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What Are Atoms, and Why Do
They Join Together?
The spheres you used to model molecules help you understand that atoms join
together in different ways. But why? What causes atoms to join together? To
answer this question, you need to know more about the structure of an atom.
The Atomic Theory
Atoms are extremely small. There is no tool that can enable scientists to
look inside individual atoms. Instead, scientists have learned about atoms
through logic and by observing how atoms behave in different conditions.
The understanding that scientists have about the existence of atoms and
their structure is known as the atomic theory. The atomic theory is the
current answer to the question, What is the structure of the atom?
In science, theories help scientists organize what they know about the world.
Theories are the big ideas in science, often developed over long periods of
time, using evidence gained through observations and experimental data.
Scientific theories are continuously studied and investigated. If new evidence is
discovered that cannot be explained by the theory, the theory must be changed.
A small part of the theory might be changed, or the entire theory might be
replaced. The development of the atomic theory has a long and interesting
history. As new experiments were designed and new technology was developed,
scientists made observations that led to changes in the atomic theory. The
development was not always smooth. In fact, scientists often disagreed about the
structure of the atom, but they continued to gather evidence to develop a more
complete description of the atom. The theory that exists today represents the
best understanding at the current time. If scientists make discoveries that are not
explained by the current theory, the atomic theory will change yet again.
atomic theory:
the idea that all
matter is formed
from atoms and
that atoms have a
unique structure.
theory: a big
idea in science,
often developed
over time,
using evidence
gained through
observations and
experimental data.
The origin of the atomic theory can be traced back more than 2000 years
ago to Greek philosophers, such as Democritus. These philosophers did
not conduct controlled experiments, but instead, they used logic to reach
conclusions about the natural world. Democritus, for example, imagined
cutting a sample of matter in half and then cutting each half again and again.
AQ 85
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Learning Set 2ÊUÊ7…>ÌÊÃʈ˜ÊˆÀ¶
He argued that eventually he would get to a particle that could not be
divided any further. He named this particle an “atom,” which is the Greek
word for “uncuttable.” Democritus’s idea was accepted, but it would be
many years before scientists could find evidence to support his ideas.
370 B.C.
Democritus imagined that all
matter is made of smaller
particles that contain the
essence of a substance and
cannot be divided. He called
these particles atoms.
370 B.C.E.
Rutherford Model
1909 Ernest Rutherford determined
that there were two areas
in each atom. He called the
center of the atom the nucleus,
which contained positively
–
charged particles. In the
–
outside area, negatively
–
charged particles moved
–
around the nucleus.
+
–
–
1803
1909
Dalton Model
1803 John Dalton developed
an atomic theory from
observations of experiments
he conducted. He pictured
atoms as tiny, solid particles
that could not be destroyed and
had no internal structure.
–
–
–
+
–
–
–
–
1913
Bohr Model
1913 Niels Bohr found
that the electrons
(negatively charged
–
particles) of an atom
move in orbits at a fixed
distance from the
nucleus.
In the early 1800s, an English scientist named John Dalton gathered evidence
that supported many of Democritus’s ideas. In his experiments, he combined
different substances to make new substances. Each time, he measured the
mass of the substances. The total mass of the substances before he mixed
them with each other was always the same as the total mass of the resulting
substances. From this and other experimental results, he gathered evidence
that supported the beginning of the atomic theory.
Dalton’s atomic theory stated the following:
Project-Based Inquiry Science
s
All matter consists of small particles called atoms.
s
Atoms of any element are identical to each other, and the properties
of each of these atoms are identical. Atoms of different elements
have different sets of properties.
AQ 86
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s
Atoms cannot be destroyed. Atoms can rearrange or combine, but
they are not destroyed.
s
When atoms of different elements combine to form molecules, they
combine in predictable ways. The number of atoms you start with
equals the number of atoms you end up with.
