Lesson Video: The Periodic Table

In this video, we will learn about the periodic table of elements: how it is laid out, the names for its parts, and the neat ways that the position of elements on the periodic table indicates some of their properties.

17:59

Video Transcript

In this video, we will learn about the periodic table of elements, how it’s laid out, the names for its parts, and the neat ways that the position of elements on the periodic table indicates some of their properties.

Chemistry is all about chemicals and how they react. But if you try to remember every detail of every chemical, you’ve got a big job on your hands. There are about 160 million chemicals that are officially identified and an infinitely greater number we could imagine. Even if all we look at are the named chemicals in use today, we’d still have way too many to remember. However, all chemicals are made out of the chemical elements. And there are a limited number of these. We can use our basic understanding of the elements to predict the bonding in chemical compounds and then predict reactivity and behavior without necessarily understanding the chemical itself.

However, 118 elements is still quite a lot. So there’s a tool called the periodic table, which helps chemists remember lots of details about the properties and behaviors of the elements. We’re not going to look at the history of the periodic table in this video. Instead, we’ll look at the theory behind the modern periodic table and how you can use it to be a better chemist.

Each element is a type of atom or ion, based on the number of protons in the nucleus. The number of protons in the nucleus determines the number of electrons needed to make it neutral and form an atom. The number of protons and the number of electrons is what determines how an atom or ion reacts. So the number of protons in nuclei is really important. It’s taken a long time for scientists to isolate and identify the chemical elements we know now. What we have is an understanding of nuclei with between one and 118 protons.

To start us off, we need to have a basic set of information for each element. A name and a chemical symbol have been chosen for each element. We can put these in a cell or box. The most basic piece of information we have about each element is the number of protons in the nuclei of that element, the number of protons in an atom or an ion. This is the atomic number. The atomic number is usually placed above the chemical symbol, but sometimes you’ll see it at the bottom or elsewhere. The other piece of information that’s really useful is the element’s average atomic mass. This is the average mass of an atom of this element, expressed in unified atomic mass units. This number has to be measured for each element based on the abundance of the isotopes of that element on Earth.

Not all elements have average atomic masses because not all elements are found on Earth in large enough quantities for these measurements to be made. So the other number you might see in a cell for an element is the atomic mass for the most common isotope, or you may see nothing at all. But you needn’t worry. You probably won’t be asked about any element with an undefined atomic mass. If you’re ever worried about which number is which, just remember that the atomic number will only ever be the same size or smaller than the atomic mass. Once we have all the cells, all 118, it’s helpful to put them in order by the atomic number of the element. But chemists also like to be able to compare elements based on how they react and how their electrons are arranged.

To keep things simple for now, let’s look only at the first 20 elements, hydrogen to calcium. The easiest thing to do is to look at the elemental forms for these elements. How they’d exist if they were pure at room temperature and pressure. In their elemental form, a number of these are gases, like hydrogen gas, helium gas, and fluorine gas. The rest will be solids, and we don’t happen to have any liquids here. We also see that some of these elements are metals, which are shiny and conductive. And now we’ve accounted for all our elements. Elements like carbon, which in some forms are conductive, are an exception to the rule that nonmetals are nonconductive.

At this point, clear patterns aren’t emerging, so we have to look deeper before we can find something useful. The next candidate is reactivity. This should be useful because, after all, we’re looking for patterns in how elements react with one another. Hydrogen is reactive. It forms compounds with other elements. For example, hydrogen will react with oxygen to form H2O, water. However, no matter how hard you try, you can’t get helium to react with other elements. The elements lithium to fluorine are reactive, but neon is not. The elements sodium to chlorine are reactive too, but argon is not. And finally, potassium and calcium are reactive. We won’t go further than that for now.

We’ve now identified behavior that three elements in a repeating fashion have in common. But even more importantly, if we look at the sequence of elements in between helium, neon, and argon, we see a repeating pattern, where lithium and sodium, beryllium, magnesium, et cetera, pair up in their behavior. We even see that pattern continue with potassium and calcium. But hydrogen is more like lithium, sodium, and potassium than it is like fluorine and chlorine. We can group hydrogen, lithium, sodium, and potassium together in one column. And we can do the same with helium, neon, and argon. And we can add in the other elements, leaving a gap between hydrogen and helium. This keeps the elements left to right in order of atomic number. But vertically, they’re above or below elements with similar behaviors.

