Check and uncheck as needed to select sections to print. Then select print preview to view your
selections and print:
Print section(s) of
Chemistry
Table of Contents
How to Cite This Article
Chemistry is the science that deals with the composition, structure, and properties of substances. It also
deals with the changes that these substances undergo. Because the study of chemistry involves every
possible kind of substance, it is central to the understanding of many other sciences.
The atomic theory is one of the most fundamental parts of the science of chemistry. This theory states
that all substances are made up of tiny particles far too small to be seen, even with the strongest
microscopes. These tiny particles are called atoms. Everything—glass, brick, iron, water, the stars in the
sky, and your own body—is made up of atoms.
There are many kinds of atoms. So far we know of at least 114 different kinds. But most of them are
quite rare. Only about a dozen kinds of atoms are common here on Earth.
Then how can there be so many different things on Earth? The answer is that atoms are like the letters
of the alphabet—all the words in the English language are built out of only 26 letters. A particular kind of
material, or substance, is formed when atoms combine. Even just a few kinds of atoms can combine into
many different arrangements. Each arrangement makes up a different substance.
Sometimes a substance is made up of combinations of only one kind of atom. Such a substance is called
an element. Iron is made up of one kind of atom, as are sulfur and aluminum. These are examples of
elements.
When a substance is made up of combinations of more than one kind of atom, it is a compound. The
substance ferrous sulfide is a compound because it is made up of pairs of two different kinds of atoms.
Each pair consists of one iron atom and one sulfur atom.
A group of two or more atoms tightly bound together is called a molecule. The pairs of atoms in ferrous
sulfide, for example, are molecules. Atoms in a molecule can also be the same element. An oxygen
molecule is made up of two oxygen atoms.
Chemical and Physical Changes
Understanding how substances can be changed is essential to understanding chemistry. But not all
changes involve principles of chemistry.
For example, you can break up a bar of iron into tiny pieces. Each piece is still iron, because the atom
arrangement has not been changed. This is a physical change. You can magnetize a piece of iron. You
can let an electric current pass through it. Or you can heat it red hot. These, too, are physical changes.
You can mix iron with something else without changing the atom arrangement. Suppose you mixed
powdered iron with powdered sulfur. In this mixture each little grain of iron is still iron. And each little
grain of sulfur is still sulfur.
You can easily separate the mixture again, because iron and sulfur have different properties. A magnet
passed through the mixture will attract the iron and leave the sulfur behind. A liquid called carbon
disulfide will dissolve the sulfur and leave the iron behind.
But what if you heated the mixture of powdered iron and sulfur? That would bring about a different kind
of change. A blackish material would form, in which you could no longer see separate little bits of
grayish iron or yellow sulfur.
The new material would have a new set of properties, unlike those of either iron or sulfur. The new
substance would not be attracted by a magnet. It would not dissolve in carbon disulfide.
You would now have a substance called ferrous sulfide. When the iron and sulfur were heated, each
sulfur atom combined with an iron atom, forming molecules of ferrous sulfide. That is, a new
arrangement of atoms was formed, and it made up a new substance with new properties. This is an
example of a chemical change, which is also called a chemical reaction.
Chemical changes go on all about us. Whenever coal or oil burns, that is a chemical change. The rusting
of iron is a chemical change. When food is cooked, it goes through many chemical changes. Chemical
changes also go on inside the body at all times. These are the many kinds of changes that interest a
chemist.
Symbols, Formulas, and Equations
Chemists often refer to the different elements and the compounds they form. They do this so often that
a special shorthand language has been worked out. Each element is represented by one or two letters
taken from its name. This is the chemical symbol of the element.
Often the chemical symbol is just the initial or the first two letters of the element. For example, the
chemical symbol of sulfur is S and for aluminum is Al. Sometimes the symbol is taken from the Latin
name. For example, the Latin name for iron is ferrum. The symbol for iron is Fe.
Symbols can be used to show the atomic makeup of a molecule. The ferrous sulfide molecule contains
one iron atom and one sulfur atom. So the molecule is written FeS. This is an example of a chemical
formula, a way of showing the atomic makeup of a molecule by means of symbols.
Whenever a chemical reaction takes place, atoms are rearranged. Therefore the details of the reaction
can be shown in chemical symbols. When iron and sulfur are mixed and heated, ferrous sulfide forms.
