Question

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

Answer 1

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:

• Aluminum is more reactive than zinc (more easily oxidized), so it SHOULD have a lower melting point than zinc, based on that logic. However:

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.