Lesson Video: Electron Shells Chemistry

In this video, we will learn about the electron shell model and how to use electron shell diagrams. We’ll describe how electrons are arranged in the shells of atoms and ions and convert back and forth between electron shell notation and electron shell diagrams.

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Video Transcript

In this video, we will learn about the electron shell model and how to use electron shell diagrams. We’ll describe how electrons are arranged in the shells of atoms and ions and convert back and forth between electron shell notation and electron shell diagrams.

We know there are many types of atom or ion called elements. Inside the nucleus, we find protons and neutrons. And it’s the number of protons that determines the element. If we count up the protons, we’ll get the atomic number for the element. We can find lots of information about the elements on the periodic table of elements. Let’s take the element carbon as an example. The atomic number of carbon is six, which means atoms or ions of carbon contain six protons. So, if we have an atom or ion of carbon, we know for sure it contains six protons, but we don’t know for sure the number of neutrons, but we are going to look at that in this video.

What’s more important is to remember that protons are positively charged, which make the nucleus positively charged. And the nucleus strongly attracts negative electrons. Electrons and protons have equal but opposite charge, meaning that to form a neutral atom, we need exactly the same number of electrons as we have protons. If we have more electrons or fewer electrons than the number of protons, then we have an ion with an overall charge. If there are more electrons than protons, then we have a negatively charged anion. But if we have fewer electrons than protons, then we have a positively charged cation.

Here are the steps you’d have to follow to work out the number of electrons in an atom or ion. The first thing you’ll have to do is determine the number of protons in the atom or ion. You might need to count them in a diagram or use the atomic number found by looking at the element’s symbol. Then, the number of electrons is simply equal to the number of protons, at least for an atom. For an ion, however, you would add or remove electrons for the charge, adding electrons for an anion or taking them away for a cation.

Let’s run this through with an example, Mg2+. We can look up Mg on the periodic table and see that the atomic number for magnesium is 12. So, we have 12 protons. Then, we convert the number of protons to the number of electrons, and we’d have 12 electrons if we were dealing with a magnesium atom. But we’re dealing with a magnesium two plus cation, so we take two electrons away, leaving us with 10 electrons.

Now we know how to count the electrons in an atom or ion, let’s have a look at how they’re arranged.

We have known for a long time that electrons aren’t just orbiting at random around the nucleus. Some electrons are closer to the nucleus than others. And the way they move and interact is very complicated. Quantum theory can describe the bizarre way that electrons move around the nucleus. However, neither of these models are the best tools for understanding the basics of chemistry, so we need a model in between.

That’s the electron shell model. In this model, electrons are said to occupy shells around the nucleus. Each shell is further from the nucleus than the last. Like layers of an onion, they get bigger and bigger and bigger. The shells further from the nucleus can fit more electrons. And lastly, electrons are more stable if they’re in a shell closer to the nucleus. This model is only really effective for the first 20 elements. Beyond this, it doesn’t tell us enough about how electrons really behave.

The very first electron shell is the one that’s closest to the nucleus, and it’s quite small and can fit only two electrons. We can draw an atom of hydrogen or an atom of helium like this. The electron configuration is how we write out the number of electrons in each shell. For hydrogen, it’s simply one. For helium, it’s two. There’s not enough space in the first shell for any more electrons. So, for particles with three or more electrons, we need to go to the second shell.

The second electron shell is further from the nucleus, it’s bigger, and it can fit eight electrons. We can draw atoms of lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon like this. Lithium has an atomic number of three, so an atom of lithium has three electrons: two in the first shell and one in the second. For atoms of the other elements here, we’re simply adding one electron to the outer shell until we have a total of eight. Now that we filled the second electron shell, it’s time to move on to the third.

The third electron shell is even bigger. In this simple model, it can contain a maximum of eight electrons. At higher levels, you may be told that it can fit 18. This is done when we include the first row of the D-block of the periodic table, scandium to zinc. But this is where the model breaks down. You don’t need to worry about this for now. Just assume you’ll never need to use this model beyond calcium. Atoms of sodium to argon have their outer electrons in the third electron shell, with configurations two, eight, one to two, eight, eight. Now, we can move on to the fourth shell.

