Chapter 5
Atoms and Elements
ALABAMA 8TH GRDE SCIENCE STANDARDS COVERED IN THIS CHAPTER INCLUDE:
Describe the structure of atoms, including the location of protons, neutrons
and electrons.
.
.
Identifying the charge of each subatomic particle
Identifying Democritus and Dalton as contributors to the atomic
theory
EARLY IDEAS THAI MATTER
What are objects made of? For thousands of years, humans have
tried to understand the world around them by asking this question.
Ancient people came up with many different ideas about the things
that surrounded them.
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The ancient Greek philosophers made the first recorded steps in
understanding the universe through their use oflogic. Democritus
was one ofthern. He lived from 460 370 B.C. He was the first to
describe an “indivisible unit” of matter he called atoma. The Figure 5.1 Democñtus
English word atom comes from this ancient Greek term.
He reasoned: ifyou take a glass ofwater and divide it into two, you have two glasses of
water. Even though each glass only contains half the volume of the original glass, the
substance in each glass is still water. Ifyou further divide each glass ofwater in half, you
would then have four glasses of water. If you continued to divide the water into smaller
and smaller portions, at some point you would not be able to separate the water any
further. The Greek idea was that matter could not be reduced to infinity. Eventually, you
would reach some stopping point. Some part of matter that could no longer be divided.
This base unit ofmatter was called an atom.
—
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.
I
H
Figure 5.2 Divisible Matter
47
H
Atoms and Elements
DALTON’S ATOMIC THEORY
The ideas ofthe ancient Greeks are very similar to our modern understanding ofmafter. It took
over 2,000 years before the ideas of the Greeks were put into a scientific theory. It was John
19th century. It combined the idea of atoms
Dalton who developed an atomic theory in the early
with quantitative data.
Throughout the 1 8th century, scientists identified more elements. Elements are the smallest
units of matter that cannot be divided into simpler substances. Some ofthese elements reacted
together to form new forms of matter. Their masses were measured before and after the
reaction. The measurements revealed that matter combined during chemical reactions.
To explain how this was possible, Dalton proposed a theory about atoms. It consisted of six
main points:
1. Elements consist of atoms.
2. Atoms of the same elements are identical and have the same mass.
3.
Atoms of different elements are different and have different masses.
4. Atoms of different elements can bond together (join) to form compounds.
5.
Chemical reactions occur when atoms are separated, rearranged or combined into
compounds.
6. Atoms of one element cannot change into atoms of another element during a
chemical reaction.
Dalton’s atomic theory laid down the foundation for modem chemistry. This theory explained
what matter consists of at the atomic level, and what happens when matter undergoes a
chemical change. This theory also supports an idea that was gaining more support in the late
1 8th century, the law of conservation of matter (or mass). We will look at this in more detail
later, but for now know that it serves as one ofthe most important principles in physical science.
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ELEMENTS
Ask yourselfthis:
Is all matter the same?
You already know the answer must be NO. There are many different kinds ofmatter.
Chapter 5
Different kinds of matter are made up of atoms of dfferent kinds of elements. Elements are
substances that cannot be further broken down into simpler substances. Hydrogen is hydrogen.
Gold is gold.
1
79
‘-H
Au
Hydrogen
Gold
Figure 5.3 Individual Elements: Hydrogen and Gold
You are probably already familiar with many elements, because most of them can be found in
nature. There are ill named elements, 90 ofwhich are naturally-occurring. The rest have been
synthesized (made) by scientists in a laboratory. All of these elements have a name (like
hydrogen) and a symbol (like H).
Why so many?
Think ofwhat the world would be like ifthere WEREN’T that many! The more elements there
are, the more possible combinations there are. In the end, that means more different kinds of
matter. How is one element different from another? The most basic answer is that each element
is made up of only one kind of atom. A sample of gold is made up of only gold atoms.
THE INDIVISIBLE ATOM
We’ve already talked a little about atoms, so let’s go into more detail. To do that, look no further
than the nearest pile ofpaper clips. This is a modem twist on Democritus’s ideas.
