C3aL3 Trends 1: Alkali Metals
Key Words
Alkali Metals - elements of Group I
Alkaline - has a pH greater than 7
Exothermic - Gives out heat.
Hydroxide - the OH- ion
Ionic compound - two or more elements held together by attraction of opposite charges
Oxide - a compound between an element and oxygen
Grade E
The alkali metals are a family of metals that have a single outer shell electron. They are in Group I. They have the following properties:
- They are reactive with both oxygen and water;
- They are soft, being cut easily with a knife;
- They have relatively low densities. Lithium, sodium and potassium float on water.
The group get more reactive as you go down, because the outer shell electron is more easily removed.
Watch this clip from the Braniac series* on the alkali metals. Or a more steady version.
The table shows some properties:
| Element | Symbol | Atomic Number | Melting Point (oC) | Boiling Point (oC) | Density (g/cm3) |
| Lithium | Li | 3 | 181 | 1347 | 0.53 |
| Sodium | Na | 11 | 98 | 883 | 0.97 |
| Potassium | K | 19 | 64 | 774 | 0.86 |
| Rubidium | Rb | 37 | 39 | 688 | 1.48 |
| Caesium | Cs | 55 | 29 | 679 | 1.87 |
You could melt caesium with the warmth of your hand.
There is another alkali metal, Francium. It is very radioactive, with a half life of a few minutes.
* I am told that the Braniac sequence actually used explosive charges. There were showers of water, against which they took no particular precautions. A caesium hydroxide facial would not enhance one's appearance. Nor would I want to go paddling about in the stuff.
Grade C
When alkali metals react with water, hydrogen is displaced in an exothermic reaction. With Lithium, the hydrogen does not get hot enough to ignite. With sodium, it may do if the sodium sample is big enough. The flame is yellow because sodium is being burned. With potassium, the hydrogen flame turns lilac.
Rubidium reacts violently with water; caesium explodes. The compounds formed are hydroxides that are very alkaline, with the pH being about 14. The equation below is a typical reaction:
2Rb(s) + 2H2O(l) ® 2RbOH(aq) + H2(g)
Reactions between alkali metals and non-metals produce ionic compounds. They react readily with oxygen. Freshly cut alkali metals are shiny, but tarnish rapidly. In the lab, they are kept in oil to keep air and water off.
4Rb(s) + O2(g) ® 2Rb2O(s)
The metal oxides are white, but readily react with water to make alkaline hydroxides.
The alkali metals react with halogens to make halides:
2Rb(s) + Cl2(g) ® 2RbCl(s)
Alkali metal halides are all salts. The crystals are white, and are giant ionic structures. They are soluble in water, giving colourless solutions with a neutral pH (7).
Grade A
The reason that the alkali metals are so reactive is that the single electron is given up very easily to form the noble gas structure. As the atoms get larger, the single electrons get further away from the nucleus. There are other electrons trying to repel it as well. For caesium, that outer electron is therefore given up very easily indeed, making caesium a very reactive element indeed.