VSEPR Theory

Summary

The script introduces the Valence Shell Electron Pair Repulsion (VSEPR) theory, a model used to predict molecular geometry in three dimensions. It explains how electron pairs around a central atom repel each other, influencing molecular shape. The theory covers geometries from linear (two regions) to tetrahedral (four regions), and extends to trigonal bipyramidal (five regions) and octahedral (six regions). These shapes are crucial for understanding organic chemistry and molecular behavior.

Takeaways

• 📚 The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the 3D shapes of molecules based on the repulsion between electron pairs around a central atom.
• 🔬 VSEPR theory suggests that electron regions around a central atom will arrange themselves to be as far apart as possible due to their negative charge.
• 📏 With two electron regions, a molecule adopts a linear shape with a bond angle of 180 degrees.
• 🍕 Three electron regions result in a trigonal planar shape with bond angles of 120 degrees each.
• 🔲 Four electron regions create a tetrahedral geometry, with bond angles of approximately 109.5 degrees.
• 📐 The bond angles decrease as more electron regions are added, due to increased repulsion among the electron pairs.
• 🔺 With five electron regions, a molecule forms a trigonal bipyramidal shape, combining linear and trigonal planar geometries.
• 🔶 Six electron regions result in an octahedral geometry, resembling the XYZ coordinate system with bond angles of 90 or 180 degrees.
• 🧩 VSEPR theory accounts for molecules with expanded octets, which can have five or six electron regions, leading to more complex 3D shapes.
• 🌐 The theory is fundamental to understanding molecular geometry, particularly in organic chemistry, and helps predict the spatial arrangement of atoms in molecules.

Q & A

What is the Valence Shell Electron Pair Repulsion (VSEPR) theory?

VSEPR theory is a model used to predict the shapes of molecules by considering the repulsion between electron pairs in the valence shell of the central atom.

Why is it necessary to consider 3-dimensional molecular shapes?

Molecules exist in a three-dimensional world, and considering their 3D shapes helps to understand their real-life behavior and interactions more accurately.

How does the VSEPR theory work?

The VSEPR theory works by counting electron regions around the central atom and predicting the molecular shape based on the repulsion between these negatively charged regions.

What is the molecular shape when there are two electron regions?

When there are two electron regions, the molecule adopts a linear shape with a bond angle of 180 degrees.

What is the molecular shape when there are three electron regions?

With three electron regions, the molecule forms a trigonal planar shape with bond angles of 120 degrees.

What is the tetrahedral geometry and how many regions does it have?

Tetrahedral geometry is a molecular shape with four regions around the central atom, characterized by bond angles of 109.5 degrees.

What is the significance of the bond angles getting smaller as more regions are added?

As more electron regions are added, the bond angles decrease due to increased repulsion between the electron pairs, leading to more compact molecular shapes.

What is a trigonal bipyramidal shape and how is it formed?

A trigonal bipyramidal shape is formed when there are five electron regions, consisting of two linear regions and three regions in a trigonal planar arrangement perpendicular to the linear ones.

How does the octahedral geometry relate to the XYZ coordinate system?

An octahedral geometry is similar to the XYZ coordinate system, with six regions positioned at 90 or 180-degree angles, resembling the axes and coordinate points in three-dimensional space.

What is the total number of bond angles in an octahedral geometry?

In an octahedral geometry, there are eight bond angles, with some being 90 degrees and others 180 degrees, forming a closed three-dimensional figure with eight sides.

How many electronic geometries are there in total according to the VSEPR theory?