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Figure 5.4 Pile ofPaper Clips
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$o, let’s try it together:
1.
Divide your pile ofpaper clips into two equal piles.
2.
Divide each ofthe smaller piles into two equal piles.
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49
Atoms and Elements
3
.
Keep dividing the piles equally until you are down to a pile containing only one paper clip.
Can that one paper clip still hold papers together? YES! (Can you predict what we’re going
to do next? OK, read on...)
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Figure 5.5 The Indivisible Paper Clip
4.
Bend the paper clip in halfand then make it straight again. Repeat this action until the paper
clip breaks into two parts. Can half of the paper clip still hold papers together? NO!
Figure 5.6 Fragment ofPaper Clip
When you get to the point where the paper clip no longer holds paper together, you can no
longer call it a paper clip. If you do the same thing with any element, you will reach a point
where you come to an indivisible part that has the same properties ofthe element, like the single
paper clip. This indivisible part is called the atom.
But HOW are the atoms of different elements, well
. . .
different?
Ah, yes .an excellent question! In order to describe how atoms are different from each other,
you must first know the parts ofthe atom.
. .
Atoms are made up of subatomic particles. The three main subatomic particles are the proton,
neutron and electron. The properties of each subatomic particle are shown in Table 5 1 Protons
and neutrons are located in the center area of the atom called a nucleus, and they have almost
all of the mass of the atom.
.
.
Table5.1
Subatomic Particle
Proton
Neutron
Electron
Mass
i .673 x lO_27 kg
1.675 x lO_27 kg
9. 109 x lO_31 kg
Charge
1 .602 x iO C
0C
1 .602 x 1O_16 C
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0
0.
0
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0
0
As you can see, the proton and neutron are bigger than the electron. Can you tell how much
bigger? Divide the mass of the proton by the mass of the electron and you will see that the
proton is about 1 ,837 times larger than the electron. For comparison, a commercial airliner like
the Boeing 767, when empty of cargo and passengers, weighs about 1,867 times what you do
(give or take a few pounds).
Protons have a positive (+) charge, electrons have a negative (—) charge, and neutrons have no
charge. The charge of an atom can be negative, positive or neutral. In order to be uncharged
(neutral), an atom must have the same number of electrons as protons. When an atom is
50
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Chapter 5
charged, it is called an ion. If there are more electrons than protons, the charge of the ion is
negative. If there are fewer electrons than protons, the charge of the ion is positive. Keep in
mind that the number ofprotons does not change under normal circumstances, but the number
of electrons can and often does.
Electrons are located outside the nucleus. They are much tinier and
move very fast. So fast, in fact, that we can usually only describe
their location as a likely general area, instead oftrying to pinpoint an
exact spot. Ifyou took a snapshot ofthe area around a nucleus every
second for, let’s say, 5 minutes, you would get a picture that looked
a bit like Figure 5.7. The large dot in the middle is the nucleus. The
small dots around the nucleus are the places where a single electron
has shown up over the past five minutes. Those past locations define
the general area where an electron is likely to be found. It is called
an electron cloud, but its proper name is an orbital.
Figure 5.7 Orbital
Let’s look at the arrangement of a single helium atom.
Model of a Helium Atom
Nucleus contains
7
2 protons. p, and
2 neutrons, n.
2 electrons reside in e shell
surrounding the nucleus.
Figure 5.8 Model of a Helium Atom
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The negatively charged electrons are electrically attracted to the positively charged protons in
the nucleus. This attraction holds the electrons in orbit around the nucleus. This area is the
orbital.
An electron may be found at any point within the space of an orbital and sometimes even
outside that space. These orbitals are designated s, p, d and f, each ofwhich has a unique shape.
When associated with their energy level (1, 2, 3 . . .), these orbitals define the electron
distribution of an atom. The smallest atom, hydrogen (H), consists of 1 proton and 1 electron.