Scientists still did not know why atoms combined in predictable ways. In
1911, the English scientist Ernest Rutherford performed experiments that
helped him uncover the structure of atoms. He found that the mass of an
atom is concentrated in a central part, what is now called the nucleus.
Around the nucleus was another, less dense part of the atom. Knowing
that atoms have a nucleus was the first step in understanding why atoms
combine as they do. But scientists still wondered what was in that region
around the nucleus.
nucleus: the
center part of an
atom.
Rutherford’s work was followed by discoveries made by the Danish scientist
Niels Bohr in 1913. Bohr investigated the space around the nucleus of an
atom, and the particles that exist in that space.
How Big Is an Atom?
Some ideas in science are difficult to picture. Things that are really big or
really small are often difficult to imagine. Atoms are a good example of
this. They are so small, it is difficult to grasp their size. Comparing atoms
to other things might help. For example, it would take about one hundred
million (100,000,000) oxygen atoms to form a line one centimeter long.
one centimeter
Compared with the size of the nucleus, the electrons of an atom are very
far away. For example, if the period of this sentence were the nucleus of
one type of atom, the closest electrons would be orbiting the period about
50 m (150 ft) away. There is nothing in the space between the nucleus
and the electrons.
All matter is made of atoms, but most of the volume of an atom is empty
space. This last fact is surprising, but it is true.
AQ 87
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Learning Set 2ÊUÊ7…>ÌÊÃʈ˜ÊˆÀ¶
The Parts of Atoms
subatomic
particles:
particles that make
up an atom.
proton: a
subatomic particle
of an atom found
in the nucleus.
It has a positive
charge.
electron: a
subatomic particle
of an atom found
outside the
nucleus. It has a
negative charge.
neutron: a
subatomic particle
of an atom found
in the nucleus. It is
electrically neutral.
Through the contributions of many individuals, scientists developed an
understanding of what atoms are and how they behave. In particular, they
learned that while atoms are the smallest particles that have the properties
of an element, atoms are made up of even smaller particles. These particles,
known as subatomic particles, are the proton, neutron, and electron.
Protons and electrons are electrically charged particles. The proton is a
subatomic particle that has a positive charge. The electron is a subatomic
particle that has a negative charge. The neutron is a subatomic particle
that does not have an electric charge. It is said to be electrically neutral.
Protons and neutrons are located in the nucleus of an atom. These particles
make up most of the mass of the atom. The mass of a proton is about the
same as the mass of a neutron. Every element has a specific number of
protons in its nucleus. For example, oxygen has eight protons in its nucleus.
Any atom with eight protons would have to be oxygen. The number of
neutrons in the nucleus of an element may vary. Most oxygen atoms have
seven neutrons, but some may have slightly more or less neutrons. You
cannot tell what element an atom is just by the number of neutrons.
The mass of an electron is much less than the mass of a proton or neutron,
and it would take about 2000 electrons to equal their mass. Electrons once
were thought to orbit the nucleus much like planets orbit the Sun. However,
scientists later found out that it is impossible to predict the exact location of
an electron at any specific time. Instead, the region in which an electron is
most likely to be found is described as an electron cloud.
Stop and Think
1. Describe the characteristics of the three parts of an atom.
Charged Particles
You may be wondering what electric charge possibly has to do with the
atomic theory. Quite a bit of the modern atomic theory depends on
electric charge.
Now, using what you know about opposite charges, think about protons and
electrons. Because they have opposite charges, protons and electrons attract
one another. Protons in the nucleus and electrons outside the nucleus pull
together. The force of attraction (pull) helps hold the atom together.
Project-Based Inquiry Science
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Investigation
Electric Charges
You read that protons and electrons are electrically charged particles. Protons
have a positive charge and electrons have a negative charge. Why is it
important that there are two different types of charges? You will use invisible
tape to investigate what happens when charged objects are brought together.
Materials
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Procedure
1. Cut two strips of invisible tape about 12 cm long.
Fold over a 1-cm section at one end of each
strip to make a tab that you can hold.
