The periodic table

0:00- [Presenter] Have you ever wondered why some elements behave very similarly, like how sodium and potassium both react violently with water, while other elements like, say, helium and neon, for example, wouldn't react at all? A powerful tool in chemistry to understand the elements and their properties is the periodic table.

0:20Now, I used to think that this periodic table was just a list of elements, but turns out it's so much more. It's actually a model that organizes these elements in a way that reveals patterns in their properties. But how exactly, let's find out. Let's start with the basics.

0:40Each element on the period table is represented by a chemical symbol, then there is an atomic number, and usually, there's also an atomic mass. Now, the chemical symbol is a shorthand way to represent the element. So Fe over here represents iron. Other chemical symbols involve, for example, H for hydrogen, O for oxygen, and so on and so forth.

1:09Okay, then you have something called the atomic number, that you can see over here. What does that represent? It tells you the number of protons present in the nucleus of the atom. So hydrogen has an atomic number of 1, which means there's only one proton in its nucleus.

1:26Oxygen has an atomic number of 8, that means there are eight protons in it's nucleus. And it's this number that determines which element it is. Oxygen is oxygen because it has eight protons. So if you look at your periodic table carefully, you can see that it is arranged in the order of the increasing atomic number.

1:49Okay, now you'll also often see the atomic mass listed over there. This number is basically the average mass of that elements atom, you know, based on the different isotopes found in nature. So look, even though each box on the period table is small, it is packed with a lot of information.

2:12Now let's talk about some of the patterns that we can find. The first major pattern is how the elements are grouped. We have metals, we have non-metals, and we have metalloids. Most of the elements on the table are metals. And you can see metals are mostly found on the left side of the periodic table.

2:34You can see that non-metals are found here on the right side of the periodic table. And in between them is a zigzag region of elements called metalloids, which have properties in between that of metals and non-metals. Okay, let's quickly recall what these properties are.

2:49So metals are usually shiny. They are solids. Of course, there are some exceptions like for example, mercury is a liquid at room temperature. But besides that, usually metals are solids. Then they are ductile, which means you can draw them into wires. And they're also pretty malleable, meaning, you can hammer them into very thin sheets.

3:12Besides that, metals are also pretty good conductors of heat and electricity. All right, let's talk about non-metals. They are very different. First of all, non-metals can exist as solids, liquids, or even gases for that matter. And if they are a solid, they tend to be brittle.

3:30They are usually poor conductors of heat and electricity, and they usually have a pretty dull appearance. By the way, carbon is actually an exception to all this. I mean, diamond, for example, which is made of carbon, is anything but brittle or dull. So remember, there are always exceptions, okay?

3:46Okay, that brings us to metalloids. They fall somewhere in between, so they look somewhat shiny, they are usually somewhat brittle, and they act as semiconductors, meaning they conduct electricity better than non-metals, but not as well as metals. And these are some of the commonly accepted metalloids.

4:08So we have boron, silicon, germanium, arsenic, antimony, and tellurium. Okay, now let's dig a little deeper. To see more detailed patterns, we need to understand how the periodic table is organized. This entire table is arranged into horizontal rows, which we call periods, and vertical columns, which we call groups.

4:32Okay, let's talk about the periods. There are seven periods over here. And as you move across a period from left to right, look, the atomic number increases, and the properties of the element gradually change. You go from metals to metalloids to non-metals, for example.

4:50Okay, what about the groups? Groups are especially important, because elements in the same group tend to have similar chemical properties. And there are two common ways to label the groups. One way is to just label them one to 18, but there's also an older system which uses Roman numerals along with As and Bs.

5:12Don't worry too much about them. The point is, you might see either one depending on which source you're referring to. Now, since elements belonging to the same group have pretty much the same properties, we like to give names to these groups. So the first group, this first group of elements, we call them alkali metals.

5:31These include elements like lithium, sodium, and potassium. And what are their properties? Well, alkali metals are usually soft, they are silvery, and they're pretty reactive as we saw earlier. They react so easily that they're never found in pure form in nature.

5:48And that's why they're usually stored under oil to prevent them from reacting with air or water. But of course, hydrogen is an exception. Even though it is written under first group, it's not an alkaline metal, it's actually a non-metal, and it behaves very differently from the elements below it.

