More reactive the metal, lesser the melting it has. Right?

1 Answer
Dec 26, 2016

It sounds like a coincidence to me. You're talking about chemical properties and trying to relate them to physical properties, which is usually not necessarily clear-cut.

Claim:

  • Aluminum is more reactive than molybdenum, so it has a lower melting point?

Well, it IS more reactive, since it was more easily oxidized if the reaction is correct, and it DOES have a lower melting point... (1221^@ "F" vs. 4753^@ "F")

(Though you should consider that molybdenum (VI) oxide is a solid and not aqueous, and that the oxidation state of molybdenum in it is particularly large in comparison to aluminum ion.)

Counterexample:

http://chemistry.stackexchange.com/http://chemistry.stackexchange.com/

We can see that aluminum has a higher melting point than zinc.
(1221^@ "F" vs. 787.2^@ "F")

Therefore, reactivity and melting point are not related, or there are complicating factors that we're not considering. In other words, there is no clear link between reactivity and melting point.


Instead, I can think of a few physical reasons why aluminum has a lower melting point (a physical property).

Melting point for metals has to do with:

  • How many electrons are "free" to move between metal atoms in the metallic crystal structure.
  • How well they are delocalized throughout the metal atoms.

The more half-filled orbitals a metal has, the more electrons it can contribute""^([1]) to the conduction band""^([2]), and thus, the more delocalized its valence electrons are.

The following electron configurations for "Al" and "Mo" are:

"Al": [Ne]3s^2 3p^1
"Mo": [Kr]5s^1 4d^5 (compare with "Cr" for the electron configuration, though "W" does not have the same valence electronic structure.)

Since "Mo" has six unpaired electrons, it contributes more electrons to the conduction band, so it has much more electron delocalization than "Al", so its metallic bonding is stronger, and it has a much higher melting point than "Al" (4753^@ "F" vs. 1221^@ "F").

Furthermore, its significantly higher atomic number (42 vs. 13) means it has a significantly higher effective nuclear charge (20.25 vs. 9.5 from Slater's Rules), so it has more closely-packed metal atoms, and thus stronger metallic bonding and a higher melting point.

You can refer to this answer if you want to read further about melting points, or this page if you want to learn more about band theory.