To put this quickly, we can write:
Fe + S
→
FeS
That is a chemical equation. It says that an atom of iron combines with an atom of sulfur to form a
molecule of ferrous sulfide. The little triangle over the arrow stands for heat. It means that the mixture
has to be heated before the chemical reaction takes place. (Light, electricity, and even other chemicals
can also bring about—or at least speed up—chemical reactions. Any substance that makes a reaction go
faster, without itself seeming to be changed, is called a catalyst.)
When a symbol is written by itself, it stands for one atom of a particular element. What if a molecule
contains more than one atom of that element?
The chemical reaction in which hydrogen and oxygen combine to form water cannot be written:
H2 + O2
H2O
→
This would say that 2 hydrogen atoms combine with 2 oxygen atoms to form a molecule of water. But
the molecule of water contains 2 hydrogen atoms and only 1 oxygen atom. What happened to the other
oxygen atom?
The reaction must be written:
2H2 + O2
2H2O
→
O2, you remember, stands for 2 oxygen atoms or 1 oxygen molecule. H2 stands for 2 hydrogen atoms or
1 hydrogen molecule. So 2H2 stands for 2 hydrogen molecules (or 4 atoms). Therefore the equation
shows that 2 hydrogen molecules and 1 oxygen molecule combine to form 2 molecules of water. The 2
molecules of water are made up of 4 hydrogen atoms and 2 oxygen atoms all together. All the atoms are
accounted for, and the result is a balanced chemical equation.
Structural Formulas
When different atoms combine to form a molecule, they follow certain rules. For instance, a single
hydrogen atom can combine with only one other atom. A single oxygen atom, however, can combine
with two other atoms. A single nitrogen atom can combine with three other atoms.
This combining power is called valence. Hydrogen has a valence of one; oxygen, a valence of two; and
nitrogen, a valence of three.
Sometimes formulas are written so as to show the valence. It is shown by little dashes between the
atoms. The molecules of hydrogen, oxygen, and nitrogen can be written: H—H, OO, N≡N. The hydrogen
atoms are shown held together by a single bond; the oxygen atoms, by a double bond; and the nitrogen
atoms, by a triple bond.
The formula of water (H2O) can be written H—O—H. Here each bond of the oxygen atom is shown to be
holding one of the hydrogen atoms. A molecule of a gas called ammonia (NH3) can be shown with the
three valence bonds of the nitrogen atom each connected to a hydrogen atom:
H
|
H—N—H
Such formulas show the exact way in which the different atoms of the molecule are connected by
bonds. They are called structural formulas.
Structural formulas are particularly important to chemists dealing with carbon compounds. For it often
happens that the atoms within the molecule can be arranged in more than one way. Each different
arrangement is a different compound.
As an example, imagine a molecule made up of 2 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom.
These can be put together in two different ways:
ethyl alcohol dimethyl ether
HH
||
H H
| |
H— C— C— O— H H— C— O— C— H
||
HH
| |
H H
In both compounds all the carbon atoms have four valence bonds. The oxygen atom has two, and the
hydrogen atoms have one. In each compound the number of each kind of atom is the same. But the
arrangement is different.
The compound with the molecule shown on the left is ethyl alcohol. It is the alcohol that is found in beer
and wine. The compound with the molecule shown on the right is dimethyl ether. Its properties are
completely different from those of ethyl alcohol. Ethyl alcohol is a clear liquid with a rather pleasant,
sweetish smell. Dimethyl ether is a gas with a much sharper smell. If a small piece of the metal sodium is
added to ethyl alcohol, a chemical reaction takes place. Bubbles of hydrogen gas are given off. If sodium
is exposed to dimethyl ether, nothing happens.
So you have two different substances with molecules made up of the same atoms in different
arrangements. Such substances are called isomers. The larger a molecule, the greater the number of
isomers that can be built up out of its atoms.
Atomic Structure
To understand something about how and why atoms combine, let's take a look at the way they are built.
The atom is made up of three types of particles: the proton, the neutron, and the electron. The protons
and the neutrons are heavy particles that are located at the very center of the atom. They form the
atomic nucleus. The electrons are very light particles. They are spread throughout the outer regions of
the atom and orbit the nucleus. (You can read more about atoms in the Atoms article in this
encyclopedia.)
The proton and the electron both carry an electric charge. All protons are charged the same way, and
this charge is said to be positive. All electrons are charged in another way, negative. The neutrons have
no electric charge at all.
The atomic nucleus contains all the protons of the atom. Therefore the nucleus has a positive charge.
