Melting and boiling points down group 2
After studying this page, you should be able to:
- describe how melting and boiling points change down group 2
- understand that there doesn't seem to be a satisfactory explanation for how these change
Melting and boiling points
The table shows melting points and boiling points for the common elements in group 2.
|Element||Symbol||Atomic number||Melting point /K||Boiling point /K|
The temperatures are given in kelvin, K.
You can easily convert K to °C and back again:
°C = K + 273 (e.g. 100 °C = 373 K)
K = °C – 273 (e.g. 273 K = 0 °C)
Strictly speaking it should be 273.15 rather than 273, but the less precise value is acceptable at A Level.
Notice that there is no ° sign in front of the K.
The graph shows how melting points and boiling points vary down group 2. Magnesium is excluded to begin with, but you can toggle it on and off.
What is going on?
There is a general decrease in melting point going down group 2. However, if you include magnesium, you will see that its melting point is lower than the melting point of calcium, the next element down. If you include magnesium, there is no obvious trend in melting points (see below).
There does not appear to be a trend in boiling points going down the group. Again magnesium is an anomaly. If you exclude it, the boiling points decrease from beryllium to strontium, then increase to radium.
What is going on with magnesium?
I wish I knew! There does not seem to be a satisfactory explanation for the anomalous melting and boiling points of magnesium. One explanation involves the different packing structures for the metal atoms:
- beryllium and magnesium have a hexagonal close-packed structure
- calcium and strontium have a face-centred cubic structure
- barium and radium have a body-centred cubic structure.
Unfortunately, these differences cannot properly explain what is going on. If and when I find a satisfactory explanation, I will update this page.
Metallic bonding is often described as the attraction between positive metal ions and delocalised electrons. This is incorrect because metals still consist of atoms, but the outer electrons are delocalised and are free to move through the structure.
In a similar way, graphite (a non-metal) also has delocalised electrons. However, you don't see the idea that it consists of carbon ions.