A Level Chemistry Revision "Shapes of Molecules".

Summary

This video explains how to determine the shapes of molecules using electron pair repulsion theory. The lesson covers how electron pairs surrounding a central atom repel each other and determine molecular structure. It introduces common molecular shapes such as linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, illustrating their bond angles and geometry. The explanation focuses on molecules without lone pairs of electrons on the central atom, with a follow-up video promising to cover molecules with lone pairs.

Takeaways

• 🧪 The shape of molecules can be determined using electron pair repulsion theory.
• 📐 Solid lines in molecular diagrams represent bonds lying on the plane of the page, while solid and dotted wedges indicate bonds coming out of and behind the plane of the page, respectively.
• ⚛️ Electron pair repulsion theory explains that electron pairs repel each other and move as far apart as possible to minimize repulsion.
• 🧬 The structure of beryllium chloride is linear, with bond angles of 180 degrees, because there are two bonding pairs around the central atom.
• 🌿 Carbon dioxide is also a linear molecule, with double bonds treated as single bonding areas, resulting in a 180-degree bond angle.
• 🔺 Boron trifluoride has a trigonal planar shape, with three bonding pairs around the central atom, and bond angles of 120 degrees.
• 🔷 Methane has a tetrahedral shape with four bonding pairs and bond angles of 109.5 degrees.
• 📊 Phosphorus pentachloride has a trigonal bipyramidal shape with five bonding pairs, featuring two bond angles: 90 degrees for vertical bonds and 120 degrees for those on the plane.
• 🛑 Sulfur hexafluoride forms an octahedral structure with six bonding pairs, with all bond angles being 90 degrees.
• 🔍 The next video will cover the shapes of ions and the effect of lone pairs of electrons on molecular shapes.

Q & A

What is the main focus of the video?

The main focus of the video is to teach viewers how to determine the shapes of molecules by understanding the three-dimensional representations and electron pair repulsion theory.

How does the video represent the three-dimensional structure of methane?

The video represents methane's structure with a central carbon atom surrounded by four hydrogen atoms. It uses solid lines to indicate bonds on the plane of the screen or page, a solid wedge for bonds coming out of the page, and a dotted wedge for bonds behind the page.

What is electron pair repulsion theory?

Electron pair repulsion theory states that the shape of a molecule is determined by the electron pairs surrounding the central atom, which repel each other and move as far apart as possible to minimize repulsion.

What is the significance of treating multiple bonds as single bonding areas when determining molecular shape?

Treating multiple bonds as single bonding areas simplifies the analysis of molecular shape, as it allows for the consideration of the overall electron pair repulsion without getting into the specifics of bond order.

What is the molecular shape of a molecule with two bonds or bonding areas?

A molecule with two bonds or bonding areas has a linear shape with a 180-degree bond angle.

How does the video explain the shape of boron trifluoride?

The video explains that boron trifluoride has a trigonal planar shape with bond angles of 120 degrees, as the three electron pairs repel each other and arrange themselves towards the points of a triangle.

What is the shape of a molecule with four bonding pairs around the central atom?

A molecule with four bonding pairs around the central atom has a tetrahedral shape, with bond angles of 109.5 degrees.

What is the difference between trigonal bipyramidal and octahedral shapes?

Trigonal bipyramidal shapes have three bonding pairs on a central plane with 120-degree angles and two bonding pairs above and below the plane at 90 degrees, while octahedral shapes have six bonding pairs with four on the central plane at 90-degree angles and two above and below the plane also at 90 degrees.

Why is it important to consider lone pairs of electrons when determining molecular shape?

Lone pairs of electrons can affect the molecular shape because they contribute to the electron repulsion around the central atom, influencing the arrangement and angles of the bonding pairs.

What is the next topic the video series will cover after discussing molecular shapes?

The next topic the video series will cover is the shapes of ions and how to deal with lone pairs of electrons.

Outlines

00:00

🔬 Understanding Molecular Shapes

This paragraph introduces the concept of determining molecular shapes through basic rules. It explains how scientists represent three-dimensional structures, using methane as an example with a central carbon atom surrounded by four hydrogen atoms. The paragraph details the use of solid lines, solid wedges, and dotted wedges to depict bonds in relation to the plane of the screen or page. It then transitions into electron pair repulsion theory, which posits that the shape of a molecule is influenced by the electron pairs around the central atom, aiming to minimize repulsion by moving as far apart as possible. The paragraph concludes with an introduction to linear molecular structures, exemplified by beryllium chloride and carbon dioxide, where the central atom has two bonds or bonding areas, resulting in a 180-degree bond angle.

