Alkali Metals Flashcards
Alkali metals and their physical properties
The elements in Group 1 of the Periodic Table are called the alkali metals.
The melting and boiling points of these elements are very low
As you go down the line of elements, the melting and boiling points get lower.
Their densities tend to increase down the group
The metals are very soft and easily cut with a knife.
As you go down the line of elements, the metals become softer.
Storage and handling
Reactivity
Forming oxides
Reacting rapidly with water
Why are some alkali metals stored under oil
Why do some elements such as rubidium and caesium have to be stored in sealed glass tubes
All these metals are very reactive and get more reactive as they go down the group.
They all react quickly with oxygen in the air to form oxides.
They react rapidly with water to form strongly alkaline solutions of the metal hydroxides. This is what they are commonly known as alkali metals.
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 oxygen getting to them.
Why are these specific metals in Group 1?
They all have one electron in their outer shell
They have similar chemical properties. For example, they all react with water in the same way to form a hydroxide.
Reactions with water
All these metals react in the same way with water to produce a metal hydroxide and hydrogen:
Alkali metal + water —> alkali metal hydroxide + hydrogen
The main difference between the reactions is how quickly they happen. As you go down the group, the metals become more reactive and the reactions occur more rapidly.
Sodium
The main observations you can make when this reaction cues:
- The sodium floats because it is less dense than water.
- The sodium melts into a ball because it’s melting point is low and a lot of heat is produced by the reaction.
- There is fizzing because hydrogen gas is produced.
- The sodium moves around on the surface of the water
- The piece of sodium gets smaller and eventually disappears.
- If you test the solution that is formed with universal indicator, you will see it turns blue which shows an alkaline solution has been formed.
Lithium
The reaction is very similar to sodium’s reaction except that sodium’s reaction is slower. Lithium’s melting point is higher and the heat isn’t produced so quickly so the lithium does not melt.
Potassium
Potassium’s reaction s faster than sodium’s. Enough heat is produced to ignite the hydrogen and burns with a lilac flame. The reaction often ends with potassium spitting around and exploding.
Rubidium and caesium
These react even more violently and the reaction can be explosive. Rubidium hydroxide and caesium hydroxide is formed.
Why does the reactivity increase as you go down the group?
As you go down the group, the atoms have more shells of electrons and so get bigger. As the atoms have got bigger, the outer shell of electrons are further away from the positive nucleus and is less attracted to it. This means that is it easier for the electron to be lost which makes the atom more reactive.
Reactions with air
The metals tarnish when exposed to air because the freshly exposed metal reacts with oxygen in the air.
Lithium is slower at reactions. Potassium is the fastest at reactions out of the top three elements in the group.
Use knowledge of trends in Group 1 to predict the properties of other alkali metals
Nobody has actually seen a piece of francium so we can only predict what the properties are:
- Very soft
- Lower melting point than caesium
- Higher density than caesium
- Silvery metal that will tarnish instantly in air
- Will react violently with water.