2. Place one strip on a table with the sticky side
facing down. Label the tab “B,” for “bottom.”
3. Place the second strip exactly on top of the first strip,
sticky side down. Press it down firmly to make good
contact. Label the tab “T,” for “top.”
4. Hold down the tab on the bottom strip and peel off the top strip.
Then, with the other hand, pull off the bottom strip.
5. Hold the strips about 15 cm apart, allowing them to hang down.
Slowly, bring the strips toward each other but do not let them touch.
Analyze Your Data
1. What happened when you brought the strips close together?
2. What do your observations suggest about the electric charges on the two
strips of tape?
3. How do you think the two strips of tape became electrically charged?
Predict
You are going to make a second set of top and bottom pieces of tape.
1. What do you think will happen if you bring two top pieces of tape together?
2. What do you think will happen if you bring two bottom pieces of
tape together?
Record your predictions on a piece of paper.
AQ 89
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Learning Set 2ÊUÊ7…>ÌÊÃʈ˜ÊˆÀ¶
Procedure
1. Make a second step of top and bottom strips as before. Be sure to label
them with a “B” or a “T.”
2. Bring the two top strips together without touching.
3. Bring the two bottom strips together without touching.
4. What is similar about the two top pieces of tape? The two bottom pieces?
Analyze Your Data
1. What happened when you brought the two top strips together? The
bottom strips?
2. What do your observations suggest about the electric charges on the top
pieces of tape? The bottom pieces?
Reflect
What test could you do to determine whether the top pieces of tape have a
negative charge (more electrons) or a positive charge (less electrons). Why
do you think this would work?
Before
Before
Before
After
After
After
Electrically Charged Particles
Perhaps you have walked across a carpet and received a “shock”
by touching a metal doorknob. If so, you have experienced electric
charge. Electric charge is a physical property of some objects.
There are two kinds of charge—positive charge (+) or negative
charge (–). Electric charges can build up on an object. This buildup
of charge is known as static electricity. When the charges return
to their normal, or neutral, condition, the release of charge can
be experienced as a shock. The release of electric charge on a
greater scale can be seen as lightning.
You may have heard the saying “opposites attract.” This is true
when it comes to electric charges. Opposite charges are attracted
to one another. So a positive charge and a negative charge will be
pulled together. Like charges repel one another. So two positive
charges or two negative charges will push each other apart.
Metal spheres that have no charge, hang straight down. Spheres
with opposite charges attract. Spheres with like charges repel.
Project-Based Inquiry Science
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Atomic Bonding
The number of protons, neutrons, and electrons in an atom depends
on the element. No two elements have the same number of protons.
In a neutral atom, the number of protons is the same as the number of
electrons. A carbon atom, for example, has 6 protons, 6 neutrons, and 6
electrons. An oxygen atom has 8 protons, 8 neutrons, and 8 electrons.
Atoms are most stable when they have a complete set of electrons in
their outer level. For most atoms, a complete set includes eight outer
electrons. One way atoms get a complete set of outer electrons is to share
electrons. The attraction between atoms that share electrons is known
as a chemical bond. Atoms in molecules are held together by chemical
bonds. For now, the discussion about chemical bonds will be limited to
sharing electrons. There are, however, other types of chemical bonds that
do not involve sharing.
Earlier, you used your atomic-model kit to build molecules. The gray rods
represented chemical bonds between atoms. You were told that most atoms are
unstable and combine with other atoms to become stable. Now you know that
atoms bond together to get a complete set of outer electrons.
You may have also discovered that atoms can form different numbers of
bonds. In your model of the nitrogen molecule (N2), the nitrogen atoms were
connected by three gray rods. The three gray rods represent a triple bond.
In a triple bond, two atoms share six electrons—two for each bond. In the
oxygen molecules (O2), the atoms were connected by two gray rods. These rods
represent double bonds. In a double bond, two atoms share four electrons.
