How Do Scientists Know What atoms are like?
Since atoms are too small to be seen with the eyes alone, people have wondered for
a long time what atoms look like. In fact, scientist have been studying atoms since
the 1800s. But if scientists can’t see an atom, how do they know what atoms look
like?
Using Models
Sometimes scientists can tell what things look like by studing how they act. For
example, what does wind look like. If you said wind blows things, you are
describing what wind does, not what it looks like.
Scientists use the same kind of evidence to study things they can’t see, such as
atoms. Scientists study how atoms act and then decide what an atom must look like.
Scientist make models. You have probably seen models of cars, or airplanes or
buildings. In science, a model is an idea, a picture, or an object that is built to
explain how something else looks or works.
Models of Atoms
Scientist use models of atoms to show how atoms look and act without having
actually to see them. Many scientist have developed models of atoms. In the early
1900s, a scientist developed a model of an atom like those shown below.
A
B
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Find the center of each atom. This central part of an atom is called a nucleus.
Protons are labeled with the letter p. Another symbol for a proton is a + sign. Now
look in the figures for the letter e. This letter stand for electrons. Electrons are
particles in an atom that move around the outside of the protons. Electrons are
smaller than protons. Another symbol for an electron is a minus – sign. The protons
and electrons of an atom stay together because the attract each other.
Notice that the numbers of protons and electrons in the models are different. Figure
A show a model of an atom of hydrogen. You can see that hydrogen has one proton
and one electron. Figure B shows an atom of helium, a gas that is often used to fill
balloons. How many protons and electrons des helium have?
In 1932, Scientists had evidence that the nucleus of an atom had another kind of
particle. This particle is called a Neutron. Because of the new evidence, scientist
changed the model of the atom. Figure B show has the model changed. Find the
neutrons, labeled n.
Today scientists use another model of atoms. You can see this new electron cloud
model in Figure D. This new model was developed because scientist have evidence
that electrons are not in separate layers around the nucleus. They could be
anywhere in a cloud at a given time. As scientists learn more about atoms, perhaps
the model will change again.
D
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You have looked at models showing the number of protons and electrons in
different atoms. The table below lists some other atoms and tells the numbers of
protons and electrons in each. Find the number of protons in carbon. How many
electrons does carbon lave? Compare the numbers of protons and electrons in
nitrogen. How many of each does it have? Now look at the numbers of protons and
electrons in each of the other atoms. What do you notice? The number of protons in
an atom is equal to the number of electrons in the atom.
Kind of atom
hydrogen
helium
lithium
beryllium
boron
carbon
nitrogen
oxygen
fluorine
neon
number of protons
1
2
3
4
5
6
7
8
9
10
number of electrons
1
2
3
4
5
6
7
8
9
10
Questions
1. If scientist cannot see atoms, how do they know what atoms look like?
2. What is a model?
3. How many protons are in the atom shown in the drawing?
3. How many protons are in the atoms shown in the drawing? How many
electrons?
4.
5. What is the name of the kind of atom.