According to the VSEPR theory, there are five electronic geometries around the central atom: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

Outlines

00:00

🔬 Understanding Molecular Geometry with VSEPR Theory

The paragraph introduces the concept of transitioning from 2D electron dot formulas to understanding molecular geometry in a 3D world. It explains the Valence Shell Electron Pair Repulsion (VSEPR) theory, which is a model that predicts the shapes of molecules based on the repulsion between electron pairs around a central atom. The theory suggests that electron pairs, whether they are bonding pairs or lone pairs, will arrange themselves to be as far apart as possible due to their negative charge. This repulsion is influenced by the central atom's nucleus, which holds the electron pairs in place. The paragraph outlines the basic shapes that result from different numbers of electron regions: two regions lead to a linear shape with a 180-degree bond angle, three regions result in a trigonal planar shape with 120-degree bond angles, and four regions form a tetrahedral geometry with bond angles of 109.5 degrees. The theory is foundational for understanding molecular shapes in organic chemistry.

Keywords

💡Electron Dot Formulas

Electron dot formulas are a way to represent the valence electrons of an atom in a chemical structure. In the context of the video, these formulas are a starting point for understanding how molecules are arranged in three-dimensional space. The script mentions that after mastering electron dot formulas on a 2D plane, one must consider the 3D arrangement of molecules, which is where VSEPR theory comes into play.

💡Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory is a model used in chemistry to predict the shape of molecules based on the repulsion between electron pairs around a central atom. The video script emphasizes that VSEPR theory helps to move from the 2D representation of molecules to a more realistic 3D model. It's described as a simple yet effective model for understanding molecular geometry.

💡Central Atom

The central atom is the atom around which the rest of the molecule is arranged, typically the one with the most available bonding sites. In the script, the central atom is key to understanding how electron pairs repel each other and how this repulsion influences the molecular shape, as described by VSEPR theory.

💡Electron Regions

Electron regions refer to the areas around the central atom where electrons are likely to be found. These regions include bonding pairs and lone pairs. The script explains that counting these regions is crucial for applying VSEPR theory to predict molecular geometry, as different numbers of regions lead to different shapes.

💡Linear Shape

A linear shape is a molecular geometry where the electron regions around the central atom are arranged in a straight line, resulting in a 180-degree bond angle. The script uses the linear shape as the first example of how electron regions repel each other to achieve the maximum distance possible.

💡Trigonal Planar Shape

Trigonal planar shape is a molecular geometry where the electron regions around the central atom are arranged in a flat, triangular pattern, with bond angles of 120 degrees. The script describes this shape as resulting from three electron regions trying to maximize their distance from each other while still being held by the central atom.

💡Tetrahedral Geometry

Tetrahedral geometry is a molecular shape where the electron regions around the central atom are arranged in a three-dimensional shape resembling a tetrahedron, with bond angles of approximately 109.5 degrees. The script explains that this geometry arises when there are four electron regions, and it's an example of how VSEPR theory extends into three-dimensional space.

💡Expanded Octet

An expanded octet refers to a situation where a central atom has more than eight electrons in its valence shell, which is more than the typical octet. The script mentions that this concept is necessary for understanding molecules with five or six electron regions, which lead to more complex geometries like trigonal bipyramidal and octahedral.

💡Trigonal Bipyramid

Trigonal bipyramid is a molecular geometry where the electron regions around the central atom are arranged with two in a linear fashion and three in a trigonal planar arrangement, perpendicular to the linear axis. The script describes this as a more complex geometry that arises when there are five electron regions.

💡Octahedral Geometry

Octahedral geometry is a molecular shape where the electron regions around the central atom are arranged with six atoms, resembling the coordinates of 3D space, with bond angles of either 90 or 180 degrees. The script explains that this geometry is named for the eight-sided figure formed when lines are drawn between the points, even though there are only six regions.

💡Bond Angles

Bond angles are the angles formed between the lines connecting the central atom to the surrounding atoms. The script discusses how bond angles change depending on the number of electron regions: from 180 degrees in linear molecules to 120 degrees in trigonal planar, 109.5 degrees in tetrahedral, and varying in trigonal bipyramidal and octahedral geometries.

Highlights

Introduction to the concept of electron dot formulas in 2-dimensional representation.

Transition from 2D to 3D understanding of molecules using VSEPR theory.

Explanation of VSEPR theory as a model for molecular geometry.

VSEPR theory's approach to determining molecular shapes based on electron repulsion.

