In this episode we define the acronym V-S-E-P-R and take a closer look at bonding and non-bonding electron pairs around a central atom (1:10).
In this episode we define the acronym V-S-E-P-R and take a closer look at bonding and non-bonding electron pairs around a central atom (1:10). We distinguish between electron-domain geometry and molecular geometry (1:50). The episode describes the six electron-domain geometries (2:40) as well as the molecular geometries and bonding angles that arise from “mix and matching” bonding and non-bonding electron pairs (3:45). Balloons are a great tool to visualize these molecular shapes (7:00).
Question: (8:06 )What is the molecular shape of NH3? trigonal planar, trigonal pyramidal, or tetrahedral
Thank you for listening to The APsolute RecAP: Chemistry Edition!
(AP is a registered trademark of the College Board and is not affiliated with The APsolute RecAP. Copyright 2020 - The APsolute RecAP, LLC. All rights reserved.)
Website:
EMAIL:
Follow Us:
Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap VSEPR Theory.
Lets Zoom Out:
Unit 2 - Molecular and Ionic Compound Structure and Properties
Topic - 2.7
Big idea - Structure and Properties
“Water, water everywhere, nor a drop to drink” is a rather famous quote from “the rime of the ancient mariner” by Samuel Taylor Coleridge. In chemistry we certainly have water everywhere - and we don’t drink in the lab! But what makes water so special? Part of it is the water's molecular structure and its polarity. Knowing a molecules’ molecular geometry is a central contributor to its chemical and physical properties. In this episode we will therefore recap molecular structure by taking a closer look at VSEPR Theory.
Let’s zoom in:
We are starting by talking about the acronym: V-S-E-P-R, which stands for valence shell electron repulsion and is based on the idea that the electrons in the valence shell, the highest energy level, are experiencing Coulombic repulsion - yup, that Gentleman again. We can distinguish between two different types of electrons - or electron pairs more specifically - in the valence shell of covalently bonded molecules: We either have a non-bonding or lone pair or a bonding pair connected to our central atom. No matter the type - the pairs repel each other and are therefore arranged with the greatest possible distance. This is what gives us the molecule’s geometry.
When talking about geometry, we have to distinguish between two types of geometry: electron-domain geometry and molecular geometry. Electron-domain geometry takes into account all electron pairs, no matter if bonding or nonbonding. Determining the electron-domain geometry can help us with molecular geometry. Molecular geometry is what the molecules actually look like - in 3D! And that is essential to chemists, because it influences how molecules react. To describe the molecular geometry, we focus on the bonding pairs - because that is the part that we can actually see, since it is bonded to another atom. BUT, the non-bonding pairs still have a huge role, because they repel the bonding pairs and therefore shape the molecule and influence the bonding angles.
So let’s take a closer look at the possible electron domain geometries first: There are six electron domains.
But wait - what? If we share six valence electrons, then the atoms have overall 12 valence electrons? How is that possible? These elements can have an expanded octet. There is still some discussion amongst chemists, but one hypothesis is that these electrons can be found in the d-orbitals. This is supported by the observation that only elements in period 3 and higher can form expanded octets. Reminder: the orbitals 1d and 2d do not exist.
So let’s look at the electron domains: a molecule with two electron domains - which can be bonding or non-bonding - is linear and three electron domains are trigonal-planar. Four electron domains form a tetrahedral shape and five electron domains a trigonal bipyramidal shape. With six electron domains, the molecule has an octahedral electron domain geometry.
To determine the molecular geometry, we can use the electron-domain geometry as foundation and “mix-and-match” bonding and non-bonding pairs. With a linear electron domain of 2, there is only one molecular geometry: linear, with bonding angles of 180 degrees. The trigonal planar electron domain has two molecular geometries: if all of them are bonding, you have a trigonal planar molecule, with bond angles of 120 degrees. If two of the electron domains are bonding and one is non-bonding, the molecule is bent.
The tetrahedral electron domain has three molecular geometries:
tetrahedral, with all bonding electron pairs and a bonding angle of 109.5 degrees; trigonal pyramidal with 3 bonding and one non-bonding and bent, if there are two non-bonding pairs. Since non-bonding pairs are larger than bonding pairs the repulsion is stronger. Therefore, the bond angle for trigonal pyramidal is less than 109.5 - usually around 107 degrees - and even smaller for bent, which has a bonding angle of 104.5 degrees. The term “bent” describes the molecular geometry of water! The central oxygen has 4 electron domains, 2 bonding and 2 non-bonding!
The trigonal bipyramidal electron domain has four molecular geometries: with 5 bonding pairs, the molecule is trigonal bipyramidal. The bonding angles are 120 degrees equatorial and 90 degrees axial. With 4 bonding and 1 non-bonding pair, the molecule is described as seesaw. The non-bonding pair will be in the equatorial plane, since it experiences less repulsion there, compared to the axial plane. Replacing another bonding with a non-bonding pair in the equatorial plane, you will have a t-shaped molecule. And when replacing the third equatorial bonding pair with a non-bonding pair, the molecule will only have the axial bonding pairs - and therefore be linear!
Last, but not least, the octahedral electron domain. There are three molecular geometries: octahedral with 6 bonding pairs and 90 degree bonding angles, square pyramidal when replacing one of the axial bonding pairs and square planar when replacing the second axial bonding pair.
TADA! That’s it! I know, it is a lot and sounds confusing. BUT: You really just have to keep in mind the EPR part of VSEPR: the electrons are repelling each other, trying to be as far from each other as possible - and that is the shape the molecule has!
If you have a couple of balloons: They make great models and help you visualize! Build the molecular geometry with all bonding pairs in one balloon color and replace one bonding pair after another with a balloon of a different color, representing the non-bonding pair!
To recap……
VSEPR Theory is based on the repulsion of electrons in the valence shell and can be used to determine the molecular geometry and bond angles of molecules. Electron domain geometry describes the geometry around a central atom of bonding and non-bonding pairs. Molecular Geometry describes the shape of a molecule with its bonding pairs. The shape is influenced by the non-bonding pairs. Using the molecular geometry you can determine the bonding angles.
Coming up next on the APsolute RecAP Chemistry Edition: Intermolecular Forces
Today’s Question of the day is about molecular geometry.
Question: What is the molecular shape of NH3? trigonal planar, trigonal pyramidal, tetrahedral