Melting points of pgms
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Category: Metallurgy and Materials
Subject: Melting points of pgms
Question
Why are the melting points of the platinum group metals so high?
Answer
The melting points of the noble metals of Groups 8 to 10 are indeed high, but as the following Table shows those of the metals of Groups 6 and 7 are generally even higher.
| Group number | Metal | Tm, K | Metal | Tm, K |
|---|---|---|---|---|
| 6 | Mo | 2895 | W | 3683 |
| 7 | Tc | 2413 | Re | 3440 |
| 8 | Ru | 2673 | Os | 3318 |
| 9 | Rh | 2239 | Ir | 2727 |
| 10 | Pd | 1827 | Pt | 2046 |
| 11 | Ag | 1233 | Au | 1336 |
| 12 | Cd | 594 | Hg | 234 |
Starting from the alkali metals of Group 1, as electrons are added to form the elements of the Transition Series, more of them are available for bonding, and so melting points (and many other physical properties reflecting bond strength) increase, until a maximum is reached at Group 6.
After that, further electrons pair up with those already there, so that fewer and fewer remain unpaired and able to form bonds between atoms. The melting temperature therefore falls progressively as shown in the Table.
The only anomaly occurs in Group 7, where in the second Transition Series the melting point of technetium is less than those of the elements before and after it. This is because it has the s 2d 5 structure, in other words, the d-shell is half-filled; this arrangement has some stability and the electrons therefore form weaker bonds. The effect is much more evident with manganese, the chemistry of which is quite anomalous, resembling in many ways that of elements, such as copper, which have a completely filled d-shell.
Answer posted 25 February 2005
Submitted by: Ms S. E. White
Affiliation: Hertford College, Cambridge
Answered by: Professor Geoffrey Bond
Affiliation: Emeritus Professor, Brunel University
