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/

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.