- Created by: chunks-42
- Created on: 10-05-15 14:46
Losing 2 Electrons when they React
Group 2 elements all have two electrons in their outer shell (s2)
They lose their two outer electrons to form 2+ ions. Their ions then have every atom's dream electronic structure - that of a noble gas.
Atomic Radius Increases Down the Group
This is because of the extra electron shells as you go down the group.
Ionisation Energy Decreases Down the Group
1) Each element down Group 2 has an extra electron shell compared to the one above.
2) The extra inner shells shield the outer electrons from the attraction of the nucleus.
3) Also, the extra shell means that the outer electrons are further away from the nucleus, which greatly reduces the nucleus's attraction.
Both of these factors makes it easier to remove outer electrons, resulting in a lower ionisation energy.
Reactivity Increases Down the Group
1) As you go down the group, the ionisation energies decrease. This is due to the increasing to the increasing atomic radius and shielding effect.
2) When Group 2 elements react they lose electrons, forming positive ions. The easier it is to lose electrons (i.e. the lower the first and second ionisation energies), the more reactive the element, so reactivity increases down the group.
Melting Points Generally Decrease Down the Group
1) The Group 2 elements have typically metallic strucutres, with the electrons of their outer shells being delocalised.
2) Going down the group the metallic ions get bigger - so they have a smaller charge/volume ratio. But the number of delocalised electrons per atom doesn't change (it's always two) - so the delocalised electrons get more spread out.
3) These two factors mean there's reduced attraction of the positive ions of the 'sea' of delocalised electrns. So it takes less energy to break the bonds, which means lower melting points generally down the group. However, there's a big 'blip' at magnesium, because the crystal structure (the arrangement of metallic ions) changes.
Group 2 Elements React With Water
When Group 2 elements react, they are oxidised from a state of 0 to +2, forming M2+ ions.
M---> M2+ +2e- E.g. Ca----> Ca2+ 2e-
Oxidising State: 0 +2 0 +2
The Group 2 metals react with water to give a metal hdyroxide and hydrogen.
M + 2H2O ---> M(OH)2 + H2 E.g. Ca + 2H2O ----> Ca(OH)2 + H2
0 +2 0 +2
They react more readily down the group because the ionisation energies increase.
Solubities Trends Depend on the Compound Anion
Generally, compounds of Group 2 elements that contain singly charges negative ions (e.g. OH-) increase in solubility down the group, whereas compounds that contain doubly charged negative ions (SO4 2-) increase in solubility down the group.
Compounds like magnesium hydroxide which have very low solubilities are said to be sparingly soluble.
Most sulphates are soluble in water, but barium sulphate is insoluble.
The test for sulphate ions makes use of this property.
Test for sulphate ions
If acidifed barium chloride (BaCl2) is added to a solution containing sulphate ions then a white precipitate of barium sulphate is formed.
Ba2+ + SO4 -2 ---> BaSO4
Solubilities of Hydroxides and Sulphates
Uses of Neitralising Acidity
Group 2 elements are known as alkaline earth metals, and many of their common compounds are used for neutralising acids. Here are a couple of common examples:
1. Calcium hydoxide (slaked lime, Ca(OH)2) is used in agriculture to neutralise acid soils.
2. Magnesium hydroxide (Mg(OH)2) is used in some indigestion tablets as an antacid.
In both cases, the ionic equation for the neutralisation is: H+ + OH- ---> H2O
X-rays are great for finding broken bones, but they pass straight through soft tissue - so soft tissues, like the digestive system, don't show up on conventional X-ray pictures.
1) Barium sulphate is opaque to X-rays - they won't pass through it. It's used in 'barium meals' to help diagnose problems with the oesophagus, stomach or intestines.
2) A patient swallows the barium meal, which is a suspension of barium sulphate.The barium sulphate coats the tissues, making them show up on the X-rays, showing the structure of the organs.