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Class: ____________
Date: _____________________
The History of the Atom: a timeline of Atomic Theory
Directions: Read this packet and carefully annotate the text. Annotate means to underline important words,
make notes in the margins, ask questions that you have. The first paragraph has been done for you. After you
are finished reading the first section, please answer the reflection questions in your OWN WORDS using
complete sentences. Then, we will move on to the next section as a class. After you have completed both sets
of reflection questions, you will be expected the make your own timeline of the evolution of atomic theory.
The Early Atom: Dalton’s Atomic Theory and Thomson’s discovery of the electron
Leucippus, a Greek philosopher, was the first person recorded to believe that all matter consisted
of atoms. He believed that everything was made up of tiny and indivisible bodies called atoms. The
word atom comes from the Greek word ‘atomos’, which means not divisible. According to
Democritus, another Greek philosopher who agreed with what Leucippus said, these atoms were
not all alike. He said that they differed in size and shape, and that substances were different from
each other because they were made up of different types of atoms. Other Greek philosophers, such
as Aristotle, believed that everything was instead made up of four basic elements: earth, fire, air,
and water, not atoms. Views such as Aristotle’s dominated science for many centuries, up until the
middle ages.
During the period up to the middle ages, science in general, and science regarding to the atom in
particular, halted. When interest in science was revived in Europe, around the 16 th and 17th
centuries, enough was already known about what Greek philosophers knew about the atom to
carry on their work. Scientists began to question previous ideas and conduct experiments that led
to new discoveries about matter and the atom. John Dalton was born in 1766, and developed the
ancient concept of atoms into a scientific theory that was the foundation for modern chemistry.
Throughout his life, Dalton was very interested in the atmosphere of the Earth, and he recorded
over 200,000 observations of the atmosphere in his notes. These observations led Dalton to study
and experiment with gases. From these experiments, he began to formulate his theories on atoms
and the structure of the atom.
His theory on atoms - his most
important contribution to science - was
that matter is made up of atoms that
could be distinguished by differences in
their weights. What does that mean
exactly? Dalton stated that matter is
composed of elements and that all
atoms of a given element are identical
in weight. He also believed that atoms
of one element can combine with
atoms of other elements to form
compounds. A compound is a substance made of two or more elements joined together in a
specific way. A given compound always has the same relative numbers and types of atoms. In his
book ‘A New System of Chemical Philosophy’, which he published in 1808, he listed the weights of a
few known elements relative to the weight of hydrogen. Although the weights that he proposed
were not entirely accurate, they formed the basis (or foundation) for the modern periodic table of
elements.
Dalton’s model of the atom was important because it successfully explained important
observations, such as the law of constant composition. The law of constant composition says that
a given compound always contains elements in exactly the same proportion by mass. For example,
if a compound contains one nitrogen atom and one oxygen atom, every sample of that compound
contains a 1:1 ratio of nitrogen and oxygen atoms. Dalton used his model to predict (or guess) that
a given pair of elements could combine to form more than one compound. For example, nitrogen
and oxygen might form a compound containing one atom of nitrogen and one atom of oxygen
(written NO) or one atom of nitrogen and two atoms of oxygen (written NO2). While Dalton’s
model of the atom was a good starting point, scientists soon discovered that the atom is actually
much more complicated!
Scientists that were interested in matter and atoms began
to build off of Dalton’s theory to create better, more
complicated theories about what the atom is. Sir Joseph
John Thomson, born in 1856, is recognized as the British
scientist who discovered and identified the electron. How
was he able to do this? In 1897 Thomson used a cathode
ray tube – a gas-filled tube containing high-speed electrons
emitted by a negative electrode when an electric current
passes through it - to show that atoms of any element
could be made to emit tiny negative particles. He concluded that all types of atoms must contain
these tiny negative particles, which we now call electrons. He believed that atoms were the central
part of all matter, and developed a model of an atom in which a number of negatively changed
electrons were embedded in a sphere of positive electricity, with the two charges neutralizing each
other.
Thomson’s model of the atom is called the “Plum
Pudding model.” This theory states that the atom
looks something like plum pudding, or a positively
charged “pudding” with negatively charged “plums,”
or electrons, distributed evenly throughout. He
reasoned that the atom might be thought of as
uniform “pudding” of positive charge with enough
negatively charged electrons throughout to
counterbalance the positive charge.
Reflection Questions: Please answer these questions in your own words using complete sentences
after you have read the passage above.
When did Dalton come up with his atomic theory model?
Name 3 important components of Dalton’s atomic theory.
Which scientist is responsible for the discovery of electrons? Describe the experiment that led to
this discovery.
What is Thomson’s plum pudding model? Give an explanation in words and then draw a picture.
How did Thomson’s model of the atom build on Dalton’s model of the atom? What did he discover
about atoms that Dalton’s model didn’t include?