Each proton has a charge equal to + 1. The number of protons in the nucleus is said to be the atomic
number of that atom. All atoms with the same atomic number behave exactly alike.
The electric charge on the electron is exactly as large as the electric charge on the proton. The electron's
charge is negative, however. So we call it −1.
Each atom has exactly enough electrons to balance the protons. The negative electric charges of the
electrons balance the positive electric charges of the protons. The atom as a whole is electrically neutral.
The Periodic Table
As chemists began to understand atomic structure, they realized that the atoms of some elements were
similar in certain ways to the atoms of other elements. They saw, for example, that if the elements were
arranged by atomic number, their valences changed in an orderly fashion. Let's look at this list of the
first 20 elements:
Atomic Number Element Valence
1 hydrogen +1
2 helium 0
3 lithium +1
4 beryllium +2
5 boron +3
6 carbon + or −4
7 nitrogen −3
8 oxygen −2
9 fluorine −1
10 neon 0
11 sodium +1
12 magnesium +2
13 aluminum +3
14 silicon + or −4
15 phosphorus −3
16 sulfur −2
17 chlorine −1
18 argon 0
19 potassium +1
20 calcium +2
(Elements that lose electrons in a chemical reaction have a + valence. Those that attract electrons have a
− valence.)
It is possible to arrange the elements in rows in such a way that elements with the same valences and
very similar properties fall into the same column.
VALENCES +1 +2 +3 ±4 −3 −2 −1 0
ROW 1 1 H 2 He
ROW 2 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne
ROW 3 11 Na 12 Mg 13Al 14 Si 15 P 16 S 17 Cl 18 A
ROW 4 19 K 20 Ca
By looking at the chart, you can determine that helium, neon, and argon (in the far right-hand column)
are similar elements. They are not chemically active and seldom combine with other chemicals. Each has
a valence of zero.
Hydrogen, lithium, sodium, and potassium (in the far left-hand column) are all very active. These
elements have very similar properties, and each has a valence of +1.
If you were to arrange all the elements in a chart that grouped together similar elements, you would
have created one of the most important tools in chemistry—the periodic table. The periodic table has
helped chemists and physicists explain and predict properties of many elements. It is especially
important to students learning chemistry. You can see the complete periodic table in the article
Elements, Chemical .
Electron Shells and Ions
When any two atoms combine and form a bond, it is their electrons that take part in the bond. The
protons and neutrons are not touched. When the bond is formed, the electrons may undergo changes in
position or condition. When these changes occur, a chemical reaction has taken place.
An atom's electrons are arranged in groups referred to as electron shells. Lithium, with an atomic
number of 3, has its 3 electrons arranged in two shells: 2 in the inner shell and 1 in the outer. Its
arrangement is 2, 1. Sodium, with an atomic number of 11, has three electron shells; the arrangement is
2, 8, 1. Potassium, with an atomic number of 19, has 4 electron shells, and the arrangement is 2, 8, 8, 1.
An atom tends to rearrange its electrons so that its outermost shell holds 8 electrons. (However, the
innermost electron shell can hold only 2 electrons at most.)
Knowing how an atom's electrons are arranged helps chemists predict how an atom will behave when a
bond is formed. A sodium atom, with its electrons arranged 2, 8, 1, can easily lose one electron. Its
second shell, with 8 electrons, becomes the outermost shell. A chlorine atom, with its electrons
arranged 2, 8, 7, can easily gain one electron. Its outermost shell then has 8 electrons.
If a sodium atom collides with a chlorine atom, an electron passes from the former to the latter. The
sodium atom, with an electron gone, now has only 10 electrons to balance the 11 protons in the
nucleus. There is one positive charge left over. Therefore the sodium atom now has a charge of +1.
An atom with an electric charge is called an ion. Instead of a sodium atom we have a sodium ion. The
sodium atom has the symbol Na (from its Latin name, natrium), and the sodium ion has the symbol Na+.
The chlorine atom, with an electron gained, now has 18 electrons but only 17 protons in the nucleus. It
has one negative charge too many. So it, too, is an ion. But it is a negatively charged one. Its symbol is
Cl−.
The properties of ions are different from those of neutral atoms. Sodium is a poisonous metal, and
chlorine is a poisonous gas. But their ions are mild and gentle. When sodium and chlorine react
together, they form ions. The ions in turn form crystals called sodium chloride. This is ordinary table salt.
