Video Transcript
In this video, we’re talking about
atomic structure. In this lesson, we’ll learn what an
atom is, how it’s put together, and how our understanding of the atom has changed
over time.
The story of the atom goes back a
long way. Ever since people noticed that
larger objects are made up of smaller ones, which themselves are made up of smaller
ones. There’s been the question: what’s
the smallest a piece of matter can possibly be? The idea was that if we figured out
what this smallest bit of matter was, then perhaps, as we start to understand it, we
can begin to fit those pieces together to create something new. For hundreds of years, this
question remained. What are the basic building blocks
of matter? In the early 1800s, an idea was put
forth by a man named John Dalton.
Dalton hypothesized that all matter
— every material object we see, whether our house or a rock or a tree — everything
that’s material is made of very small solid spheres. In this solid sphere model, Dalton
said that there are different types of atoms for different elements. For example, nitrogen has one type
of atom, while oxygen has a different type. And iron has yet a different type,
and so on. As we can see, in this model,
there’s nothing to the atom other than the single solid sphere. And actually, that agrees with the
name atom.
The word Atom comes from the Greek
word atomos, which means indivisible. That’s true to the idea that the
atom is the smallest possible chunk of matter. Nothing smaller can exist. And hence, Dalton came up with an
idea for the atom that was made of a solid sphere. For a while, this model of the atom
was the best game in town. It was the best way we had of
explaining what matter was at the smallest scale. In the early 1900s though,
experimental evidence started to emerge, which changed our understanding of what an
atom looks like. Thanks to this evidence, the
discovery was made that atoms contain electric charge in them. Furthermore, it was hypothesized
that both kinds of electric charge, positive as well as negative, were contained
within the atom.
The working theory in this model
was that the atom was a generally positive structure, with small bits of negative
charge embedded within that positive cloud. This came to be called the plum
pudding model of the atom. The idea was that just like bits of
fruit are embedded in a pudding, so the negative electric charges were embedded in
the large positive mass. But then, thanks to yet more
experimental evidence, something really significant happened with our understanding
of how the atom works. The discovery was made that the
atom consists of a small core with an overall positive charge with smaller
negatively charged particles that move around that core. The name given to this positive
core is the nucleus. And so, this model came to be
called the nuclear model of the atom.
Notice, though, that at this point,
with the nuclear model, the word atom no longer accurately describes what we’re
working with. As we saw, the word atom refers to
something that can’t be divided into smaller pieces. But according to this model, the
atom does consist of separable parts, the positive nucleus and the negative charges
around it. Despite this fact, the name atom
stuck. And of course, we still use it
today. At about the time that this nuclear
model of the atom was gaining in popularity, our understanding of quantum physics
and quantum mechanics was also developing. In response to this, in 1913, a
scientist by the name of Niels Bohr came up with a refined model for the atom.
According to Bohr, the atom did
indeed consist of a positively charged core, the nucleus, with negative charges
moving around it. But he said that those negative
charges moved in very particular ways around the nucleus. For example, there were certain
allowed distances that the negative charges could be from the nucleus and certain
disallowed distances. This gave rise to the idea of
electron orbitals. And that word orbit makes us think
of planets orbiting around a central star. Bohr’s planetary model of the atom
modified the nuclear model by saying that, thanks to the developments of quantum
mechanics, we now know that these negative charges moving around the positive core
can only move in certain ways and at certain distances from that nucleus.
Even though this is called the
planetary model, according to Bohr, the orbiting objects, the negative charges,
could do things that planets don’t do. Specifically, the negative charges
are capable of moving from one orbital, the one that they’re in, to another orbital
within the atom. And Bohr said it’s possible for
these negative charges to move from an orbit farther from nucleus to one closer to
it, as well as one closer to it to one farther away. The important thing to realize
about all this shifting of negative charges from one orbital to another is that each
shift involves an exchange of energy.