6:04And so this is a good reminder that although periodic table shows us very strong patterns, there are also important exceptions. Okay, onto the next group, we call them alkaline earth metals. These include elements like magnesium and calcium, for example. These metals are also pretty reactive, but compared to alkaline metals, they are less reactive.

6:26They're also harder than the alkaline metals. They are usually denser than the alkaline metals. And they're reaction with water is also slower than the alkaline metals. Okay, and in the middle of the table are the transition metals. This includes familiar elements like iron, and copper, and gold and so on.

6:46Now these transition metals are extremely important in everyday life. But their patterns are more complex than those of the main group elements. So for now, we'll focus on the clear patterns found on the left and the right edges of the periodic table. So let's move to the far right.

7:02Here, we find the halogens. This includes fluorine, and chlorine, and bromine, and iodine. Halogens are highly reactive nonmetals, and they're rarely found in pure form. Instead, they're usually found in compounds, especially combined with metals to form salts like NaCl, which is the most common example.

7:26And at the far right edge of the table, we find the noble gases. These include helium, neon, argon, and so on and so forth. And noble gases are very different from the other groups we have discussed. They're almost completely unreactive. And now that brings us to the central question, why are some elements so reactive while other elements so stable?

7:50See, so far we mostly describe what kind of patterns we find on the periodic table. But now let's talk about why those patterns exist in the first place. So for that we have to talk about reactivity. What does it mean to say an atom is very reactive? It means that an atom can more readily lose an electron to become stable or more readily gain an electron to become stable, or it is more likely to share electrons to become stable.

8:15And when we say an element is stable, we mean its electrons are arranged in a way that don't typically change. And the most important electrons for determining the reactivity, are the outermost electrons, as you can see over here. These are called valence electrons.

8:35And if you look at noble gases like helium and neon, they are so stable because they already have a full set of valence electrons. What does that mean? Well, for helium, it means two electrons in his outermost shell, and for most other elements, it means having eight electrons in their outermost shell.

8:52This arrangement makes them very unlikely to react. Now, most other elements do not have this stable arrangement. For example, sodium has one electron in its outermost shell, and chlorine has seven in its outermost shell. So these elements will react in ways to try and get that eight electrons in its outermost shell.

9:12For example, if sodium were to lose one electron, then look, its outermost shell would end up with one, two, three, four, five, six, seven, eight electrons just like neon. On the other hand, if chlorine were to gain one electron, then it too, would have eight electrons in its outermost shell just like neon.

9:31So this explains their reactivity. This idea is summarized in what we call the Octet Rule. The Octet Rule pretty much states that atoms tend to either lose or gain electrons, or sometimes even share electrons in order to have eight electrons in their outermost energy level.

9:51Now this is not a perfect rule, and there are of course exceptions, but it describes a very common and useful pattern. So look, knowing the valence electrons, it's pretty important to understand reactivity, right? And one of the most useful features of the period table is that you can figure out the number of valence electrons just by knowing the group.

10:12Group 1 has one valence electrons, group 2 has two, group 13 has three, group 14 has four, all the way to group 18, which has eight valence electrons making them stable. Of course, helium is an exception. It only has two valence electrons, but that still makes it stable.

10:30And so now we can understand why elements in the same group behave alike. For example, sodium and potassium are both in group 1. Each one has one valence electron. That's why both are soft metals that react vigorously with water. Fluorine and chlorine, for example, both belong to group 17.

10:51So each one has seven valence electrons, and therefore, they're highly reactive. So these are highly reactive non-metals. So as we go down the group, the total number of electrons does increase, but the number of valence electrons, that stays the same. That's why they tend to have the same properties.

11:11So putting it all together, the periodic table is a model that organizes elements by atomic numbers in a way that reveals the patterns in its physical and chemical properties. And elements position on the table gives us clues about its behavior, including whether it's metal or it's non-metal, or whether it's a metalloid.

11:31Elements in the same group tend to behave similarly because they have the same number of valence electrons. And reactivity mainly depends on the number of these valence electrons. Noble gases are stable because they have a full set of valence electrons, eight.

11:47And the Octet Rule describes the common pattern that atoms tend to lose or gain electrons, or even share electrons, all in order to obtain eight electrons in its valence shell. So look, the periodic table is much more than a chart. It's a map that helps us understand how the building blocks of the matter behave.

12:04And once you know how to read it, you can begin to predict the chemistry of the world around you.