The figure that follows shows ball-and-stick drawings of...

The figure that follows shows ball-and-stick drawings of three possible shapes of an $\mathrm{AF}_{3}$ molecule. (a) For each shape, give the electron-domain geometry on which the molecular geometry is based. (b) For each shape, how many nonbonding electron domains are there on atom $\mathrm{A} ?$ (c) Which of the following elements will lead to an $\mathrm{AF}_{3}$ molecule with the shape in (ii): $\mathrm{Li}, \mathrm{B}, \mathrm{N}, \mathrm{Al}, \mathrm{P},$ Cl? (d) Name an element A that is expected to lead to the $\mathrm{AF}_{3}$ structure shown in (iii). Explain your reasoning.

Key Concepts

Axial and Equatorial Positions

In molecules with geometries such as trigonal bipyramidal, the electron domains can occupy different positions (axial or equatorial) that are not equivalent in terms of repulsive interactions. Lone pairs tend to prefer the equatorial positions, where the repulsive interactions are minimized, thus playing a significant role in determining the observed molecular geometry as opposed to the simple electron-domain geometry.

Central Atom Electron Domain Count

The count of electron domains around a central atom—including both bonding domains (ligands) and nonbonding domains (lone pairs)—is critical for predicting molecular geometry. Understanding the central atom's total number of valence electrons allows one to determine how many bonds and lone pairs it will have, which in turn influences the specific shape of the molecule.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is the fundamental principle used to predict the geometry of molecules by considering the repulsions between electron pairs around a central atom. The theory posits that electron pairs, whether they are bonding or nonbonding, will arrange themselves as far apart as possible to minimize repulsive interactions, which in turn determines the overall electron-domain geometry of the molecule.

Electron-Domain Geometry

Electron-domain geometry refers to the spatial arrangement of all electron density regions (bonding and nonbonding electron pairs) around a central atom. This concept is essential because it not only guides the shape of the molecule but also provides the framework on which molecular geometry is based. The electron-domain geometry is determined solely by the number of electron pairs present around the central atom.

Nonbonding Electron Domains

Nonbonding electron domains, commonly known as lone pairs, are pairs of valence electrons that are not involved in forming bonds between atoms. These lone pairs occupy space, affecting the molecular shape since they exert greater repulsive forces compared to bonding electron pairs. Their presence often leads to distortions in the ideal bond angles predicted by the electron-domain geometry.

Transcript

00:01 All right, so this is a question on molecular geometry and vsepr theory and shape.

00:09 We're given the shape of the molecules of af3, the possible shapes of the af3 molecule, and here's it.

00:20 To answer the first question, a, for each shape we're asked to determine the possible, to give the electron domain geometry.

00:32 Remember, there are five basic electron domain geometries.

00:37 You have linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral.

00:44 And usually, the shapes of this molecule, of these five main geometries, is based on the fact that all of the electron groups, you know, on the number of electron groups on the central atom, right? the number of electron groups on the central atom is what would determine the electron domain geometry.

01:06 And so, if you look at this, this shape, the five basic entries looks like a bond angle of 120, right? that's what that looks like.

01:16 So, a is definitely the first one, i, is definitely trigonal planar, trigonal planar, definitely, right? the second one, if you look at that, that looks like the tetrahedral structure, because that bond angle is smaller.

01:31 You can see it's smaller than what you have in the first one.

01:34 And so, that's definitely tetrahedral, that's tetrahedral.

01:38 So, the trick here is to be able to see how the bond angles are decreasing, right? the angles between this, this, of course, the third one, you would see, it's a further decrease, and it looks like a 90 and a 120 situation, sort of, or something like a 180, almost.

01:59 And so, that's going to be trigonal planar.

02:05 In fact, that, the third one has sort of like a t -shape.

02:09 This looks, has a t -shape, you know, if you look at it, and that's one that's corresponding to a vsepr notation of ax3, which is telling you that there are five electron groups on the central lateral.

02:24 Right, so, that's for part a of that question.

02:27 The first shape is trigonal planar, the other one is tetrahedral, the third is trigonal bipyramidal.

02:34 I'm so sorry, not planar, i meant bipyramidal, bipyramidal, right? trigonal bipyramidal, that's what should be there, bipyramidal, right? trigonal bipyramidal.

02:48 Okay, so the b part says for each shape, how many non -bonding electrons and domains are there on atom a? well, definitely, right, we can see that, i can see for sure that for the ball and stick model of one, there will be no, it's definitely zero, right? so, zero, no non -bonding electron.

03:15 However, for two, that tetrahedral electron geometry looks like there is one, one non -bonding, so one, right? one non -bonding.

03:25 Of course, if you look, remember what i said for three, it looks, it has a t shape, you know, so that's where you have two, right, from that vsepr notation, the vsepr notation.

03:37 The vsepr notation just makes it easier to relate the electron geometry to the shape...

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