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Atomic Theory
Study Guides
Big Picture
The atomic theory has taken centuries to get to where it is today. The observations from several important experiments
have shaped the development of the atomic model. John Dalton first formulated the modern concept of atoms, J. J.
Thomson discovered the electron using a cathode ray tube, and Ernest Rutherford used his famous gold foil experiment
to identify the nucleus. Today’s atomic model is built upon the work of these scientists and many others.
Key Terms
Atom: Basic unit of matter. Made up of protons, neutrons, and electrons.
Chemistry
Development
Proton: Positively charged particle.
Neutron: Neutrally charged particle. Helps hold together the nucleus of large elements by minimizing repulsion
between protons.
Electron: Negatively charged particle orbiting the nucleus. Much smaller than a proton or neutron.
Model: A descriptive, graphic, or three-dimensional representation used to help enhance understanding.
Law of Multiple Proportions: Two or more elements can be combined to form more than one product. In these
different reactions, the ratio of the reactants will always be in small, whole numbers.
Nucleus: The center of the atom, made out of protons and neutrons. Held together by very strong forces that
overcome the repulsive forces of the proton.
History of Atomic Theory
Much of the atomic theory was developed without scientists ever observing individual atoms! In fact, individual atoms
can only be observed with a scanning tunneling microscope, which wasn’t developed until the 1980s.
Instead, scientists proposed models that tried to explain the results of scientific experiments. Over the years, the
atomic model has been modified and changed in order to explain new evidence. The table below outlines some of the
major discoveries that contributed to the development of the atomic theory.
Scientist
Contribution
Democritus
(circa 460 BCE
- circa 370 BCE)
First to propose that all matter in the universe are made up of atomos - tiny, indivisible,
solid objects
John Dalton
(1766-1844)
Developed Dalton's atomic theory to explain his observations
Joseph John Thomson
(1856-1940)
Discovered the electron and proposed the plum-pudding model (atom has electrons
studded all over the surface)
Ernest Rutherford
(1871-1937)
Discovered the nucleus and developed the nuclear model of the atom (positively
charged nucleus makes up tiny part of atom, negatively charged electrons orbit the
nucleus)
• Lacked experimental support.
• Part of Dalton's theory is the law of multiple proportions
• Disproved Thomson's plum pudding model
Found the charge of an electron
Niels Bohr
(1885-1962)
Proposed the Bohr model (electrons have distinct energy levels and travel in circular
orbits)
Edwin Schrödinger
(1887-1961)
Developed a mathematical equation that described the behavior of electrons in atoms
James Chadwick
(1891-1974)
Confirmed the existence of the neutron
Dalton’s Atomic Theory
Dalton was the first to develop an atomic theory to explain his observations. His atomic theory stated:
1.Matter is made of tiny particles called atoms.
2.Atoms are indivisible. During a chemical reaction, atoms are rearranged, but they do not break apart, nor are they
created or destroyed.
3.All atoms of a given element are identical in mass and other properties. Atoms of different elements differ in mass
and other properties.
4.Atoms of one element can combine with atoms of another element to form compounds in simple whole-number
ratios. This is also known as the law of multiple proportions.
Not all of Dalton’s theory is correct. An atom is actually divisible and is made up of protons, neutrons, and electrons.
This guide was created by Steven Lai, Rory Runser, and Jin Yu. To learn more
about the student authors, visit http://www.ck12.org/about/about-us/team/
interns.
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Disclaimer: this study guide was not created to replace
your textbook and is for classroom or individual use only.
Robert Andrews Millikan
(1868-1953)
Chemistry
Development
of
Atomic Theory
cont .
Thomson’s Cathode Ray Experiment
Thomson developed his plum-pudding model based on the results of his experiments with the cathode-ray tube. His
experimental setup included a glass tube with a fluorescent coating painted onto one end. Two metal disks called
electrodes are placed halfway down the tube.
When the electrodes are connected to a source of electricity, one of the electrodes will be negatively charged (called a
cathode) and the other will be positively charged (called an anode).
A ray of electricity, called a cathode ray, flowed from the cathode to the anode. The anode had a small slit that allowed
the cathode ray to pass through and hit the coating. The spot where the ray hit the coating would glow.
When one positively charged metal plate and one
negatively charged metal plate were placed near the
cathode ray tube, the spot hitting the coating moved
closer to the positively charged plate.
