Home A Level Inorganic Chemistry - Core Periodicity Melting and boiling points down group 2

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Melting and boiling points down group 2

Learning outcomes

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
beryllium Be 4 1551 2742
magnesium Mg 12 922 1363
calcium Ca 20 1112 1757
strontium Sr 38 1042 1650
barium Ba 56 998 1910
radium Ra 88 973 2010

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.

Description

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.

When you click on the download symbol, you will be able to download the graph as an image file or pdf file, save its data, annotate it, and print it. Note that graphs will be watermarked.
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Click on the key underneath the graph to toggle each set of bars on and off.
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Toggle the magnesium bars on and off using the button and dropdown menu under the graph.
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What is going on?

Melting point

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).

Boiling points

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.