The proton is in the nucleus and the electron orbits the nucleus in its is orbital. The next largest
atom is helium (He), which consists of 2 protons, 2 neutrons and 2 electrons. As shown in
Figure 5.8, the 2 protons and 2 neutrons of the helium atom are held together in the nucleus,
while the 2 electrons orbit the nucleus in its is orbital.
51
Atoms and Elements
The first orbital will only hold two electrons, however.
The next element in the periodic table is lithium (Li),
which has 3 protons, 4 neutrons and 3 electrons. Two of
lithium’s electrons will go into first orbital, and the third
will go into the next highest orbital, as shown in Figure
5.9. This orbital is located farther from the nucleus and is
higher in energy. The greater the distance between two
charged particles, the weaker the forces holding them
together. Therefore, since the lithium electron in the
second orbital spends most of its time farther away from
the nucleus than the atom’s 1St two electrons, it is less
tightly bound to the nucleus.
MODEL OF A LITHIUM ATOM
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Figure 5.9 Lithium Atom Model
Including Electron Shells
The quantum number and orbital designation ofeach electron can be accounted for in an atom’s
electron configuration. The electron configuration is the arrangement of electrons in an atom
or molecule. One atom can have multiple orbitals surrounding the nucleus. Each orbital can
contain up to two electrons. Each type of sublevel (s, p, d and f) holds a different number of
orbitals and, therefore, a different number ofelectrons. The first sublevel, called s, has only one
orbital, which, as you know, can hold up to 2 electrons. The second sublevel, called p, has three
orbitals. Each of these can hold up to two electron as well, for a total of six electrons. Table 5.2
below summarizes the orbital and electron capacity for each sublevel.
Table 5.2 Orbital and Electron Capacity for Sublevels
Sublevel
# of Orbitals
Maximum # of Electrons
5
1
2
p
3
6
d
5
10
f
7
14
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Co
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This electron configuration is the organizational concept behind the periodic table, and will be
discussed more in the next chapter. For now, it is sufficient to recognize that electrons
sequentially fill various energy levels (1,2,3, etc. . .), and the various orbitals (sometimes called
shells) within those energy levels. As each energy level becomes full, electrons begin to fill the
next highest level. The highest energy level orbital containing electrons is the atom’s valence
shell. It contains the electrons that exist farthest away from the nucleus of the atom. These
“outer electrons” are called valence electrons and are free to participate in bonding with other
atoms. The number of valence electrons is easily determined by the position of the element on
the Periodic Table. The element’s position on the Periodic Table also indicates many other
properties of the element, which we will look at in the next chapter.
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52
Chapter 5
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Many times, atoms gain and lose electrons and become ions. The list below contains some
common ions ofelements. Determine the correct number ofprotons and electrons contained
in each ion. Assume the atom only gains or loses electrons.
Number of Protons
l.Br
2.Au3
3$2_
j14.Cu2+
5. C1
6.A13
7.Fe3
8.I
9.N3
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11. Ni4
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15. Cu
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53
Number of Electrons
Atoms and Elements
CHAPTER 5 REVIEW
1.
Which ofthe following is the heaviest?
A
B
C
B
the electron
the proton
the neutron
the nucleus
2. Neutral lithium has three protons. During a chemical reaction, one electron is
removed. How many protons does lithium have now?
3
A
2
B
Cl
You must know what element lithium reacted with to determine this.
D
3. Orbitals contain
A
B
C
B
thousands of fast-moving electrons.
a few fast-moving protons.
a few fast-moving electrons.
many nuclei.
4. Which of the following is a positive ion?
lithium (Li), with 3 protons and 2 electrons
A
neon (Ne), with 10 protons and 10 electrons
B
oxygen (0) with 8 protons and 1 0 electrons
C
fluorine (F), with 9 protons and 10 electrons
D
5.
The nucleus of an atom is
A
B
C
B
the most massive.
theleastmassive.
without any mass.
negatively charged.