Group 1: The Alkali Metals
Alkali metals are metals that are part of group one. They are extremely reactive metals, and reactivity increases DOWNWARDS – in other words, lithium is the least reactive and francium.
Some Basic Physical Properties
| Metal | Melting Point (0C) | Boiling Point (0C) | Density (g/cm3) | |
| Lithium | 181 | 1342 | 0.53 | |
| Sodium | 98 | 883 | 0.97 | |
| Potassium | 63 | 760 | 0.86 | |
| Rubidium | 39 | 686 | 1.53 | |
| Francium | 29 | 669 | 1.88 | |
You can see that as reactivity increases, the melting and boiling points decreases; however, density increases. These points are very low for metals. Remember that potassium, sodium and lithium would float on water due to their densities. But why are they so reactive? Well they only have one electron to lose!
The metals are also very soft and easy to cut, becoming softer as you go down the group. They are shiny and silver when cut, but tarnish within seconds on exposure to air.
Storage and Handling
All these metals are extremely reactive. Anyways the metals will quickly react with air to form oxides, and react between rapidly and violently with water to form strongly alkaline solutions of metal hydroxides.
To stop them reacting with oxygen or water vapour in the air, lithium, sodium and potassium are stored under oil. Rubidium and caesium are so reactive that they have to be stored in sealed glass tubes to stop any possibility of oxygen getting at them.
Great care must be taken not to touch any of these metals with bare fingers. There could be enough sweat on your skin to give a reaction producing lots of heat and a very corrosive metal hydroxide.
Reactions with Water
All these metals react with water to produce a metal hydroxide and hydrogen.
Metal + Water -> Metal Hydroxide + Hydrogen
All the hydroxides are bases and turn pH paper purple.
With Sodium
The sodium floats because it is less dense than water. It melts because its melting point is low and a lot of heat is produced by the reaction. Observations would be that the sodium would turn into a ball and whiz around the surface of the water. It may form a white trail which is sodium hydroxide. This dissolves to make a strongly alkaline solution with the water. When lit, it produces a yellow flame.
With Lithium
The reaction is very similar to sodium’s reaction, except it is slower. The lithium does not melt due to its higher melting point. When lit, it produces a red flame.
With Potassium
Potassium’s reaction is faster than sodium’s. Enough heat is produced to ignite the hydrogen, which burns with a lilac flame. The reaction often ends with the potassium spitting around.
With Rubidium and Caesium – The Two Baddies
The reaction is so violent it can be explosive. When lit, Rubidium forms a red flame and Caesium forms a blue flame.
Explaining the Increase in Reactivity
The differences between reactions depend in part on how easily the outer electron of the metal is lost in each case. That depends on how strongly it is attracted to the nucleus. The more electron shells an atom has, the less powerful the attraction forces are. For example, Lithium is a lot less reactive than Potassium. This is because there are less shells which shield the full attraction of the nucleus from the This makes the electron harder to lose. However, potassium has a lot more electron shells which shield the outer electron from the nucleus. This weakens the attraction in compared to lithium, and therefore, the electron is easier to lose.
Compounds of Alkali Metals
All group one metal ions are colourless. That means that their compounds will be colourless or white unless they are combined with a coloured negative ion (remember metals would become positive ions because they lose electrons, whereas, most non-metals gain electrons). Potassium dichromate is orange, for example, because the dichromate ion is orange. Group one compounds are typical ionic solids and are mostly soluble in water.
Alkali Metals: Quick Notes
- Group One so +1 charge
- One electron on outer shell
- Reactivity increases downwards
- Density increases downwards
- Melting and Boiling points both decrease downwards
- Very soft and tarnish quickly in air
- Li, Na and K are stored under oil, whilst Rb and Cs are stored in sealed glass tubes
- Reacts with air to form oxides
- Reacts with water to form alkaline hydroxides, which turns pH paper purple
- Positive ions are formed and they are colourless
- Flame Colours: lithium, red; sodium, yellow; potassium, lilac; rubidium, red; caesium, blue.
- Forget about Francium you don’t need to know much about it.