2.1 Alkali Metals
The alkali metals are all the elements in the first group of the periodic table except for hydrogen. They are all very reactive metals. Weirdly though, they do not match the physical description of a metal that you would except. They are soft and can be cut easily with a plastic edge. They have low densities; therefore float on water. They have low boiling and melting points relative to other metals. But they are metals since they are silvery and shiny in appearance and conduct electricity.
These elements are very reactive; they have to be stored away from oxygen and water vapor in air. Therefore, they are usually kept in mineral oils in airtight containers. If you have done the experiment where lithium, sodium or potassium is exposed to water you would understand how reactive and dangerous they can be. The remaining two elements are too dangerously reactive for you to perform experiments on in a school lab. Therefore, lets focus on the chemistry behind lithium, sodium and potassium.
Why are these metals so reactive? If we look at the configuration above we can try to understand. They have 1 valence electron; therefore, are in group 1 of the periodic table. They are desperate to react with anything that will pull away this electron from them and give them a full shell to become stable. Moreover, this valence electron is preceded by a full shell which means the valence electron is not as attracted to the nucleus. Therefore, it can give away this electron without much of a hassle. As you go down the group, the distance and obstructions between the valence electron and nucleus increases. Therefore, the attraction between the valence electron and nucleus weakens. As a result reactivity increases down the group (Li < Na < K < Cs < Fr).
You may be wandering why they are called alkali metals. Well alkali metals react with oxygen to form metal oxides. When they react with water, instead of metal oxides they form metal hydroxides. Metal oxides turn into metal hydroxides when they come into contact with water. The aqueous solutions of these metal hydroxides are alkaline; therefore, they are called alkali metals.
They form strong bases that are very corrosive. Therefore proper precautions must always be taken when handling these metals as moisture on your skin is enough for it to react with and cause burns and damages to skin.
Lets look at the chemical reaction equations for alkaline metals.
Reaction in Air
As stated earlier, alkali metals react with oxygen when exposed to air. Therefore, they are stored in oils to prevent them from coming into contact with oxygen and water vapor.
Alkali metals react with oxygen to form metal oxides. These oxides appear in the form of thin crusts on the metal surfaces exposed to air. Lithium is the slowest to form this layer when exposed; the speed of reaction increases down the group. Therefore, if you take a piece of Lithium and Sodium out of a container and expose it to air, Lithium will have a shiny surface for a longer period of time than Sodium. Eventually though, Lithium will also form an oxide layer which turns the surface dull. We can increase the intensity of this reaction by introducing heat.
If we introduce a piece of an alkali metal to an open flame it will rapidly burn to form a white metal oxide powder. This time, rather than just the surface, the whole sample of metal is converted to a metal oxide. Moreover, depending on the metal, they burn with different colors. Lithium produces a red flame, sodium produces a yellow flame, potassium produces a lilac flame, rubidium produces a red-violet flame and caesium produces a violet flame. Francium is radioactive and not stable in nature; collecting samples of this metal for analysis is very difficult. Therefore, the color this metal produces when exposed to a flame is uncertain.
The chemical equation for lithium's reaction with oxygen is shown below:
Each oxygen atom has 6 valence electrons;therefore, they need only 2 to become stable. They can react with 2 lithium atoms since each lithium has only 1 valence electron. Lithium oxide, an ionic compound, is formed. Now is this equation balanced? No, because there is only 1 lithium atom on the reactant side but there are 2 lithium atoms in the product side. Similarly there are 2 oxygen atoms in the reactant side while there is only 1 oxygen atom in the right hand side. After balancing, the final equation obtained is shown below:
We can generalize this equation so that it fits all the elements in group. This generalized form is shown below with 'M' denoting the type of alkali metal:
Remember that the metal oxide is an alkali.
Reaction in Water
Alkali metals react vigorously in water. They react to form metal hydroxides and hydrogen. Since their degree of reactivity increases down the group, the intensity with which they react also changes.
Lithium, which has the lowest reactivity, will float and move around in water. Note that all alkali metals have a lower density that water and will float in water. The fizzing and bubbling is from the hydrogen gas produced. No flame is produced in this reaction.
Sodium reacts more vigorously and fizzes around more rapidly. It produces so much heat that the sample of sodium melts into a ball. It will sometimes ignite to form a yellow flame. The oxide produced dissolves in water to form hydroxide as the reaction progresses.
Potassium reacts more vigorously than sodium. It's so vigorous that it explodes and spatters around. It will burn vigorously with a lilac flame on the water. It is dangerous so be careful when observing this experiment in class.
The rest of the alkali metals react more rapidly and is very dangerous.
They all react to form oxides that dissolve in water to form metal hydroxide solutions which are alkaline. You can use an indicator to check for this after the reaction has been completed.
The chemical equation for lithium reacting with water is given below:
LiOH is aqueous because it dissolves in water. After balancing this equation you get:
We can generalize this equation as well so that it can fit any other alkali metals
As we discussed earlier in the previous chapters, elements in a group show trends. We can predict the properties of one element in the group by observing the properties of the other elements in the same group by using these trends. Is this useful? Yes it is especially with unstable elements or compounds that are not readily available for testing. For example, Francium is very unstable and radioactive. Therefore, whenever it is formed it almost immediately decays and turns into another element like radium or astatine. In these sorts of cases being able to use trends to determine the properties of these untested elements is very useful. Since alkali metals group shows trends we can use these to better understand the physical and chemical properties of Francium. For example, as you go down the group their densities increase while their melting and boiling points decrease. They also get more reactive as you go down the group. Therefore, we can predict that Francium will have higher boiling and melting point than Caesium. We can also predict that it will immediately react with air and react violently with water. Since all alkali metals are soft, Francium will also be soft.