• Since
opposite charges attract and like charges repel, Thomson hypothesized that the cathode ray was
made up of negatively charged particles.
• These negatively charged particles were later named
electrons.
Thomson also measured the ratio of the charge of the
electrons to its mass. He found that:
• The ratio was constant — it did not depend on the gas
in the tube or the metal in the electrodes. Conclusion:
All atoms (of all elements) have electrons.
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CC-BY-NC-SA 3.0
• The ratio was so large, the particles either must carry a huge charge or must be very small. Conclusion: Electrons
are really small.
Plum-Pudding Model of the Atom
Thomson knew that if the atom has negative particles, it must also contain positive material. He
proposed the plum-pudding model, where the negative electrons are embedded in a positively
charged sphere.
Rutherford’s Gold Foil Experiment
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CC-BY-NC-SA 3.0
Gold Foil Experiment
Rutherford tested the plum-pudding model by shooting a
narrow beam of alpha particles toward a thin gold foil. Alpha
particles were helium atoms that lost two protons (atoms
with two protons, two neutrons, and no electrons). The gold
foil was surrounded by a fluorescent screen that would light
up whenever an alpha particle hit the screen.
Nuclear Model of the Atom
Rutherford found that most of the particles passed straight
through the gold atoms without deflection. A small number
of the particles were deflected greatly, and some of the
particles even bounced straight back toward the source.
Based on his observations, Rutherford proposed a new
atomic model:
• The atom is mostly empty space. This is why most of the
alpha particles passed straight through the foil.
• The
center of the atom contains the positive charge and
most of the mass - this region is called the nucleus.
When alpha particles collide with the positively charged
nucleus, the particles bounce back toward the source.
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of
Atomic Theory
cont .
Bohr and the Atomic Spectrum
Bohr modified the Rutherford model by proposing that
the electrons can only be found in specific orbits set at
certain distances away from the nucleus. According to
the Bohr model:
• Each
Support for the Bohr model came from the atomic emission spectrum.
• When
an atom absorbs energy, the atom loses that
energy by emitting light made up of a mixture of
specific colors.
orbit has a specific energy called an energy
level.
• Electrons
further away from the nucleus have more
energy than electrons closer to the nucleus.
• Electrons
cannot continuously lose energy; they
must gain or lose a certain amount of energy (called
a quantum of energy) before they can move from
one allowed orbit to another.
• This
amount of energy is not constant. It takes
more energy for an electron to move between
energy levels closer to the nucleus.
• The
emission spectrum shows the specific colors
as a series of colored lines. Each line has a specific
frequency.
• The emission spectrum is unique for every element.
Bohr proposed that the emission spectrum was a
result of electrons moving between energy levels. He
mathematically calculated the energy levels for the
hydrogen atom. The differences between the calculated
energy levels matched the energies of the frequencies
in the emission spectrum. See the Wave-Particle Duality
chemistry study guide for more details.
Image Credit: Merikanto, Public Domain
Figure: This is the unique emission spectrum for hydrogen.
Quantum Mechanical Model
The Bohr model was later shown to be inaccurate.
• The model did not explain why certain energy levels
existed.
• It could not explain the emission spectrum of atoms
with more than one electron.
However, it helped pave the way for the development
and discovery of quantum mechanics. The quantum
mechanical model (also known as the electron cloud
model) kept the idea that electron energies were limited
to certain values.
Edwin Schrödinger was able to describe the behavior of
the electron in a hydrogen atom with a complex mathematical equation. The quantum mechanical model
comes from the solutions to the Schrödinger equation.
The quantum mechanical model differed from earlier
models in one key way: the quantum mechanical model
did not predict the exact path the electron traveled.
Instead, the solutions to the Schrödinger equation
described the probability of finding an electron at a
given point.
• The electron is found in a volume of space called an
electron cloud.
• The
cloud is often drawn as a fuzzy region around
the nucleus. It has variable densities.
• The
denser the cloud, the higher the probability
of finding the electron there.
• Away from the nucleus, the probability of finding
the electron decreases (the cloud is less dense).
• There is no real boundary to the cloud — there is no
distance from the nucleus where the probability of
finding an electron is zero.
• Chemists
often chose to draw an arbitrary
boundary so that the probability of finding the
electron inside the boundary is 90%.
For more information see the Quantum Mechanical
Model chemistry study guide.
Notes
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Chemistry
Development