We could think of it this way. The energy of an orbiting negative
particle increases as it moves farther and farther away from the nucleus. This means that a negative particle
orbiting out here, farther away from the nucleus, has more energy than a negative
particle orbiting in here, closer to the nucleus. But once the more energetic
negative charge moves into a closer orbital to the nucleus, it needs to take on the
lower energy level characteristic of that orbital. In other words, to make this
transition, this negative charge needs to lose or release some energy. The normal way this happens is when
the electron makes this transition to a lower energy level. It releases some energy in the form
of light.
The way this works is that when our
negative charge moves to a lower energy level, it releases its excess energy as
light. And then, in order to enable a
lower energy negative charge to move to an orbital farther away from the nucleus, it
needs to be given some energy. And that typically happens with the
negative charge absorbing some light energy. So, as we can see, Bohr’s planetary
model of the atom allowed for negative charges in the atom to transition from one
level to another. And when these transitions
happened; some exchange of energy was involved.
As helpful as the planetary model
was, about 15 years later, even it was refined into a new model. This newer model retained the
positively charged nuclear core. But it said that that core is
orbited by negative charges called electrons, which move in a cloud-like
formation. This new model said that at any one
moment, it was impossible to tell with absolute certainty where any of the negative
charges, the electrons, were. But the model could tell where it
probably was with some degree of confidence. Often this model, the quantum
model, is represented this way. By showing that these negative
charges, the electrons, are more likely to be closer in towards the nucleus and less
likely to be farther away. But again, at any one instant, we
can’t say exactly where one is.
This new model, then, is based on
probability rather than certainty about where an object is located. When the quantum model of the atom
was developed, the assumption was that atoms are made up of two types of electric
charge, positive charges and negative charges. Each negative charge was called an
electron, and each positive charge was called a proton. As time went on, though, and
experiments continued with atomic structure, we soon began to realize that this
doesn’t tell the whole story of the parts of an atom. In the early 1930s, thanks to a set
of experiments that involved uncharged radiation. That is, atomic radiation that
could penetrate into material but it wouldn’t give it a charge. It was discovered that there’s a
third kind of particle involved in atoms. This particle had no electric
charge. That was what made it so hard to
find and was therefore called a neutron.
Before the discovery of the
neutron, we thought about the nucleus, the core of an atom, like this. We thought that the nucleus
consisted of a bunch of positive charges, protons. But that in order for the nucleus
to stick together and for its mass to agree with experimental mass calculations,
there must be a bunch of negatively charged electrons also embedded in there. But not as many electrons as
protons because we knew that the nucleus still had an overall positive charge. But then when the neutron, this
uncharged particle, was discovered, we saw that actually the core of an atom, its
nucleus, looks something like this. It looks like a cluster of these
positively charged protons and these neutrally charged neutrons.
So by this point, we believed that
the atom was composed of three separate elements: electrons, protons, and
neutrons. And regarding these three
constituent particles, there are a number of interesting facts. First off, the spacing between
them. If we look at this picture of the
quantum model of an atom, we might think that the electrons orbit the nucleus very
close by relative to the size of the nucleus. But actually, most of an atom is
empty space. That empty space is between the
cloud of orbiting electrons and the solid nuclear core. In fact, here’s how extreme the
situation is.
Imagine that you have a pea, a
single solitary unit of that green vegetable. If this pea represented the
nucleus, the core of an atom, then the orbiting cloud of electrons would be like a
racetrack on a full-length Olympic-size course. So really, relative to this
quarter-mile-long racetrack, our pea would be very, very small, too small to see
actually. This is what the nucleus of an atom
is like compared to its orbiting cloud of electrons. And this is what we mean when we
say that an atom is mostly empty space. Thinking of an atom this way, we
might think that the electrons are much bigger in size than the protons and
neutrons. Since they take up so much more
space, essentially this racetrack compared to this tiny pea-sized core.
