Video Transcript
Although trends in the melting
point are hard to define when considering all of the period four transition metals,
a smaller trend within the data can be observed. For example, the melting points and
boiling points rise in tandem from scandium to vanadium but then drop at chromium
and further for manganese before rising again. Which of the following statements
might explain this drop in the melting and boiling points at chromium and
manganese? (A) Chromium and manganese form
different crystalline structures from scandium, titanium, and vanadium. (B) The metallic radii of chromium
and manganese are less than those of scandium, titanium, and vanadium. (C) Chromium and manganese have
half-filled 3d orbitals, unlike the preceding three elements. (D) The densities of chromium and
manganese are greater than those of the preceding three elements.
This question is asking us to
explain why chromium and manganese have unusually low boiling points compared to the
other metals. To give ourselves more room to work
with, let’s temporarily erase the answer choices.
In general, transition metals have
high melting and boiling points. These elements form strong metallic
bonds that strongly hold together the substance, raising its melting and boiling
points. Specifically, the bonds are strong
due to the delocalized nature of its electrons, which are shared between the metal
atoms.
To know why chromium and manganese
have different boiling points, we need to know about their electrons. Each of the period four transition
metals contains two electrons in a 4s orbital and some number of electrons in the 3d
subshell. For example, scandium’s electron
configuration is argon 4s2 3d1.
Let’s take a look at the electron
configurations of the other period four transition metals. Going from scandium to titanium to
vanadium across the period adds electrons one by one to the d orbitals in the 3d
subshell. When we get to chromium, instead of
having a full 4s orbital, the electrons spread out to half-fill the 3d subshell. Manganese also has a half-full 3d
subshell. At iron and beyond, the added
electrons fill each subsequent orbital in the 3d subshell. So the notable characteristic of
the electron configurations of chromium and manganese is the half-filled 3d
subshell.
Electrons in full or half-full
subshells are more stable, meaning they are harder to share or remove. Since they are harder to share and
the strength of a metallic bond depends on delocalized electrons, the metallic bonds
of chromium and manganese are relatively weak. Weaker metallic bonds means lower
melting and boiling points, since the substance isn’t held together as strongly.
With this logic fully explained,
let’s erase our work and bring back the answer choices. Looking at our four answer choices,
only one matches the correct explanation. Choice (C) — chromium and manganese
have half-filled 3d orbitals, unlike the preceding three elements — is the correct
answer. As we’ve just explained, the
half-filled orbitals of chromium and manganese contain stable electrons, which
create weaker metallic bonds that result in a lower melting and boiling point for
the substance.
So, what explains the drop in the
melting and boiling points at chromium and manganese? That’s choice (C): chromium and
manganese have half-filled 3d orbitals, unlike the preceding three elements.