Chemguide: Core Chemistry 14 - 16

The Periodic Table - the Alkali Metals

This page introduces the Alkali Metals in Group 1 of the Periodic Table.

The Alkali Metals

Francium is so radioactive and short-lived that nobody has ever seen a lump of it! We can, however, predict what its properties might be by exploring the trends in the group.

Physical properties of the Alkali Metals

These are very reactive metals and have to be stored out of contact with air.

Melting points

Their melting points decrease as you go down the group

Notice that caesium melts not far above a normal room temperature.

Densities

These are all light elements. Lithium, sodium and potassium are all less dense than water, and so float on water. Rubidium and caesium are both a bit denser than water, and so sink in water.

You are most likely to need to know this in their reactions with water.

Appearance and storage

All of these metals react with air and water, and so you will find the commonest of them (lithium, sodium and potassium) stored under oil. Rubidium and caesium are more normally found in sealed glass phials under an inert atmosphere of argon.

Reactions of the Alkali Metals with air or oxygen

All the metals react with gases in the air. The sequence of videos coming up shows this happening, and also illustrates the way the metals are stored.

Lithium

Lithium is unique in Group 1 because it reacts with nitrogen in the air as well as oxygen.

The red flashes when the lithium is heated in air are due to lithium compounds affecting the colour of the flame. We call these colours "flame colours" and the compounds of some metals have this sort of effect on flames. Flame colours can be used to help to identify some metals in compounds.

But when lithium burns, it burns with a while flame.

You get a mixture of lithium oxide and lithium nitride. At this level, you are unlikely to need to know about the nitride, but you could be expected to write an equation for the formation of the oxide.

4Li(s) + O₂(g)     2Li₂O(s)

Sodium

Sodium is softer than lithium and tarnishes faster when it is cut. Sodium and the rest of the group only react with oxygen and not nitrogen.

Sodium burns in oxygen with an orange-yellow flame. The next video shows this happening.

Chemistry gets a bit more complex now, because the sodium reacts to form two oxides - sodium oxide, Na₂O, and sodium peroxide, Na₂O₂.

At this level you are unlikely to need to know about the formation of the peroxide. If you are ever asked, just talk about sodium oxide being formed, and give the simple equation:

4Na(s) + O₂(g)     2Na₂O(s)

Potassium

Potassium is similar to sodium, but softer and more reactive. The first video just shows a piece being cut.

The second one shows it burning in oxygen.

The video mentions three oxides of potassium being formed - potassium oxide, K₂O, potassium peroxide, K₂O₂ and potassium superoxide, KO₂. You would not be expected to know about these at this level.

Rubidium and caesium

You are very unlikely to come across these in a school lab. The video shows them coming into contact with air.

With these metals, the main oxide formed is the superoxide - again, you don't need to know that.

Reactions of the Alkali Metals with water

All the Alkali Metals react with water to give a solution of the metal hydroxide and hydrogen gas. For example:

2Na(s) + 2H₂O(l)     2NaOH(aq) + H₂(g)

For the other metals, you can just change the Na for Li or K or Rb or Cs.

The solutions formed (sodium hydroxide and the rest) are all alkaline because ot the presence of hydroxide ions in solution - hence the name of the group, Alkali Metals.

Watch this bit of video once for an overall impression, and then I will talk about some things that you need to notice before you watch it a second time.

Lithium

Notice that the lithium floats but doesn't melt. Its melting point is high enough to keep it solid even though heat is given out during the reaction.

Sodium

This time sodium does melt into a ball. It is a faster reaction so that the heat is given out more quickly, and the melting point of the sodium is lower.

Notice the white trail which falls away under the sodium in the water. This is the sodium hydroxide being produced, and you see this trail just before it dissolves in the water.

The sodium moves around the surface because it is being pushed around by the hydrogen being produced.

Ignore the slightly more complicated looking equation. All it is doing is showing the ions present in the sodium hydroxide solution formed.

Potassium

This is more reactive still, and enough heat is produced to ignite the hydrogen. It isn't the potassium burning - it is hydrogen burning.

Rubidium

This just gets more violent still. Students often wonder what the technician tossed into the trough at the very end of this part of the video. I think it is probably the broken glass phial which originally contained the rubidium.

Caesium

The video speaks for itself!

Summary of trend in reactivity

Reactivity increases as you go down the group. This is best illustrated by the reactions with water we have just looked at.

The reason is fairly easy to see.

In each case, when the metal reacts with water, it loses an electron to make a metal ion. For example:

Na(s)     Na⁺(aq) + e^-

Those electrons are picked up by the water molecules to give hydroxide ions and hydrogen.

2H₂O(l) + 2e^-    2OH^-(aq) + H₂(g)

Obviously, the first equation has to happen twice to give enough electrons for the second one.

I want to concentrate on the metal reaction, because the formation of the hydroxide ions and hydrogen is exactly the same in every case.

Consider lithium and sodium.

Their electron structures are 2,1 and 2,8,1.

Lithium has 3 protons and sodium has 11.

In each case, it is the single outer electron which is lost.

In the lithium case, that electron feels a net pull of 1+ from the nucleus - the 3 protons offset by the 2 inner electrons.

In the sodium case, the outer electron feels a net pull of 1+ from the nucleus - the 11 protons offset by the 10 inner electrons.

That is true of every element in Group 1. The outer electron feels a net pull of 1+ from the nucleus.

But every time you add an extra layer of electrons, the outer one is further away from the nucleus and the attraction drops because of the increasing distance.

Reactivity depends on the loss of the outer electron, and so it increases down the group, because the outer electron is less firmly held as you go down the group.

Predicting the properties of francium

Because francium is very radioactive with only a short existence, nobody is going to be able to check most of this, but you can make predictions.

Based on the trends in the properties in the group, you would expect:

• Francium would be a very soft metal with a melting point not far above room temperature.

• Francium would react immediately with oxygen if it was exposed to air, forming a complicated oxide.

• Francium would have a density greater than caesium and sink if it was dropped into water.

• Francium would react explosively with water to form francium hydroxide solution and hydrogen.

• Francium hydroxide solution would be alkaline, containing Fr⁺ ions and OH^- ions.