05:01

📐 Exploring Various Molecular Geometries

The second paragraph delves into different molecular geometries based on the number of bonding electron pairs around the central atom. It starts with trigonal planar shapes, using boron trifluoride as an example, where three bonding pairs arrange in a triangular pattern with 120-degree bond angles. The paragraph then moves on to tetrahedral molecules, like methane and the ammonium ion, characterized by four bonding pairs and 109.5-degree bond angles. The concept of trigonal bipyramidal shape is introduced with phosphorus pentachloride, where five bonding pairs arrange with two pairs opposite each other and the remaining three in a triangular plane, featuring bond angles of 90 and 120 degrees. Lastly, the paragraph touches on the octahedral shape of sulfur hexafluoride, with six bonding pairs, two above and below the central plane at 90 degrees and four in the plane at 90-degree angles. The paragraph ends with a teaser for the next video, which will cover ion shapes and the impact of lone pairs of electrons.

Keywords

💡Molecular Shape

Molecular shape refers to the three-dimensional arrangement of atoms within a molecule. It is a central theme in the video as it explains how the shape of a molecule is determined by the positions of atoms and the electron pairs around the central atom. For instance, methane is described as having a tetrahedral shape with bond angles of 109.5 degrees, illustrating the concept of molecular shape in a practical example.

💡Electron Pair Repulsion Theory

Electron Pair Repulsion Theory is a fundamental concept in the video that states the shape of a molecule is influenced by the repulsion between electron pairs surrounding the central atom. These electron pairs, including those involved in covalent bonds, move as far apart as possible to minimize repulsion, thus determining the molecule's shape. The theory is crucial for understanding why methane has a tetrahedral shape and why carbon dioxide is linear.

💡Central Atom

The central atom is the atom in a molecule that is directly bonded to other atoms and around which the molecular geometry is centered. The video uses the central atom concept to explain how different numbers of bonding electron pairs around the central atom lead to different molecular shapes, such as linear for two pairs and tetrahedral for four pairs.

💡Lone Pairs

Lone pairs are pairs of electrons that are not involved in bonding and are found on the central atom. The video mentions that the presence of lone pairs can affect molecular shape, but a detailed discussion is reserved for a subsequent video. Lone pairs are important for understanding molecular geometry because they contribute to the electron pair repulsion that influences shape.

💡Bonding Pairs

Bonding pairs are pairs of electrons that are shared between two atoms in a covalent bond. The video explains that the arrangement of bonding pairs around the central atom is key to determining molecular shape. For example, in methane, the four bonding pairs arrange themselves in a tetrahedral geometry around the central carbon atom.

💡Linear Structure

A linear structure is a molecular shape where the atoms are arranged in a straight line, with a bond angle of 180 degrees between the bonding pairs. The video uses carbon dioxide as an example of a linear molecule, where the two double bonds to oxygen atoms are treated as single bonding areas for the purpose of determining shape.

💡Trigonal Planar

Trigonal planar is a molecular shape where the central atom is bonded to three other atoms in a flat, triangular arrangement with bond angles of 120 degrees. Boron trifluoride is given as an example in the video, where the three fluorine atoms are positioned at the corners of a triangle around the central boron atom.

💡Tetrahedral

Tetrahedral is a molecular shape characterized by four bonding pairs around the central atom, with bond angles of approximately 109.5 degrees. Methane is used as a classic example in the video, where the central carbon atom is surrounded by four hydrogen atoms in a tetrahedral arrangement.

💡Trigonal Bipyramidal

Trigonal bipyramidal is a more complex molecular shape where the central atom is bonded to five atoms, with two atoms opposite each other and three on the same plane, forming a triangular base. The video mentions phosphorus pentachloride as an example, where the molecule has a trigonal bipyramidal shape to minimize electron pair repulsion.

💡Octahedral

Octahedral is a molecular shape where the central atom is bonded to six atoms, with two atoms above and below the central plane and four atoms in the plane, all positioned at 90-degree angles to each other. Sulfur hexafluoride is described in the video as having an octahedral shape, which is a significant concept in understanding the geometry of molecules with six bonding pairs.

Highlights

Introduction to determining the shapes of molecules using basic rules.

Representation of three-dimensional shapes of molecules, using methane as an example.

Explanation of solid lines, wedges, and dotted wedges in molecular diagrams.

Electron pair repulsion theory as the basis for molecular shape determination.

The concept that electron pairs repel each other and move to minimize repulsion.

Linear structure of molecules with two bonds, like beryllium chloride and carbon dioxide.

Treating multiple bonds as single bonding areas for shape determination.

Trigonal planar shape of molecules with three bonds, exemplified by boron trifluoride.

Tetrahedral shape of molecules with four bonds, using methane and ammonium ion as examples.

