Video Transcript
In this video, we will learn about
many of the atomic models that have been used throughout history to help us
understand the composition of matter. And we’ll learn how the model of
the atom has evolved over time as new experimental evidence has emerged. The idea that matter is composed of
tiny entities called atoms is not new. The first atomic theory was
proposed in 400 BCE by the ancient Greek philosophers Leucippus and Democritus. They suggested that all matter is
composed of tiny, indivisible particles that they called atoms, which means
uncuttable. And the different properties of
matter reflect the kind of atoms that make it up.
However, the atomic theory of
matter wasn’t accepted in mainstream science until 1808 when a meteorologist named
John Dalton proposed the first atomic model. Dalton was trying to understand the
physical properties of gases by exploring recent research in mass laws that seem to
be true for all forms of matter. His atomic model, often referred to
as the hard- or solid-sphere model of the atom, was able to explain these mass laws
with great success. His atomic model has four basic
assumptions.
The first is that all matter
consists of atoms that cannot be created or destroyed. The second is that atoms of one
element cannot be converted into atoms of another element. In chemical reactions, atoms don’t
change from one element to another. Atoms of the original substance
simply recombine to form new substances. This assumption has actually since
been proven false due to research in radioactivity pioneered by Marie Sklodowska
Curie since in radioactive processes, atoms of one element can turn into
another.
The third states that atoms of an
element are identical and are different from atoms of any other element. So all atoms of hydrogen always
have the same mass and the same properties, but hydrogen atoms and oxygen atoms are
different from each other. Finally, the last assumption is
that compounds result from the chemical combination of a specific ratio of atoms of
different elements. For example, the compound water, or
H2O, is always composed of a two-to-one ratio of hydrogen and oxygen because each
unit of H2O is made of two hydrogen atoms and one oxygen atom.
The assumptions provided by Dalton
in his atomic model were unchallenged until 1987 through experiments performed by
J. J. Thomson on cathode-ray tubes. A cathode-ray tube is a glass tube
with metal electrodes at the end. When an electric current is
applied, it creates a ray that strikes the end of the tube. While J. J. Thomson was studying these
cathode-ray tubes, he observed that the ray bends in a magnetic field, which
indicated that the ray consists of charged particles. Second, he observed that the ray
bends towards the positive plate in an electric field, which indicated that the ray
consists of negatively charged particles. Finally, he observed that the ray
is identical for any cathode, so these particles must be found in all matter.
He determined that these negatively
charged particles, which he called electrons, were far lighter than the atom. This meant that the atom could not
be the smallest unit of matter, so the atomic model needed to be revised. So Thomson proposed a new model of
the atom, featuring the newly discovered electrons in a sea of positive charge, like
the dried fruit in a plum or a bread pudding, hence the name the plum pudding model
of the atom.
Based on J. J. Thomson’s research, there were two
additional atomic models proposed. In 1902, Gilbert Lewis suggested
that the electrons in an atom were positioned on the corners of a cube. And in 1904, Han Tao Nagaoka
suggested that electrons revolved around a massive center like the rings around
Saturn. With the discovery of this new
subatomic particle, the electron, scientists were interested to know more about its
properties.
In 1909, Robert Millikan set out to
determine the charge of this new particle by watching tiny charged oil droplets fall
in a special chamber. He was able to determine the charge
on each drop of oil that fell. He noticed that all the charges
were multiples of negative 1.6 times 10 to the minus 19 coulombs. So that must be the charge of a
single electron. Amazingly, the charge of the
electron that Millikan was able to measure is within one percent of the value that’s
measured today.
Though the plum pudding model of
the atom successfully incorporated the electron, it was quickly proven false by the
results of experiments performed by Hans Geiger and Ernest Marsden under the
guidance of Ernest Rutherford. Geiger and Marsden shot alpha
particles at a very thin sheet of gold foil and measured the deflection of the alpha
particles with a screen surrounding the foil that flashed when an alpha particle
struck it. Since electrons are incredibly
small and the alpha particles were moving very fast, they expected that all of the
alpha particles would just go straight through the foil. But instead, they observed that a
small fraction of the particles were deflected back the direction they came.
