In this lesson, we're going to focus on alkali metals. Alkali metals can be found in the first column of the periodic table. Now, listed are some properties of alkali metals.
Like all metals, they can conduct heat and electricity. Alkyl metals are very soft. They're so soft that you can cut them with a knife, which is not typical of many metals.
They also have a low density. For instance, lithium is less dense than water. Lithium can actually float on water.
They also have low melting points compared to most other common metals. Cesium, for instance, has a melting point of around 29 degrees Celsius. Now they also have a large atomic radius. Atomic radius increases towards the left. So sodium is larger than magnesium.
As you go to the left, the atomic radius increases. So alkali metals, they have a relatively large atomic radius for their period. That is, if you compare them with other elements in the same row. Alkaline metals also have a low ionization energy.
Ionization energy increases to the right across the periodic table. So that means that sodium has a lower ionization energy than magnesium. Now alkaline metals are highly reactive. They can react with water producing hydrogen gas and at the same time they can create hydroxides so they can form alkaline solutions. All of the alkaline metals they possess one valence electron and so they have a very low electronegativity.
They're in fact electropositive. which makes them strong reducing agents. They really want to give up their valence electrons to have a complete octet.
Lithium is one of the strongest reducing agents out there. It has a standard reduction potential of negative 3.05 volts. Now let's get rid of that.
So if you go to the periodic table, In the first column, the first element you're going to see is hydrogen. Below hydrogen, we'll have the elements lithium, sodium, potassium, rubidium, cesium, and francium. The elements below hydrogen are considered the alkali metals. They can conduct electricity, they're soft, they have these properties.
Hydrogen is considered to be a non-metal. At room temperature, hydrogen is actually a gas, so it's not a solid metal. So on a test, if you're asked if hydrogen is considered an alkali metal, for the most part, it's not.
However, hydrogen, like the other alkali metals, do contain only one valence electron, since they're all part of group one. Now, as you go down this group, the reactivity of the alkali metals increases. So what this means is that francium is more reactive than cesium. Cesium is more reactive than rubidium.
In fact, cesium reacts violently with water. Now as you go down a group, the melting point decreases. Cesium has a very low melting point, and the same is true for francium.
The boiling point also decreases as you go down a group. The density increases as you go down group 1. The first ionization energy, let's call it IE1, that decreases as you go down the group. The electronegativity also decreases as you go down. So lithium is more electronegative than cesium, which means cesium is more electropositive than lithium. The atomic radius I'm going to put this here.
The atomic radius increases as you go down and the same is true for the ionic radius. That increases as you go down a group. So those are some trends that you need to be aware of when dealing with the alkaline metals. Now let's go over some reactions.
with regard to the alkaline metals. Sodium can react with water to produce an alkaline solution. It will create sodium hydroxide and hydrogen gas. So to balance this equation, we need to put a 2 in front of sodium, water, and sodium hydroxide. Lithium can react with water the same way.
It will produce lithium hydroxide and hydrogen gas. So the alkali metals are strong reducing agents. They like to give away their electrons to form metal cations. When sodium gives away its electron, it'll form a positive metal cation. And the reduction potential for that, I wrote it down somewhere, it's 2.71 volts.
If you reverse the reaction, it'll be negative 2.71. That'll give you the standard reduction potential. Lithium is also a strong reducing agent. The way to written the reduction potential here is 3.05.
If you reverse it, you get the standard reduction potential of negative 3.05. But notice that lithium is a stronger reducing agent than sodium. It has a higher potential.
Now, sodium can also react. with chlorine gas to produce table salt, sodium chloride. So to balance this we need to put a 2 in front of Na and NaCl.
Now the alkali metals, in addition to reacting with water, they can also react with air. Lithium can react with excess oxygen gas to create lithium oxide, Li2O. The oxide ion is represented by this formula, O with a negative 2 charge.
Sodium can also react with the oxygen gas in air to produce sodium oxide. Very similar to the previous reaction. But there's one slight difference here. When lithium reacts with excess oxygen, it creates lithium oxide. When sodium reacts with limited oxygen, it'll create sodium oxide as well.
But when sodium reacts with excess oxygen gas, we're going to get a different product. We're not going to get sodium oxide, we're going to get sodium peroxide. So to balance this, all we need is a 2 in front of sodium.
Pert oxide has this formula. It's O2 2-. Now, if we take potassium and react it with oxygen gas, We're going to get potassium superoxide.
So superoxide is O2 with a negative 1 charge. Whereas peroxide is O2 with... a 2-charge.
So keep that in mind. If you react sodium with limited oxygen, you get sodium oxide, but with excess oxygen, you get sodium peroxide. But when you react potassium with oxygen, you get super potassium superoxide. So those are some of the reactions that you might be tested on with regard to alkali metals. Now let's talk about the reactivity of alkali metals.
The reactivity increases as you go down the group. So what that means is that cesium is more reactive than lithium. Now let's understand why. So lithium Let me draw this differently. Let's say this is the nucleus of lithium.
This is the first shell and this is the second shell. Lithium has one valence electron in the second shell. Cesium on the other hand is going to be much bigger. So this is the first energy level. Let's say that's the second, third, fourth, fifth, and here is the sixth one.
We can see why cesium can give up its valence electron a lot easier than lithium. One reason for that is that this electron is very far away from the nucleus. And so the pull that the nucleus have on this electron, it's a lot weaker since they're further away. In the case of lithium, the nucleus has a stronger pull on the valence electron because it's so close.
So that's one reason why cesium is much more reactive than lithium. Another reason, in fact a more important reason, has to do with the melting points of these two metals. Molten lithium is much more reactive than solid lithium. because in a molten state there's more surface area for the molecules to react, or rather for the atoms to react.
Lithium has a much higher melting point than cesium. The melting point for lithium is about 180 degrees Celsius. The melting point for cesium is around 29 degrees Celsius.
Both of these alkali metals can react vigorously with water. In fact, they're exothermic. They generate a lot of heat.
Lithium, because it has a higher melting point, it's going to take a longer time for it to melt. Cesium is going to enter the molten state a lot faster than lithium because the melting point is so much lower. As a result, When it enters the molten state, it's going to have a greater surface area.
So there's going to be more contact between the atoms of cesium and the water molecules. So with that increased surface area leads to an increased reaction rate. And as the reaction proceeds faster, it's going to generate even more heat, which will further accelerate the rate of reaction.
Because when the temperature goes up... the rate of the reaction goes up as well. And so all of these factors will increase the rate at which cesium reacts with water, but primarily it's due to the fact that it has a low molten point, and so it can enter the molten state faster, and that's why it can react faster.
Metals in the molten state will react faster than metals in a solid state. The standard reduction potential for lithium, we mentioned it was negative 3.05 volts. For cesium, it's negative 3.02 volts. So even though lithium is a stronger reducing agent than cesium, because cesium is bigger and because it has a lower melting point, it turns out that cesium reacts with water a lot faster.
But a big reason is due to the fact that it has a little melting point and so it can quickly enter the molten state. Thus increase in the surface area. increase in the rate of the reaction, and as the reaction proceeds faster, more heat is generated, the temperature goes up rapidly, which also further accelerates the rate at which the reaction proceeds.
So what you need to know is that as you go down the group, the melting point decreases, and so the reactivity of the alkali metals will increase. So that's basically it for this video. Hopefully it gave you a good introduction into the properties of alkaline metals