Video: Atomic Structure

In this lesson, we will learn how to describe the Bohr model of the atom and the particles that make up atoms.

14:07

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.

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