Based on these results, in 1911,
Rutherford concluded that the atom is mostly made of empty space occupied by the
electrons, but there is a small and very dense, positively charged nucleus in the
center of the atom. He suggested that the nucleus of
the atom is composed of positively charged particles that he called protons. The atomic model he proposed based
on these conclusions is often called the planetary model of the atom since the
negatively charged electrons orbit the positively charged nucleus of the atom like
planets orbit the Sun in our solar system. The electrons in this model aren’t
confined to a plane. They can move in orbits around the
nucleus in three dimensions.
The planetary or nuclear model of
the atom included the newly discovered nucleus, but there were some immediate
problems with it. The most obvious problem is that
positively charged things are always attracted to negatively charged things. So what was keeping the negatively
charged electrons orbiting the nucleus? Additionally, the planetary model
suggests that the electrons in the atom should release energy in a continuous
spectrum, which would create a rainbow. But when scientists looked at the
spectrum produced by exciting gaseous atoms, they observed a series of lines of
different colors of light separated by black spaces. This line spectra that was produced
by these excited atoms was unique for each element.
To explain this, Niels Bohr
proposed a new model for the atom in 1913, which is often called the electron shell
or orbital model of the atom. This model features discrete energy
levels that the electrons are restricted to, which correspond to fixed orbits around
the nucleus. When an electron is traveling in an
orbit, the total energy of the atom does not change. If the atom gains energy, the
electron will jump to a higher energy orbit further from the nucleus. If energy is then lost, the
electron will return to a lower energy orbit closer to the nucleus.
Using this model, we could
understand that the distinct lines found in the hydrogen line spectra are due to the
transitions between the fixed energy orbits within the hydrogen atom. When electrons gain energy, they
jump to a higher energy orbit. And they release energy when they
return to a lower energy orbit. This energy that’s released
corresponds to a specific color on the line spectrum, so we see lines for each of
the possible transitions between orbits as the electrons return to a lower energy
orbit. For example, the transition from
the third orbit in the hydrogen atom to the second releases energy that has a
wavelength of 656 nanometers, which corresponds to the color red. And the transition from the fourth
orbit to the second has a wavelength of 486 nanometers, which is light blue in
color.
But despite the success, there was
an immediate problem with this model as well. It was unable to explain the
behavior of atoms that had many electrons. In 1920, shortly after proposing
the existence of the proton, Rutherford proposed that there was another subatomic
particle that exist in the nucleus of the atom that was neutrally charged, which he
called the neutron. He suspected this due to recent
research into atoms of the same element that have different masses, which are called
isotopes. But it wasn’t until the 1930s that
new experimental evidence emerged that could prove the existence of Rutherford’s
proposed particle.
In 1930, Walter Bothe and Herbert
Becker observed that if you bombard beryllium with alpha particles, a neutral form
of nonionizing radiation was produced. They weren’t sure what this
radiation might be, but they thought it could be gamma rays. In 1932, James Chadwick realized
that this neutral radiation that was being created was not gamma rays, but instead a
new particle that has the same mass as the proton, the neutron. This meant that the nucleus of the
atom is composed of both positively charged protons and neutral neutrons.
In 1924, Louis de Broglie was
trying to understand why electrons had fixed energy levels. He had the idea that perhaps matter
had wavelike properties. If electrons had both fixed orbits
and wavelike properties, they would only have certain allowable energies similar to
the vibrations of a guitar string that’s fixed at both ends. As it turned out, de Broglie was
correct. Electrons and other subatomic
particles do, in fact, have wavelike properties. In 1926, Erwin Schrodinger was able
to describe the wavelike properties of matter, which founded the branch of physics
called quantum mechanics.
According to quantum mechanics, the
electron is not simply a small particle that orbits the nucleus. Rather, it is spread out around the
nucleus in all kinds of funny shapes that correspond to the electron’s energy. We refer to this density of
negative charge as an electron cloud. With this, we have arrived at the
modern view of the atom, a nucleus that’s composed of positively charged protons and
neutral neutrons surrounded by a negatively charged electron cloud. Now that we’ve gone through each of
the atomic models, let’s take a look at some example problems.
How was the plum pudding model
different from the hard-sphere model of the atom? (A) The plum pudding model showed
electrons making up the corners of a cube. (B) The plum pudding model
described electrons orbiting a central nucleus. (C) The plum pudding model included
negatively charged particles known as electrons. (D) The plum pudding model included
positively charged particles known as protons. Or (E) the plum pudding model
showed electrons occupying different energy levels.