The fourth electron shell can contain 32 electrons, but this simple model only really works for the first two. Using this simple model, we can draw atoms of potassium and calcium like this. The electron configuration of a potassium atom is two in the first shell, eight in the second, eight in the third, and one electron in the fourth. Remember, you can look up the element’s symbol in the middle of the diagram on the periodic table and use the atomic number to determine the number of protons in the nucleus. Once you’ve drawn your diagram, you can check you have the right number of electrons by adding up the number of electrons in each shell.

Before we go any further, it’s worth at this point having a look at what use the electron shell model is.

Here are the first 20 elements from hydrogen to calcium. And we have the atomic numbers for each element, which tell us the number of protons there are in an atom or ion of that element. In chemistry, we want tools that will quickly and reliably tell us how elements will react with one another. To start off with, we can have a look at how these elements pair up. For this, we’re going to simplify things a lot and just look at the number of bonds formed by individual atoms or the charge of the ions they’ll form.

Helium, neon, and argon are unreactive noble gases, so they have a combining power of zero. Hydrogen atoms tend to form single bonds or singly charged ions. The metals lithium and beryllium commonly form cations, lithium plus and beryllium two plus, while boron and carbon will tend to form three or four covalent bonds, respectively. Beyond carbon, the combining power goes down, with nitrogen tending to form three bonds, oxygen forming two, and fluorine forming one. We can also get O2− and F− anions. Between neon and argon, we get the same pattern, where the combining power increases to a maximum of four for silicon and decreases down to zero again. And looking beyond argon, we get K+ and Ca2+ ions.

If you look carefully, you should be able to see a pattern that corresponds to the maximum number of electrons that can fill the electron shells: two, eight, eight, and so on. We can express this pattern in a simple principle. Atoms of any element will gain or lose electrons or form covalent bonds until they have a full outer shell. There are some exceptions, but you can still understand a great deal of very interesting chemistry using this simple model. We can use the periodic table to predict how a particular element will react.

Carbon atoms will tend to form four covalent bonds, filling up their outer shell. On the other hand, atoms of magnesium will tend to lose the two outer electrons in the third electron shell, leaving a full second electron shell underneath. You don’t need to remember all this detail, but what is helpful to remember is that this model is useful for predicting basic chemical behavior. Now, let’s have a look at the diagrams when we add or remove electrons.

This is how we might draw an atom of sodium, with 11 protons in the nucleus and 11 electrons in the electron cloud. Using our electron shell model, we can describe in more detail how those electrons are arranged: two in the first shell, eight in the second shell, and one in the third.

Sodium, the element, is in group one of the periodic table. So, we would predict that an atom of sodium is quite likely to lose its outer electron and form an Na+ ion. When removing electrons, we usually remove the least stable electrons first, the ones in the outer shell. In the case of sodium, we produce an Na+ ion with an empty third shell and a full second shell. The outer shell is this shell furthest from the nucleus that still has a least one electron in it.

We see a similar situation but in reverse for chlorine on the other side of the periodic table. The element of chlorine in group 17 of the periodic table, otherwise known as group seven, will commonly react to gain one electron or form a single covalent bond to fill that empty space in the third electron shell. And this is how you would draw the chloride ion, with an electron configuration of two, eight, eight. And for the sodium ion, the configuration is two, eight. It’s quite common to see these diagrams combined, where we see an electron move from the outer shell of the sodium atom to the outer shell of the chlorine atom. As you can see, the total number of electrons, 28, stays the same.

Before we look at some examples, I’m going to show you how electron shell diagrams might be used to illustrate various forms of bonding. Let’s look at the covalent bonding between two hydrogen atoms. You might see electron shell diagrams illustrate the two electrons in between the hydrogen nuclei. Since they’re sharing electrons, the two individual hydrogen atoms effectively have full outer shells.

A good example of an ionic attraction is what occurs between lithium and fluorine atoms after they react. When the atoms come close enough together, an electron from the outer shell of lithium hops over to the outer shell of fluorine, forming Li+ and F− ions. The second electron shell of the Li+ ion is empty, so it doesn’t need to be drawn.

