♪ [patriotic music playing] ♪ [fireworks exploding] >> host: Look familiar? Since the seventh
century in China, people have been using fireworks in festivals & celebrations.
Have you ever wondered how different fireworks give off different colors? Or why some fireworks
seem to explode right away while others take awhile? Fireworks are a great example of chemistry in action.
So to answer these questions it will help to understand the structure of the atom.
I know it might not sound so exciting right now, but this is chemistry knowledge that you'll be able to use
to predict characteristics of many everyday substances such as calcium and carbon. So, let's get started!
♪♪
Welcome to Chemistry Matters. In this unit, we're going to focus on
different models of the atom and their connection to the periodic table.
As you know from the science and engineering practices, a model is a physical, conceptual,
or mathematical representation of a real phenomenon whose purpose is to explain and predict
the observed phenomenon.
Scientists use observations and data from experiments to develop models, and models can change over time
with new information. So let's visit our classroom to learn about models of the atom.
♪♪ >> professor: We're going to start by looking at the first model
of the atom. And believe or not, this marble represents this model. Picture it smaller
than even a grain of sand. What can you tell me about it, as a model of the atom,
of course? >> student: It's circular. >> student: It's a solid. >> professor: Good.
So this model's a sphere and it's solid. This model of the atom was proposed by John Dalton.
Dalton was a scientist working in the early 19th century, who based his model off the work
of the Greek philosopher Democritus. Democritus coined the term
which means uncuttable. Democritus' model of the atom lasted for hundreds of years before scientists started
to consider alternate ideas. One of those scientists was John Dalton. Dalton believed that atoms
were tiny spheres that could not be cut into anything smaller. He thought all atoms
of a particular element were identical and that these atoms could be combined in specific ratios
to form new substances. Many of his ideas are found in our current model of the atom,
but not all of them. Now, let's take a look at the next model of the atom. How does this model
look different or similar to the Dalton model? >> student: It's still a circle >> student: But it's different
because it has negative charges spread throughout it. >> professor: Right. >> student: It's still a solid.
>> professor: Good. So, this is JJ Thomson's model of the atom. It's still spherical.
But instead of being completely solid, it's now made up of different parts,
known as subatomic particles because they are inside the atom.
In the early 1900s physicist JJ Thomson was experimenting with a cathode ray tube,
and discovered the electron. An electron is a negatively charged particle in an atom.
This discovery led to the Plum Pudding model of the atom. Of course most of us have never seen a plum pudding,
so I like to call it the chocolate chip cookie model, or the mint chocalate chip cookie model, as this case.
Thomson's model included positive and negative charges. Throughout his experiments, Thomson found
that the negative charges take up less space than the positive charges. The negative charges
spread throughout the positive area like the mint chocolate chips in this cookie,
or like the plums in the plum pudding model. ♪♪ >> host: As you can see,
our students are looking at different models of the atom to determine how they've changed over time.
As science and technology advance, it becomes easier for us to study the substances around us.
These students have looked at models through the early 1900s, but a lot has changed since then.
Scientists will keep using a model as long as the model explains their observations.
As technology and tools improve, sometimes models break down and have to be revised.
That's what science is like; we keep using the best explanation
until that explanation doesn't work anymore. Then we have to come up with a better explanation,
or model, of our observations. Now, let's go back to our classroom
to see how the atomic model kept changing. ♪♪ >> professor: We've explored
the first two models of the atom, but as with most science, improvements to the model
continue to happen as new information is available. Around the same time that Thomson was working,
another model hit the scene. Look at this model. How is it different
from Thomson's model? >> student: It has rings that go around it. >> student: Yeah, but there's
something in the middle of it. >> student: It still has positive and negative charges though.
>> professor: Right. This is called the solar system model because the electrons
orbit the positive charge area in the center, like planets going around the Sun.
The positively charged area is known as the nucleus.
Ernest Rutherford developed an experiment where he shot small positively charged particles
at a very thin piece of gold, which is made up of lots of gold atoms.
Remember, at the time, Thomson's model was the accepted model of the atom.
Thinking about the Thomson model, what would Rutherford have expected to happen
when he shot the particles at the gold foil? >> student: Well, he probably expected them
to all bounce back. >> student: Wait, why would they all bounce back? >> professor: That's a good
question. What is it in the model that makes the particles seem to bounce back?
>> student: Well, the model's made up of all positive charges, so I'd think that those positive charges
would make the particles bounce back. >> professor: That's an interesting hypothesis.
Rutherford expected the particles to go straight through the gold atoms
and only bounce back if they hit the negative charges that were spread throughout Thomson's model.
What really happened was that many of the particles went straight through the foil,
while others bounced back or off at different angles. Remember that the foil was made up of tons of atoms of gold,
so the particles were also going through the atoms themselves. So, now, why did the particles bounce off?
>> student: They hit something? >> professor: Okay. What did they hit? >> student: The nucleus.
>> professor: Good. Rutherford thought that the particles must be hitting something
with enough mass to cause the particles to deflect, or bounce off. Rutherford called this mass
the nucleus. The particles that passed straight through the foil must have moved
through empty space between the nucleus and the electrons. I'm gonna give you guys
all models. Just like a real atoms, you can't see inside it very well, or at all.
I put one marble inside each box. You can roll it around the inside if you tilt it.
Tilt the model and let the marble inside roll around.
Use your observations to determine what's on the inside of the box.
>> student: This one seems to have something right in the middle. >> student: This one seems like
it has a bunch of objects stuck all over the inside of the box. >> professor: Right!
Even though you can't see what's on the inside of the box, you can still use observations to figure out
what the atoms must be like. The marble rolling around represents the particles
hitting the atom in Rutherford's experiment. Because of the way they strike objects in the box,
you can tell what the inside of the box must be like. Since Rutherford knew from Thomson
that the electron had very little mass and a negative charge, Rutherford determined
that the nucleus must be positively charged and have a lot of mass. Eventually, the positively
charged particle in the nucleus became known as a proton. Later on, James Chadwick
discovered another particle within the nucleus, called the neutron. Neutrons have mass
similar to the protons, but have no charge. Neutrons will become important for us later on.
Now, let's look at the next model. This is the Bohr model. How is this model different
from Rutherford's? >> student: Well, It has circles instead of orbits. >> student: It has a nucleus
in the middle. >> professor: Good! You'll notice that the Bohr model
is very similar to the Rutherford model, except in the Bohr Model, some electrons are closer
to the nucleus than others. These electrons are found in the circles you mentioned. These circles represent
specific energy levels.
Bohr used mathematics to study light, gases, and energy. He found that electrons move within specific energy levels,
each of which requires different amounts of energy. This set amount of energy is known as a quantum.
The different color of fireworks are one way to demonstrate these quantum energy levels.
We'll investigate quantum energy levels later, and, although, newer models of the atom do exist,
we'll continue to use this Bohr model because it's most useful for this course.
♪♪ >> host: Now we know how the model of the atom came about, and you've seen
the Bohr model, which you'll continue to use throughout this course. Remember, a model can be
a concept, or a thinking tool, or an equation, or a physical model like these. Even something as simple
as this cardboard box can be a useful model of what's inside of an atom.
I love how scientific ideas change over time as we have more technology and information.
From here, we can investigate the different parts of the atom like protons, neutrons, and electrons.
We'll use the information about these sub atomic particles to figure out exactly what it is
that makes those fireworks explode in such vivid colors. Join us as we continue our investigation
of atomic structure in our next video.