Later in this unit, you will form water molecules (H2O) by attaching the atoms
by single gray rods. These rods represent single bonds. In a single bond, two
atoms share two electrons.
chemical bond:
an attraction
between atoms
that share
electrons.
triple bond: a
bond where 2
atoms share 6
electrons (3 gray
rods).
double bond:
a bond where 2
atoms share 4
electrons (2 gray
rods).
single bond:
a bond where 2
atoms share 2
electrons (1 gray
rod).
Hydrogen atoms
in H2 bond to each
other with a single
bond. Oxygen atoms
in O2 bond to each
other with a double
bond. Nitrogen
atoms in N2 bond
to each other with a
triple bond.
AQ 91
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Learning Set 2ÊUÊ7…>ÌÊÃʈ˜ÊˆÀ¶
Reflect
1. Models allow you to see some features of an object well, but they are not
completely accurate. What is accurate about the models you made?
2. What parts of the atomic-model kit are not accurate? Identify from the
reading how you know which parts are not accurate.
3. Imagine that you had to describe an atom to your friend. You need to
make a comparison using the word “like.” What could you compare
an atom to that would help your friend better understand what you are
talking about? Describe also how your comparison is incomplete.
Update the Project Board
On the Project Board, record what you now know about what matter
is made of, the subatomic particles, and how atoms combine to make
molecules. Record this information in the What are we learning? column.
Be sure to record your evidence in the What is our evidence? column.
You may have some new ideas about what additional information you
need to answer the Big Question and address the challenge. Record those
questions in the second column, What do we need to investigate?
What’s the Point?
Atoms are the smallest particles of matter that have the properties of an
element. Every atom of an element has the same properties and the same
number of protons. Molecules are made up of two or more atoms.
Much like molecules are made up of small particles called atoms, atoms
are made up of even smaller particles—protons, neutrons, and electrons.
Neutrons have no charge, protons have a positive charge, and electrons
have a negative charge. The opposite charges in atoms attract each other
and help hold the atom together. Charges also hold molecules together.
Project-Based Inquiry Science
AQ 92
More to Learn
Periodic Table of the Elements
By the middle of the 1800s, chemists had identified a large number
of elements. To keep track of all the elements, they needed a way
to sort them into categories. Chemists had observed that some
elements shared similar chemical properties. They looked for a way
to arrange the elements according to these properties. The chemists
that are given credit for successfully grouping the elements into a
pattern according to their properties are Dimitri Mendeleev and
Julius Meyer. Individually, they were responsible for arranging the first
version of the Periodic Table of the Elements.
While the first periodic table grouped the elements according to
properties, years later, the arrangement also revealed information
about the structure of the atoms of those elements. For example, the
modern periodic table, which has been updated since Mendeleev’s
time, shows that the elements are arranged according to their
numbers of protons.
The element with the smallest number of protons, one, is hydrogen (H).
You can find hydrogen on the top row in the left column. Another gas,
helium (He), with two protons, is in the same row but in the column on
the right. The pattern goes on like this for the rest of the table. Each
element has one more proton than the element that came before it.
Because each element has a different number of protons, the number
of protons can be used to tell one element from another. The number
of protons in an atom is called its atomic number. The atomic
number of hydrogen is 1, and the atomic number of
helium is 2. Each element has a unique atomic number.
In the periodic table shown at the back of this Unit,
6
the atomic number of each element can be found in
the upper left corner of each square. Remember that
the protons and neutrons make up almost all the mass
of an atom. The sum of the number of protons and
Carbon
neutrons in an atom of an element is the element’s
12
atomic mass. This can be found below the chemical
symbol on each square.
C
AQ 93
Periodic Table
of the Elements:
a table listing
all the known
elements and their
properties.
atomic number:
the number of
protons in an
atom’s nucleus.
atomic mass: the
average number
of total protons
and neutrons in an
atom’s nucleus.