The hard-sphere model of the atom
was the first atomic model. It was proposed by Dalton. In this model, the atom is the
smallest division of matter. The plum pudding model of the atom
came afterwards. It was proposed by J. J. Thomson who discovered the
negatively charged electrons inside atoms. So the plum pudding model is
different from the hard-sphere model of the atom because in the hard sphere model of
the atom, the atom is the smallest division of matter. But in the plum pudding model,
there is a negatively charged electron which is smaller than an atom. So let’s go through our answer
choices to see which of them reflects that.
Answer choice (A) says that the
plum pudding model shows electrons making up the corners of a cube. In the plum pudding model, the
electrons are embedded in a sea of positive charge. They don’t make up the corners of a
cube. This is actually a reference to
Lewis’s cubic model of the atom. So it’s not the correct answer
choice. The second answer choice says that
the plum pudding model described electrons orbiting a central nucleus. The nucleus of the atom wasn’t
discovered until after the plum pudding model was proposed. This answer choice is actually a
reference to Rutherford’s planetary model of the atom. So it’s not the one we’re looking
for.
The next answer choice says that
the plum pudding model included negatively charged particles known as electrons. This answer choice sounds like it’s
exactly what we’re looking for. The plum pudding model is different
from the hard-sphere model because it includes these negatively charged
electrons. But let’s go ahead and take a look
at our other answer choices just so we know why they’re incorrect. The fourth answer choice says that
the plum pudding model includes positively charged particles known as protons. This answer choice is, again, a
reference to Rutherford’s planetary model of the atom, as he suggested the term
proton to describe the positively charged particles that made up the nucleus.
Our final answer choice says that
the plum pudding model showed electrons occupying different energy levels. This answer choice is referring to
Bohr’s model of the atom, where electrons occupy fixed energy orbits around the
nucleus. So as we discussed before, the plum
pudding model is different from the hard-sphere model of the atom because the plum
pudding model included negatively charged particles known as electrons.
Let’s go ahead and take a look at
another example.
Which diagram most closely
represents Rutherford’s nuclear or planetary model of the atom?
Rutherford’s planetary or nuclear
model is based of observations from the gold foil experiment. In the planetary model, there is a
positively charged nucleus in the center of the atom. And the electrons orbit the
nucleus. So let’s see which of the diagrams
that we have most closely represents this description of the atom. In diagram (A), we have electrons
in orbits around the nucleus. This certainly matches what we’re
looking for. So let’s keep this diagram in
mind. In answer choice (B), we have
positively charged particles in a negative charge. This diagram certainly doesn’t
match what we’re looking for. In fact, it doesn’t match any model
of the atom.
In answer choice (C), we have
particles arranged on the corners of a cube. This diagram is referencing Lewis’s
cubic model of the atom. In answer choice (D), we have
electrons orbiting a central nucleus, which is what we’re looking for. So let’s keep this answer choice in
mind as well. Our final answer choice has
negatively charged particles embedded in positive charge. This certainly doesn’t describe
what we’re looking for, but it does describe the plum pudding model of the atom.
So we’ve eliminated the possible
diagrams to represent the planetary or nuclear model of the atom down to answer
choice (A) or answer choice (D). But what’s different between these
two diagrams? Both of them show electrons
orbiting a central nucleus. Well, the circles in diagram (A)
are meant to represent fixed orbits for the electrons that correspond to discrete
energy levels. So this diagram is actually meant
to represent Bohr’s model of the atom, which features fixed orbits for the electrons
due to the electrons having discrete energy levels that they occupy.
So answer choice (A) isn’t what
we’re looking for either. Which leaves us with answer choice
(D), which is the correct diagram to represent Rutherford’s planetary or nuclear
model of the atom.
So now that we’ve learned about all
the different atomic models and worked some example problems, let’s conclude with
the key points for this video. The first atomic model was Dalton’s
hard-sphere model, which established the atom as the building block of matter. J. J. Thomson’s plum pudding model, which
features negative charges in a cloud of positive charge, added the electron. Rutherford’s planetary or nuclear
model of the atom added the positively charged nucleus of the atom that was
discovered in the gold foil experiments.
Bohr’s electron shell or orbital
model added discrete energy levels for electrons. James Chadwick confirmed the
existence of the neutron. In the modern model of the atom, an
atom is composed of a nucleus made of positively charged protons and neutral
neutrons, which is surrounded by a negatively charged electron cloud.
