GROUP 2

Here we go for the hundredth time. Group 2 elements, you must be sick to death of them by now. I certainly am. But you wan that A or even A* next year lets get to work!

First of all we need to have a look at the trend of group 2. You may have noticed but as you go down group 2 the reactivity increases. This may seem familiar to you by now but its all about three factors:

• Electron shielding: As you go down group 2 there is a new shell added each time. Now group 2 metals want to LOSE the 2 electrons as it is the easiest way to get a full outer shell. As a new shell is added there is more electron shielding so the attraction on the outer electrons is weaker and the 2 outer electrons can be lost much easier.
• Atomic radius: Each element down the group has an extra shell which means the 2 outer electrons are further away. This means that there is a weaker attraction on the outer electrons and they can be lost much easier.
• Nuclear charge: As you go down group 2 the nuclear charge increases so the attraction on the outer electrons increases. HOWEVER, (before you ask) the other two factors out weight this increased attraction.

Overall it is much easier to lose the 2 electrons as you go down the group so the reactivity increases.

There are several reaction that you need to understand, they are fairly basic so don't worry too much.

First, we have a group 2 metal and oxygen:

This is a REDOX reaction (more about these on another post). When a group two element is reacted with oxygen a metal oxide forms.

2Ca (s) + O2 (g) ---> 2CaO (s)

The calcium here is oxidised: goes from 0 to +2

The oxygen is reduced: goes from 0 to -2

Second, we have a group 2 metal and water:

In this reaction when the metal reacts with water a metal hydroxide forms and hydrogen gas.

Ca (s) + 2H2O (l) ---> Ca(OH)2 (aq) + H2 (g)

You would be able to see in the reaction that the metal dissolves (the calcium goes from solid to aqueous. Also gas is produced so there would be fizzing. Furthermore Ca(OH)2 is an alkali (a soluble base) therefore if you added an indicator it would show that you have an alkali present (the pH would be around 10-12 depending on the strength of the alkali).

In this case:

Calcium is oxidised: Goes from 0 to +2

Hydrogen is reduced: Goes from +1 to 0

As you go further down the group each metal will react more violently with water.

Another fact to mention (which you need to know) is that metal hydroxides are used to neutralise acidic soil, however too much will turn the soil too alkali (which is bad).

Third, we have a group 2 oxide and water:

Here a metal oxide is reacted with water to from a solution of metal hydroxide.

CaO (s) + H2O (l) ---> Ca(OH)2 (aq)

Group 2 hydroxides dissolve in water to form alkaline solutions. Now, as you go down the group the solubility increases and the resulting solution in also more alkaline.

There is also a more ugly side to Group 2 that you need to remember: Thermal decomposition.

THERMAL DECOMPOSITION: The breaking up of a chemical substance with heat into at least two chemical substances.

Group 2 carbonates can be decomposed (broken down) by heat to form a metal oxide and carbon dioxide gas.

CaCO3 (s) ---> CaO (s) + CO2 (g)

The carbonates become more difficult to decompose as you go DOWN the group.

Here is a summary of all the reactions I have mentioned above, as well as reactions with acids. Memorise these as some will sneakily pop up in the exam and ruin your awesome chemistry exam day.