientists realized that certain groups of elements had similar physical and chemical properties. The classification of elements began in the early 19th century when the German chemist Johann Dbereiner arranged elements with similar properties in groups of three, called triads. He also was the first to investigate the numerical relations between the atomic weights of elements.

By the mid-1850s larger groups of related elements had been discovered. It also was found that certain numerical relations exist between the atomic weights of related elements. The study of atomic weights was further developed in the late 1850's largely through the work of William Odling in England and Jean Baptiste Dumas in France. Their contributions laid the basis for the modern system called the periodic classification of elements.

In 1865 John Newlands, an English chemist, found that if the elements were arranged in order of increasing atomic weights, similar properties tended to be repeated at intervals of eight. This relationship, called the law of octaves, did not hold for all elements, and therefore found no acceptance at that time. However, Newlands' idea of a periodic recurrence of properties was basically correct, and its importance was realized a few years later after the discovery of the periodic law.

The Russian chemist Dmitri Mendeleev and the German chemist Julius Lothar Meyer independently discovered the periodic law. It states that the properties of elements are periodic functions of their atomic weights. Mendeleev published his report in 1869-a year earlier than Meyer. Both scientists, who were unaware of Newlands' work, arranged the elements in order of increasing atomic weights and in groups, or families, of related elements. In this way they obtained a natural classification based on the properties of elements.

Mendeleev went even further and predicted with great accuracy the properties of undiscovered elements. Within 20 years three new elements-gallium, scandium, and germanium-were discovered; they had properties almost exactly as Mendeleev predicted.

Probably the most significant implication in the periodic law was that atoms must have similarities in their structure, which are responsible for similarities in properties. Progress in physics and chemistry was so rapid that by 1914 it became apparent that the correct basis for putting the elements in order was the atomic number, rather than the atomic weight. This basis for classification permits the chemist to deduce something about an atom's structure from its position in the periodic table. Vertical groupings of elements that have similar properties are called families. The properties of elements in a horizontal grouping, or period, show gradual change from metallic on the left to nonmetallic on the right.

Inorganic chemists put many of the compounds they study into one of four classifications: (1) oxides; (2) acids; (3) bases; and (4) salts.

An oxide is a chemical compound formed by oxygen and another element, either metallic or nonmetallic. The compound so formed is said to be an oxide of that element. For example, when zinc (Zn) combines with oxygen (O) it forms an oxide called zinc oxide. The balanced equation is:

2Zn + O2 2ZnO

When carbon (C), a nonmetallic element, combines with oxygen, carbon dioxide (C + O2 CO2) or carbon monoxide (2C + O2 2CO) is formed.

The oxides of metallic elements, such as zinc oxide, are called basic anhydrides. In solution with water, these compounds form bases. The oxides of nonmetallic elements, such as carbon dioxide, are called acid anhydrides. When these oxides are dissolved in water, they form acids.

Acids are hydrogen-containing compounds whose water solutions have certain characteristic properties. These solutions usually taste sour, react with metals to release hydrogen gas, and conduct electric current. The simplest test for an acid is to use an indicator, such as litmus. A piece of blue litmus paper turns red when moistened with an acid.

The ability of a water solution of an acid to conduct electricity is due to the presence of ions in the solution. The way in which the ions are formed is represented in the following equation using hydrochloric acid (HCl):

HCl + H2O H3O+ + Cl-

The ion H3O+ consists of a hydrogen ion (H+) fastened to a water molecule. It is called a hydronium ion.

An example of the reactiob between a metal and an acid is:

Zn + H2SO4 ZnSO4 + H2

One way of judging the strength of an acid is to note the speed with which bubbles of hydrogen gas are produced by this reaction.

Bases are compounds that are also called alkalies. Chemists usually apply the name alkali to the strongest bases, which are caustic, bitter, soapy substances. Bases that dissolve in water make solutions that will also carry electric current. This ability is due to the presence of OH ions. This charged pair of atoms is an example of a radical ion and usually acts as a single unit. As with acids, indicators show a characteristic color with bases. Red litmus paper turns blue.

An acid contains hydrogen that can be exchanged for a metal. A base, or alkali, contains a metal, or a combination of elements that acts like a metal, to exchange for hydrogen. When acid and base meet, the exchange occurs, and sometimes it is quite violent.