Electron shell diagrams aren’t typically used to illustrate metallic bonding, but let’s have a go anyway with two beryllium atoms. When metal atoms bond and come together, they lose their outer electrons and they become delocalized. However, it gets a little messy to use electron shell diagrams to illustrate this. Instead, you will more commonly see metal ions surrounded by a sea of delocalized electrons. Now, let’s get on with that practice.

What is the maximum number of electrons in the first electron shell?

An electron shell is a place around the nucleus where an electron could be. The first electron shell is simply the electron shell that is closest to the nucleus. The first electron shell is very small, so it can only fit two electrons. There aren’t very many ways of simply remembering this, but you can nudge your memory by looking at the periodic table.

There are only two elements in the first period, the first row, of the periodic table. A hydrogen atom has one electron, and a helium atom contains two. When we go to lithium, we have to go down a row, and we have three electrons per atom. And the third electron has to go in the second shell because the first is full. When we move down a row in the first few rows of the periodic table, it means we’re adding the outer electrons to a new shell. So, that’s how we can remember that the maximum number of electrons in the first electron shell is two.

Now, let’s do a question where we’re looking at analyzing an electron shell diagram.

Which picture shows the arrangement of electrons in an atom of oxygen?

What we’ve been given are five electron shell diagrams. In the middle is a drawing of a nucleus, much, much bigger in relation to the atom than it would be in real life. The red circles with p are protons, and the white circles with n are neutrons. The black circles are electron shells, which can fit a limited number of electrons. And the blue dots are the electrons.

What the question is asking for is the picture that shows the arrangement of electrons in an atom of oxygen. Oxygen is an element that we can find on the periodic table. There, we see that the atomic number of oxygen is eight. This means that every atom or ion of oxygen has eight protons in its nucleus. Atoms are neutral, which means we need an equal number of electrons and protons. The first thing we can do is check all of our diagrams depict eight protons and eight electrons.

All the nuclei look identical, and they each have eight protons. This means we’re dealing with nuclei of oxygen, and we can proceed to the next test. The easiest way to count out electrons is to work out the electron configuration of each diagram. We do this by counting the electrons in each shell, starting with the first shell.

In the first diagram, there are two electrons in the first electron shell, there are two in the second, and there are four in the third. This is the right number of electrons, but we’ll come back to this in a moment. The second diagram has configuration two, six. And the third diagram has configuration zero, eight. Remember, the zero still matters because it’s an inner shell. The fourth diagram has configuration eight. And the last diagram has configuration four, four.

Each electron shell has a fixed maximum number of electrons that it can fit. You can fit two electrons in the first electron shell and up to eight in the second. The other principle we’re going to use to find the answer is that electrons occupy the most stable space. For electron shell diagrams, that means the lowest available shell.

So, in the first diagram, we can see that there are two electrons in the lowest available shell. That’s good. However, there are four electrons in the third shell when there’s still space in the second. So, this is not the correct diagram. In the second diagram, there are again two electrons in the first electron shell. And the remaining six out of the eight electrons fill the second shell as they should. So, this is the correct diagram. But let’s look at the other three just in case.

In the third diagram, there are eight electrons in the second shell when there are two available spaces in the first. So, this is not the correct configuration. For the fourth diagram, all the electrons in the first electron shell, which is six too many. And in the last diagram, there are two too many electrons in the first electron shell. Meaning that the picture that shows the correct arrangement of electrons in an atom of oxygen is the one with eight protons in the nucleus, two electrons in the first shell, and six in the second.

Now, before we finish, let’s review the key points. An electron shell is a place around the nucleus that can fit electrons. Electrons in shells closer to the nucleus are more stable. A maximum of two electrons can go in the first electron shell. And a maximum of eight electrons can go in the second shell.

In this simple electron shell model, we can only add eight electrons to the third shell before we start adding electrons to the fourth. In electron shell diagrams, electrons typically occupy the lowest energy positions first, unless we’re dealing with something like an excited state. And we can convert between electron shell diagrams and electron configurations by counting the number of electrons in each shell. In this case, there are two in the first shell, eight in the second, and there’s one in the third. And if you know the element you’re dealing with, you should be able to convert between these forms.

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