The fact that chemical behaviors are periodic as you increase atomic number is the reason for the name of the periodic table. When we go beyond calcium, things get a little bit more difficult. We have to jump to atomic number 31 to see the repeating pattern of behavior with boron and aluminum. So we have another gap in our periodic table. Along the road between calcium and gallium, we place 10 metals that have more in common with each other than they do with any other element before them. Scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc. Since this is the first time we’ve encountered elements that behave the way these elements do, there’s nothing above them. The elements after gallium ⁠— germanium, arsenic, selenium, bromine, and krypton ⁠— all fit the patterns of their groups.

Now, the modern periodic table is much more complicated than this. And it isn’t possible to go through all the reasons and all the details in this video. If we expand to all known elements, we get a table like this. As the number of protons inside an atom increases, the differences in reactivity get less distinctive. So the way the electrons are arranged is used instead. This makes it easier to agree on where elements should go. This form of the periodic table is known as the long form. And it’s very wide and awkward to use. So this collection of elements is often moved to below the table, giving us the form of the periodic table we’re most used to.

This form of the periodic table is easier to print and use. In this floating block, the element to the top left is lanthanum. So this row is called the lanthanides or lanthanoids. For the row below, the first element is actinium. So this collection of elements is known as the actinides or actinoids. There are many other labels applied to areas of the periodic table, which help us to group elements by the behavior or electronic structure. The simplest division on the periodic table is the line between metals and nonmetals. In their elemental form, metals tend to be shiny and conductive, while nonmetals tend to be not shiny or conductive. Metals tend to be found to the left of the periodic table, with hydrogen as an exception, behaving more like the nonmetals than the metals.

On the nonmetal side of the boundary, we have elements like boron, silicon, arsenic, and tellurium. As we get to the bottom right of the periodic table, elements become rarer and more radioactive, so the metal–nonmetal status isn’t as important. On the metal side of this exact boundary, we have the metals aluminum, germanium, and antimony. However, if you examine the properties of the elements more closely, some elements seem to be like metals and like nonmetals at the same time and have properties that are in between the extremes of the two groups. These elements are called metalloids. Depending on who you ask and how specific you’re being, some elements might be described as a metalloid or as a metal or nonmetal.

For instance, silicon is broadly classified as a nonmetal, but it’s more specifically classed as metalloid. On the other hand, germanium is generally classed as a metal. But it’s also specifically classified as a metalloid as well. These are the other elements generally considered to be metalloids. But some people include polonium and astatine, and some even include aluminum and carbon. Remember that these words are used to categorize elements, so there may be some disagreement. But generally, that’s okay. Just be sure you know which elements you’re talking about when you use those words.

The next keyword for the periodic table is the word “group.” A group is simply a collection of chemically and/or electronically similar elements in a column of the periodic table. For instance, this column or group here contains the highly unreactive noble gases helium, neon, argon, krypton, xenon, and radon and oganesson. Some of the groups are given numbers, and we count left to right. Some people only count eight of the columns, the two to the left and the six to the right. This can be useful when we’re determining valency, which we’ll come to later. An alternative numbering system uses 18 columns, groups one to 18. Some of these groups have special names.

The metals in group one are known as the alkali metals. This includes lithium, sodium, potassium, rubidium, cesium, and francium but does not include hydrogen. The elements in group two of the periodic table are also known as the alkaline earth metals. These are beryllium, magnesium, calcium, strontium, barium, and radium. Some groups don’t have special names or have rarely used names, like the group 15 and group 16 elements that are known as the pnictogens and chalcogens, respectively. And the elements in groups 17, otherwise known as group seven, are called the halogens. These are fluorine, chlorine, bromine, iodine, astatine, and tennessine.