How electron regions around a central atom influence molecular geometry.

The formation of linear molecular shape with two electron regions.

Trigonal planar shape resulting from three electron regions.

Tetrahedral geometry with four electron regions and 109.5-degree bond angles.

The significance of electron regions being planar in trigonal planar geometry.

The concept of expanded octet and its role in molecular geometry.

Trigonal bipyramid molecular shape with five electron regions.

Octahedral geometry resulting from six electron regions.

The relationship between octahedral geometry and XYZ coordinate planes.

Bond angles in octahedral geometry being either 90 or 180 degrees.

The importance of VSEPR theory in understanding electronic geometries in organic chemistry.

Summary of the five electronic geometries central to VSEPR theory.

Transcripts

after you've mastered how to write

electron dot formulas on a piece of

paper which is a flat sheet and we're

kind of limited to the 2-dimensional

world you have to really start thinking

about how molecules really look in a

three-dimensional world because that's

the world that molecules actually live

in the simplest model for modeling this

type of behavior and it actually works

rather well is called the valence shell

electron pair repulsion theory and the

names probably scarier than it actually

is as far as it works we often just call

it the VSEPR theory and then we kind of

just say it all together Vesper theory

so you'll hear me say Vesper theory from

now on when I mean the previous so the

way it works and the name kind of gives

it away is you take your electron dot

formula and you simply count electron

regions around the central atom and you

realize that each of those regions is a

negatively charged region and so it's

going to repulse all the other

negatively charged regions and they're

going to try to get away from each other

they can't completely get away though

because the nucleus of the central atom

is holding them in so what they do is

they try to get away from each other but

they're still held by the central atom

this leads to distinct molecular shapes

and the easiest one is two regions if

you only have one atom on one side and

one on the other and they try to get

away from each other you get a fairly

linear type molecule and so that's the

first one two regions gives you linear

shape the bond angle is 180 stepping up

to the next one three regions you count

the regions around the central there's

three if three regions try to get away

from each other you basically get a nice

kind of pie shape it's like taking a pie

and cutting it into perfect thirds so

you get a 120 degree angle all the way

around for those three regions both of

those regions are still planar they

still look just fine on sheet of paper

it's when you get to four regions that

you have to go to the third dimension

you

have to go three-dimensional and what

this gets you is a shape that we call a

tetrahedron or tetrahedral geometry it's

four regions around a central all

perfectly symmetric and if you take a

look and spin the molecule around and

look at it every region is equivalent

and the bond angles are now down to

109.5 degrees that's a tetrahedral

geometry so as you can see the bond

angles are getting smaller because we're

bringing in more regions we started at

180 for linear 120 for trigonal planar

now we're down to 109.5 for tetrahedral

so that really sums up most of what we

will use especially in organic chemistry

now in addition to those you can have

expanded octet which gets you five and

six regions so to handle those you got

to go one step further the five regions

is a little bit complicated but it's

really just a combination of two and

three you're going to have two atoms

linear and then the other three are

going to be trigonal planar

perpendicular to that when you look at

the shape you see what we call a

trigonal bipyramid we call it a bi

pyramid because you can look at the

pyramid going up to the top atom and

down to the bottom atom those are two

little pyramids pointing in opposite

directions

that's trigonal bipyramid

the last one is six regions and when you

get up to six regions you get something

that should look somewhat familiar if

you've ever studied XYZ coordinate

planes and coordinate points

everything's 90 degrees apart or 180

it's like the XY and the z axis of

three-dimensional space you put one atom

in each space you get six total bond

angles are either 90 or they're 180 you

might go why is it octahedral when

there's six well if you close up if you

take every point and make a line you get

a closed three-dimensional object with a

total of eight sides eight

sided enclosed figure is an octahedron

and so we call it an octahedral geometry

so those are the other two that are part

of Vesper theory when you put all five

of those together you've got all five of

what we call our electronic geometries

around the central atom and that is at

the heart of Vesper theory