The result of such a reaction is the formation of water and a salt. For example:

HCl + NaOH H2O + NaCl

acid base water salt

This reaction can be written in terms of the ions present, as follows:

H2O+, Cl- + Na+, OH- 2H2O + Na+, Cl-

water solution of the acid + water solution of the base water + water solution of the salt

This reaction is called neutralization because the ions typical of the acid, H3O+, are removed from the solution by the ions typical of the base, OH-, to form water, H2O.

The pair of ions formed in a neutralization reaction is called a salt. Salts in general are composed of the positive ions of a base and the negative ions of an acid. Examples in addition to NaCl are Na2SO4, sodium sulfate, formed from NaOH and H2SO4; and NH4Cl, ammonium chloride, formed from NH4OH and HCl.

Oxidation and redaction are two fundamental chemical reactions. Oxidation originally meant the union of a compound with oxygen. Reduction meant the opposite, the loss of oxygen by a compound. The meanings of both terms have been extended, however, and now apply even when oxygen has no part in a reaction. Oxidation refers to any increase in the positive valence of an atom (caused by a loss of one or more electrons to an atom of a different element). Likewise, reduction occurs when there is a decrease in the positive valence of an atom because of electrons gained from another element.

An important industrial use of reduction is in the separation of metals from their ores. Carbon monoxide is commonly used as the reducing agent in separating iron (Fe) from iron ore having the formula Fe2O3. The carbon monoxide removes the oxygen from the ferric oxide, leaving iron and carbon dioxide: Fe2O3 + 3CO = 2Fe + 3CO2

Oxidation and reduction always take place together. It will be noticed that while the iron decreased its valence from +3 (in Fe2O3) to 0 (in the free element), the carbon increased from +2 (in CO) to +4 (in CO2).

Sometimes the combination of oxygen with a substance is so rapid and vigorous that heat and light are given off. This form of oxidation is called combustion. Other forms of oxidation are so slow that the release of heat is not easily noticed. Oxidation of this kind occurs when a metal rusts. In the living body, energy is released by the slow oxidation of foodstuffs. These contain C and H, which the body converts to CO2 and H2O.

All organic compounds contain carbon. There are more than 1,000,000 different organic molecules whose formulas are known and thousands more are identified each year. The reason for this tremendous variety is the unique ability of carbon atoms to hook together into rings or long chains. The bonds they form are covalent in type. Hydrogen, nitrogen, oxygen, and other non-metals, such as chlorine, are most frequently combined with carbon; the metals seldom. Relatively few organic molecules are soluble in water. Inorganic materials, by contrast, frequently are water-soluble. Among the most important classes of organic compounds are hydrocarbons and plastics. Other important classes of organic compounds include alcohols, aldehydes, amino acids, ethers, and ketones.

The spatial arrangement of atoms in a molecule is called the structural formula of a compound. For many organic compounds, this structure is well known and much information about the properties and ability of a molecule to react can be inferred. Organic chemists build many molecular models to help them visualize structure. It is often possible to show the arrangement of atoms graphically by representing an atom with the element's symbol and a bond by a dash (-).

There is a group of organic compounds consisting only of carbon and hydrogen that are called hydrocarbons. These are the chief constituents of petroleum. Many series of similar compounds exist. The simplest such series is the alkane series. The smallest molecule of this series is the gas methane, which has a molecular formula of CH4. The structural formula for this compound is as follows:

The second compound in the alkane series is ethane, the molecular formula of which is C2H6. The structural formula shown below represents two atoms of carbon, each with 4 valence bonds, and six atoms of hydrogen, each with a single bond.

Propane, the third in the alkane series, has the molecular formula of C3H8. The structural formula is as follows:

One of the largest industries based on organic chemistry involves the building of giant molecules commonly known as plastics. The ability of carbon to link into long chains is taken advantage of by the organic chemist who makes carbon-containing molecules which will hook together into tremendously long chains, sheets, or blocks.

A relatively simple plastic, vinyl chloride, can be represented as:

The symbol n means that the structure in parentheses is repeatedly bonded together with other identical structures for what may be millions of times.