But actually, when it comes to
relative sizes, it’s the other way around. It’s the proton and the neutron
which are much, much bigger than the electron. The mass of the proton, we can call
it 𝑚 sub proton, is approximately equal to the mass of a neutron. And that’s approximately equal to
1800 times the mass of an electron. It’s for this reason that when we
calculate the overall mass of an atom — some combination of protons, neutrons, and
electrons — it’s normal to neglect the mass of the electrons because they’re so much
smaller than the mass of the other constituent particles. They’re so small; we could say
they’re negligently small. We can neglect them in the mass
calculation.
Not only do protons, neutrons, and
electrons have a mass property, but we could also say that they have an electric
charge property. Electrons have an overall negative
electric charge, we symbolize that using negative one, and protons of an overall
positive electric charge. We symbolize that relative charge
with plus one. And neutrons, as their name
implies, have no electric charge. By the way, when it comes to atoms,
it’s fairly standard for an atom to have no overall electric charge, even though it
does have protons and electrons within it. Looking at this planetary model of
an atom, we count five protons in the nucleus and then five electrons orbiting
it. This means all the positive charge
in the nucleus is balanced out by the charge of the electrons. Overall, this atom is called
neutral.
One way to change this neutral atom
to a charged atom, called an ion, would be to add or subtract electrons to it. Now that we’ve gotten a bit of a
sense for the structure of an atom, let’s get some practice working with these ideas
through an example.
If a neutral atom has 12
protons in its nucleus, how many electrons does it have?
To figure out the answer of
this question, let’s recall the basic structure of an atom. Every atom has a nucleus made
up of some number of protons and neutrons. And that nucleus is surrounded
by orbiting electrons. In this question, we’re told
that we have a neutral atom that has 12 protons in its nucleus. If we consider the protons to
be these rose-colored spheres in our nucleus, then we can count one, two, three,
four, five, six, seven, eight, nine, 10, 11, 12 of them. For this atom to be considered
a neutral atom as it is, that means the charge of these protons must be balanced
out by the charge of the orbiting electrons. We can recall then what the
relative charge of a proton is and what the relative charge of an electron
is.
Now, independent of one
another, a proton does have a specific amount of electric charge, and so does an
electron. But in this case, we’re not
concerned with that specific amount, but rather just how the charges compare to
one another. From that perspective, we can
recall that the charge of a proton is equal and opposite the charge of an
electron. So if we let the relative
charge of a proton be plus one, then the relative charge of an electron would be
negative one. This tells us that if we add
together a proton and an electron, the combined electric charge of that
combination is zero. In other words, together, the
particles are overall electrically neutral. That helps us figure out the
answer to our question, which asks, how many electrons does this neutral atom
with 12 protons have?
If the charge of one electron
effectively cancels out the charge of one proton, then that must mean our
neutral atom has the same number of protons as electrons. We have 12 protons in our
nuclear core. And right now, we have one,
two, three, four, five, six, seven, eight, nine electrons. We’ll need to add three more to
this picture so that the total negative charge, negative 12, balances out the
total positive charge of positive 12. And this then tells us our
answer. We need to have 12 electrons
for this to be a neutral atom. That’s how many will balance
out the charge of the 12 protons in the nucleus.
Let’s take a moment now to
summaries what we’ve learned about atomic structure. In this lesson, we’ve seen that
atoms are the building blocks of matter. In other words, they’re the
smallest constituent parts of material objects. Moreover, we saw that the models of
what an atom looks like had been refined over time. Thanks to new discoveries and new
experimental evidence, the solid sphere model of the atom became the plum pudding
model. Which became the nuclear model
which transitioned to the planetary model. And then finally to our current
model, the quantum model of the atom.
In this nuclear model, we’ve seen
that atoms consist of a positive core, called the nucleus, which is made up of
protons and neutrons as well as a negative orbiting cloud of electrons. Furthermore, we saw that protons
have a positive charge. Neutrons have no electric
charge. And electrons have a negative
charge. And finally, we saw that while most
of an atom is comprised of empty space, the nucleus, which is made of protons and
neutrons, accounts for essentially all the mass of the atom. That is, compared to the mass of a
proton and the mass of a neutron, the mass of an electron is negligibly small.