Rutherford, Bohr, and Chadwick: moving toward current understanding of Atoms
Sir Ernest Rutherford was born in 1871 in New Zealand. Perhaps more than any other scientist, he
formed modern-day views about the nature of atoms and matter (This sounds important, doesn’t
it???) Around the year 1898 Rutherford discovered a number of radioactive atoms. Although others
had begun the earliest research into radioactivity with atoms, Rutherford soon dominated this
field. He found two different types of radiation, which he called alpha (α) and beta (β). α particles
are positively charged particles with a mass approximately 7500 times that of an electron.
Rutherford made his greatest discovery in 1909. In
studying the flight of α particles through air, Rutherford
found that some of the α particles were deflected by
something in the air. Puzzled by this observation, he
designed an experiment that involved directing α
particles toward a thin metal foil. Surrounding the foil
was a detector coated with a substance that produced
tiny flashes whenever it was hit with an α particle.
Although most of the α particles passed straight through
the foil, some of them were deflected at large angles.
Rutherford later responded to his experiments by saying
that "It as if you fired a fifteen-inch shell at a piece of
tissue paper and it came back and hit you." In early 1915 he announced his version of an atom
structure: a very small, very tightly packed, charged nucleus which had electrons orbiting it. The
deflected alpha particles were those that had come into close proximity with the positively charged
nucleus and had rebounded in various directions. Rutherford’s constant research and questioning
led to an amazing discovery!
Rutherford’s atomic model of the atom
However, Rutherford’s model was not perfect. One of the amazing things about science is that
theories and ideas are never completely finished. Many scientists took Rutherford’s theories and
developed them even further. One of these scientists was Niels Henrik David Bohr. Niels Bohr was
a Danish physicist who was born in 1885. He is known mainly for his work in the field of atomic
theory. Bohr worked under Ernest Rutherford, however, Bohr’s problem with Rutherford’s model
of the atom was that, according to the theories of electrodynamics (something we will get to later),
the electrons should radiate (or release energy and drop down into the lower energy levels) and
therefore lose energy and crash into the nucleus of the atom. Bohr then worked relentlessly for
three months, and was successful in obtaining a formula which correctly described how helium
nuclei were absorbed by other atomic nuclei. He believed that the energy that was lost by a helium
atomic nucleus in flying through an atom did not depend on the size of the atom, rather, it
depended on the distances between the nucleus and the various electrons in the atom.
Niels Bohr
In 1913 Bohr produced a model of the atom in which radiation was emitted only when an electron
jumped from one orbit, or shell, to another. The frequency of light that an atom emitted was not
related to the frequency of or in the atom, rather, it was based on the difference between two
energy levels within the atom.
In Bohrs model of the atom, electrons can jump from a low-energy orbit near the nucleus to orbits
of higher energy by absorbing energy. When the electrons return to a lower energy level, they
release the excess energy in the form of radiation of a characteristic wavelength, such as visible
light.
However, Bohr’s model of the atom was not quite complete! Sir James Chadwick was a British
physicist who was born in 1891. Scientists were having trouble with the structure of the atom
before Chadwick discovered the neutron. Although the protons and electrons were equal in
number so that the atomic charge in the atom was neutral, the atoms did not weigh the same
relative to each other. For example, a helium nucleus has twice the charge but four times the mass
of a proton. Clearly, the scientist thought, nuclei contain something more than just protons. In
1932 Chadwick solved this problem, with the discovery of the neutron - a particle with no electric
charge and slightly more mass than a proton, which sits in the nucleus of the atom. In 1935
Chadwick received the 1935 Nobel Prize, and he was knighted for his work on atoms and atomic
theory in 1945.
Chadwick’s model of the atom is the most similar to the current understanding of what the atom
looks like. An atom is composed of a dense nucleus, containing positively charged protons and
neutrons without electric charge. The nucleus makes up the majority of the mass of the atom.
Electrons are much, MUCH smaller than protons, but they have the same magnitude of charge as a
proton (except they are negatively charged, not positively charged). Surrounding the nucleus, the
electrons surround the nucleus in “electron clouds” occupying different energy levels. We will get
more into what this means later. An important question arises at this point: If all atoms are
composted of these same components, why do different atoms have different chemical properties?
Think about this as we move into the study of the atom and the periodic table!
Reflection Questions: Please answer these questions in your own words using complete sentences
after you have read the passage above.
1. Which scientist is responsible for the discovery of an atomic nucleus? Describe the
experiment that led to the discovery.
2. Explain Bohr’s problem with Rutherford’s model of the atom.
3. Would Bohr have been able to make his discovery without Rutherford’s ideas? Explain why
or why not.
4. After you are finished reading and answering the questions above, use the space below to
create a “timeline” of the history of the atom. Going from earliest theory to the most
recent,
a. write the date that each scientist proposed their theory
b. Include the name of the scientist
c. Include the experiment that led to the discovery
d. Write a description about what each theory stated about the atom
e. Make a connection between the scientific theory you are listing and the previous theory
about the atom (Example: Bohr used Rutherford’s model for the atom but realized that
the model was missing quantized energy levels)
Be sure the complete this before starting the homework! Completing this “outline” of a timeline
will help you be successful on your homework.