And as we mentioned earlier, the elements in group 18, otherwise known as group eight or sometimes even group zero, are called the noble gases. These are helium, neon, argon, krypton, xenon, radon, and oganesson. Each group has its own chemical character. For instance, noble gases are unreactive and alkaline metals are highly reactive and get significantly more so as you go down the group. We can use groups to predict valency for the first few rows. Valency is the combining power of atoms of an element. Atoms of hydrogen have a valency of one, while atoms of oxygen have a valency of two. So when hydrogen and oxygen combine, we need twice as much hydrogen. What we produce is H2O.

These are the common valencies for atoms of the elements in groups one and two and 13, 14, 15, 16, 17, and 18. However, sometimes valency is also used to indicate the number of electrons we’d expect in the outer shell of an atom. In this case, the valency of atoms of the elements in these groups are these numbers, one for group one and eight for group 18. The number of valence electrons of an atom has a significant impact on its chemical behavior. So it makes sense that groups of elements that react in similar ways would have atoms with the same number of valence electrons.

We’ve had a look at vertical groups. Now, let’s see what we can learn by looking at horizontal rows. We call rows on the periodic table periods. A period is simply a collection of elements with the same highest occupied electron shell. That sounds pretty complicated, so let me just explain what I mean. Every atom consists of a nucleus surrounded by an electron cloud. Using a fairly simple model, we can describe electrons as occupying distinct electron shells at various distances from the nucleus. An atom of hydrogen only has one electron, so that electron goes into the first electron shell. So we say that the element hydrogen is in period one.

Atoms of helium, on the other hand, have two electrons, but they’re still only in the first electron shell. Atoms of lithium, on the other hand, have three electrons. And there’s not enough space in the first electron shell for the third one. So that goes in the second electron shell. So lithium is in period two. We see something similar for sodium, where the outer electron is in the third shell rather than the second. And this pattern continues all the way down the periodic table. In an atom of francium, the highest electron shell containing electrons is the seventh electron shell. So we can use the row or period an element is in on the periodic table to tell us the highest occupied electron shell.

Electrons behave in more complicated ways than the electron shell model can account for. So sometimes you need to go deeper. Individual electron shells can actually contain what we call subshells, types of orbital that differ in their shape. The subshell types are s, p, d, and f. There are others, but they aren’t relevant to the periodic table. As you get further from the nucleus, each shell can fit more subshells. The first electron shell contains just an s-type subshell. The second contains an s-subshell and a p-subshell. The third contains s-, p-, and d-subshells, while the fourth subshell contains s-, p-, d-, and f-subshells. You don’t need to worry about what these subshells really are. They’re just types of orbital for electrons to sit in.

We can group elements based on where we find the outer electrons in an s-, p-, d-, or f-type subshell. For instance, hydrogen and helium are said to be in the s-block. And the metals in group one and group two are also said to be part of the s-block. Atoms of elements in groups 13 to group 18 apart from helium all have their outer electrons in a p-type subshell. So they form the p-block. And in between, we have the d-block and the f-block. So to sum up, blocks group elements based on subshells. So we have the s-block, the p-block, the d-block, and the f-block.

Before we finish, there’s one last collection of elements we need to address. This collection of compounds are generally hard and dense in their elemental form. And they’ll form colored compounds and multiple types of ion. A good example is iron. Iron is hard, dense, forms colored compounds, and forms Fe2+ or Fe3+ ions. Typically, these elements are considered transition metals, although there is some disagreement from source to source.

Elements like scandium are sometimes not considered transition metals because they don’t form colored compounds. But there is some disagreement over what should be a transition metal and what shouldn’t be. The whole of the f-block is generally considered to consist of transition metals. And these are collectively called the inner transition metals. When discussing transition metals on their own, it’s generally assumed that people will be talking about the transition metals in the d-block.

So to sum up, the periodic table condenses lots of information about the elements. Each element has a cell that gives its name, element symbol, atomic number, and atomic mass. Element cells are arranged by atomic number and chemical and/or electronic characteristics. Elements are classified as metals, nonmetals, or metalloids. And we can use the group and period to determine the valency and the highest occupied electron shell. And the table’s broken into s-, p-, d-, and f-blocks. And we’ll find transition metals in the d- and f-blocks.

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