Atomic Number
Chemical Symbol
Name
Atomic Mass
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Learning Set 2ÊUÊ7…>ÌÊÃʈ˜ÊˆÀ¶
chemical family:
group of elements
found in a column
of the periodic
table. These
elements exhibit
similar properties.
noble gases: a
family of elements
with full electron
energy levels.
These elements
do not undergo
chemical reactions
and are found in
column 18 in the
periodic table.
period: a word to
describe something
that repeats in a
regular pattern.
In addition to being arranged in order of atomic number, the elements
of the periodic table are organized into rows and columns. Each column
represents a group or chemical family. The elements in a chemical
family all have similar properties. The reason for this is that the
elements in each column have the same number of outer electrons.
Therefore, they are likely to behave in similar ways to become stable.
The elements in the first column of the periodic table (all the way to the
left) have just one outer electron. They are very likely to combine with
other atoms to become stable. On the contrary, the elements in the last
column of the periodic table (all the way to the right) all have a complete
set of outer electrons. Because they have a complete set of outer
electrons, these elements are already stable and do not combine with
other atoms. These elements are known as the noble gases. Notice
that argon is a noble gas. In your atomic-model kit, the sphere for argon
did not have any holes in it. Now you know why.
The rows of the periodic table are known as periods. Things that are
periodic repeat in a regular pattern, such as time on a clock or ocean
tides. The properties of the elements change in a pattern that repeats
itself with each new row. This is how the table got its name. Mendeleev
and other scientists observed periodic changes in the properties of
the elements when they were arranged in rows and columns. The two
rows of elements at the bottom of the periodic table, elements 57–71
(Lanthanide Series) and 89–103 (Actinide Series), actually fit into the
sixth and seventh rows. They are separated from the rest of the table to
make the table fit on a page. You will learn more about the periodic table
later in this Unit and in future science courses.
Currently, there are more than 10 0 known elements. Each element has
properties that make it unique. Scientists continue to search for new
elements and for ways the periodic table might be improved.
Stop and Think
1. Find oxygen in the periodic table. What is its atomic number? What
is the next element to the right of oxygen? How many protons does
that element have?
2. Find nitrogen in the periodic table. What is its atomic number?
3. Find two other familiar elements in the periodic table. List their
atomic numbers and atomic masses.
Project-Based Inquiry Science
AQ 94
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Development of the Periodic Table
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Numbers after the elements are atomic weights.
English chemist œ…˜Ê i܏>˜`ÃÊarranged the
56 known elements in order of increasing atomic
weight. He observed similarities that repeated after
each set of eight elements. As a result, he proposed
the law of octaves, because an octave describes a
group of eight notes in music.
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German chemist Johann Döbereiner arranged the
55 known elements by atomic weight into groups
of three, called a triad, based on similar properties.
Döbereiner noticed that he could organize elements
into groups of three in such a way that the atomic
weight of one element was an average of the atomic
weights of the other two elements.
1864
An octave of eight musical notes.
1869
Russian chemist Dmitri Mendeleev developed a
table of 63 known and predicted elements in order
of atomic weight but arranged them in columns
based on similar properties. He left spaces for
elements that had not yet been discovered based
on the properties he predicted. Mendeleev’s table
had columns and rows so that the properties of
elements changed from left to right across a row
and then repeated in a similar way in the next row.
German chemist Lothar Meyer also observed
patterns of properties when he arranged the 56
known elements in order of atomic weight and
developed his own periodic table. However, Meyer
did not predict the existence of missing elements.
Numbers after the elements are atomic weights.
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British chemist Henry Moseley determined the
atomic number of each element, which is the number
of protons in the nucleus. He changed the periodic
table to arrange the 92 known elements in order of
atomic number rather than atomic weight.
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1914
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Numbers after the elements are atomic numbers.
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Nine new elements discovered by Seaborg.
Numbers after the elements are atomic numbers.
AQ 95
American chemist Glenn Seaborg discovered nine
new elements after uranium, and his discoveries led
to the current arrangement of the periodic table. He
won the Nobel Prize in chemistry for his work.
AIR QUALITY