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Question Video: Explaining Why the Melting and Boiling Points of Chromium and Manganese Are Lower than the Preceding Period 4 Transition Metals Chemistry

Although trends in the melting point are hard to define when considering all of the period 4 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.

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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.

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