When two chlorine atoms collide, something different happens. Each can pick up an electron, but
neither is likely to give one up. Instead, each contributes an electron to be shared with the other. The
two atoms combine, sharing a pair of electrons. They form the molecule Cl2. Here a chemical reaction
takes place without the forming of ions.
The number of electrons transferred or shared depends on the arrangement of the electrons in the
atom. It depends especially on the number of electrons in the outermost shell. Each atom can transfer
or share a particular number of electrons. (That is, each element has a valence.)
Isotopes and Atomic Weight
All the atoms of a particular element have the same atomic number. But they may not all be completely
alike. In addition to the protons in the nucleus, there are neutrons as well, and the neutrons may vary in
number.
For instance, all chlorine atoms have 17 protons, and all have the atomic number of 17. Some chlorine
atoms, however, have 18 neutrons in the nucleus, while some have 20.
All protons and all neutrons have the same mass. If we consider the mass of each proton and neutron to
be 1, we can work out the mass of an entire atom just by adding up the number of protons and neutrons
in its nucleus. (The electrons are so light that they can be ignored.)
Chlorine atoms with 17 protons and 18 neutrons in the nucleus have a mass number of 35. Those with
17 protons and 20 neutrons in the nucleus have a mass number of 37. These two varieties of chlorine
atom can be written as chlorine-35 and chlorine-37. Their symbols are Cl35 and Cl37.
Atoms of the same element that differ only in their number of neutrons are called isotopes. In other
words, Cl35 and Cl37 are two isotopes of chlorine. Because isotopes have the same atomic number
(number of protons), they have the same chemical properties.
These isotopes are well mixed throughout nature. In any sample of chlorine gas, there will be a mixture
of these two isotopes—approximately three atoms of Cl35 for each atom of Cl37. For that reason the
average mass of the chlorine atoms in an actual sample of chlorine gas is about 351/2, and that is said to
be the atomic weight of chlorine.
Some elements are made up of only a single variety of atom. Or they may have several varieties, but
with one common and the others very rare. In that case the atomic weight of the element is very close
to the atomic weight of the common variety.
Branches of Chemistry
This article lists just a few of the subjects that interest chemists. In fact, chemists' interests are spread so
widely that chemistry overlaps many other sciences. For example, some chemists want to know how
fast reactions go and what can be done to change their speed. They might want to know how a salt
solution can carry an electric current. Questions like these are answered by using methods similar to
those used by physicists. This branch of chemistry is called physical chemistry.
Chemists also study the many chemical processes that take place inside living organisms. They want to
know how foods are broken down and digested, for example, or how oxygen binds with hemoglobin in
the blood. Their field of study is called biochemistry.
The branch of chemistry that deals with compounds containing carbon is called organic chemistry. The
study of other compounds takes place in inorganic chemistry. Analytical chemistry is concerned with
identifying chemical substances, determining their amounts in a mixture, and separating mixtures into
their individual components.
Specialized branches of chemistry include astrochemistry, the study of the origin and interaction of
chemicals in space. Geochemistry is related to the science of geology and is applied in areas such as
mineral-ore processing. Nuclear chemistry deals with the use of nuclear power and the safe disposal of
nuclear wastes. Environmental chemistry focuses on the impact of the environment.
Chemistry is put to use in the search for new energy sources, in efforts to fight disease, and in efforts to
improve agricultural yields and increase the world's food supply. Chemistry in fact touches many parts of
our lives and has been put to use since human history began.
Isaac Asimov
Boston University School of Medicine
See also: Atoms; Biochemistry; Body Chemistry; Elements, Chemical.
How to cite this article:
MLA (Modern Language Association) style:
Asimov, Isaac. "Chemistry." The New Book of Knowledge. Grolier Online, 2011. Web. 5 Oct. 2011.
Chicago Manual of Style:
Asimov, Isaac. "Chemistry." The New Book of Knowledge. Grolier Online
http://nbk.grolier.com/ncpage?tn=/encyc/article.html&id=a2005270h&type=0ta (accessed October 5, 2011).
APA (American Psychological Association) style:
Asimov, I. (2011). Chemistry. The New Book of Knowledge. Retrieved October 5, 2011, from Grolier
Online http://nbk.grolier.com/ncpage?tn=/encyc/article.html&id=a2005270h&type=0ta
™ & © 2011 Scholastic Inc. All rights reserved.
Print Preview Print Reset