New Year's Countdown Day 7: Seventh Element

Chemistry Level 2

Let α \alpha be the non-convex angle between two N − F \text{N} - \text{F} bonds in NF 3 . \text{NF}_3. Which of the following is true?

This problem is part of the set New Year's Countdown 2017 .
α = 12 0 ∘ \alpha = 120^{\circ} α = 109. 5 ∘ \alpha = 109.5^{\circ} 109. 5 ∘ < α < 12 0 ∘ 109.5^{\circ} < \alpha < 120^{\circ} α < 109. 5 ∘ \alpha < 109.5^{\circ} α > 12 0 ∘ \alpha > 120^{\circ}

Steven Yuan by Steven Yuan , 20, USA

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1 solution

Steven Yuan
Dec 7, 2017

The Lewis dot structure of NF 3 \text{NF}_3 look like this:

Image source: [ChemEd DL](http://www.chemeddl.org/alfresco/service/chemeddl/molecules/search.html?guest=true&amp;pubchem=24553) Image source: ChemEd DL

Since the middle nitrogen is surrounded by four electron pairs, the electron geometry of NF 3 \text{NF}_3 is tetrahedral. So, the bond angle between N − F \text{N} - \text{F} is at most 109.5°.

Now, note the lone pair of electrons on the nitrogen. It repulses the fluorines on the other ends of the tetrahedral structure, causing them to get closer together. In fact, NF 3 \text{NF}_3 has a molecular geometry that is trigonal pyramidal, resulting in a flattened pyramid shape.

Thus, the angle between the N − F \text{N} - \text{F} bonds is actually less than 109.5° , due to the repulsion of this extra pair of electrons. The accepted value for the bond angle is 102.5°.

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