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Trend in melting points of Group 2 elements

What is here?

Melting points generally decrease going down Group 2. You can:

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Table of physical data

Element

Proton number

Symbol

Melting point
(K)

beryllium

4

Be

1551

magnesium

12

Mg

922

calcium

20

Ca

1112

strontium

38

Sr

1042

barium

56

Ba

998

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Graph of physical data

Graph of melting points of Group 2 elements

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Explanation of this trend

The Group 2 elements are all metals with metallic bonding, so you expect their melting points to be high. In metallic bonding, metal cations in a metal lattice are attracted to delocalised electrons.

Going down Group 2:

  • the number of delocalised electrons remains the same ...
  • the charge on each metal cation stays the same at 2+, but ...
  • the ionic radius increases ...
  • so the attraction between the delocalised electrons and the metal cations decreases.

Notice that, although in general the melting point decreases going down the group, the melting point for magnesium is anomalously low. This is because beryllium and magnesium have different metallic structures from the other elements in the group:

  • beryllium and magnesium have a hexagonal close-packed structure
  • calcium and strontium have a face-centred cubic structure and
  • barium has a body-centred cubic structure.

You won’t be expected to know this last bit at AS Level, though!

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