Trigonal bipyramidal shape of molecules with five bonds, as seen in phosphorus pentachloride.

Octahedral shape of molecules with six bonds, demonstrated by sulfur hexafluoride.

The importance of bond angles in determining molecular geometry.

The impact of lone pairs of electrons on molecular shape, to be discussed in the next video.

Practical application of electron pair repulsion theory to predict molecular shapes.

The significance of molecular shape in understanding chemical behavior.

Anticipatory guidance for students on the next topic: shapes of ions and lone pairs.

Transcripts

[Music]

hi and welcome back to free science

lessons

by the end of this video you should be

able to determine the shapes of

molecules

now i should just point out that many

students find this tricky however it's

quite straightforward if you learn a few

basic rules

first we need to look at how scientists

represent the three-dimensional shapes

of molecules

i'm showing you here the structure of

methane as you can see methane has a

central carbon atom

surrounded by four hydrogen atoms solid

lines such as these two

tell us that these two bonds lie on the

plane of the screen or the page

so what that means is that these two

hydrogen atoms also lie on the plane of

the page

a solid wedge like this one tells us

that this bond is coming out of the

plane of the page

so that means that this hydrogen atom is

also coming out of the plane of the page

and finally a dotted wedge tells us that

this bond is projecting back

behind the plane of the page so that

means that this hydrogen is also behind

the plane of the page

we're going to be using these bonds

whenever we need to show the

three-dimensional shape of a molecule

now in order to work out the shapes of

molecules we need to look at electron

pair repulsion theory

electron pair repulsion theory states

that the shape of a molecule is

determined by the electron pairs

surrounding the central atom and i

should point

out that we're only referring to the

outer shell in this case

this is based on the fact that pairs of

electrons repel

all of the other electron pairs the

electron pairs now move as far apart as

possible to minimize this repulsion

now we've been referring to pairs of

electrons but you need to remember that

a covalent bond is a pair of electrons

now one thing i need to point out is

that in this video we're only looking at

molecules with no

lone pairs of electrons on the central

atom we'll be looking at how to deal

with lone pairs in the next video

okay let's look at a simple example i'm

showing you here the molecule beryllium

chloride

in this molecule the central atom is

beryllium this is covalently bonded

to two atoms of chlorine so this means

that we've got two electron pairs around

the central atom

these two pairs of electrons repel each

other and move as far apart as possible

now the furthest that they can move

apart is in a straight line like this

scientists say that this molecule has a

linear structure in other words a

straight line

and the angle between these two bonds is

180 degrees

here's another linear molecule this is

carbon dioxide

now this illustrates a really important

point as you can see the central atom is

carbon

and this has two double bonds to oxygen

atoms

when looking at multiple bonds such as

double or triple bonds

to determine the shape of the molecule

we treat a multiple bond as a single

bonding area

in other words we treat a double bond

the same way we treat a single bond

so these two bonding areas repel and

move as far apart as possible

and again in this case the angle between

the bonding areas is 180 degrees

so you need to learn that if a central

atom has two bonds or bonding areas

then it will have a linear shape with a

180 degree bond angle

however this is not the case if the

central atom has a lone pair of

electrons

and as i said we look at those in the

next video

in the next section we're going to look

at other examples of the shapes of

molecules

[Music]

okay this molecule is boron trifluoride

and we've seen this in the videos on

covalent bonding

this has a central boron atom bonded to

three fluorine atoms

again the electron pairs in these three

covalent bonds repel

and move apart as far as possible in

this

case the bonds arrange themselves

towards the points of a triangle like

this

and the bond angle between them is 120

degrees

scientists call this shape trigonal

because it's based on a triangle

now if you viewed the molecule from the

side you would see that it's flat

scientists call this planar which means

flat

so this shape is called trigonal planar

and you'll see this whenever you've got

a central atom with three pairs of

bonding electrons around it

as long as the central atom has no lone

pairs of electrons

okay what if this central atom has four

pairs of bonding electrons around it

well in this case we've got a

tetrahedral molecule

and a good example is methane all of the

bond angles and tetrahedral molecules

are 109.5 degrees

here's another tetrahedral molecule this

is the ammonium ion

and again the bond angles are all 109.5

degrees

okay so what if the central atom has

five pairs of bonding electrons around

it

a good example is phosphorus

pentachloride

well in this case in order to minimize

repulsion two of the bonding pairs move

to opposite sides of the molecule

and we can see these here the other

three bonding pairs now take up a

central position

lying on the same plane and they spread

themselves out as far as they can

now there are two bond angles to

consider here the bonds pointing up and

down about 90 degrees to the central

plane

whereas the angle between the bonds

lying on the central plane is 120

degrees

now this shape is called trigonal

bipyramidal

trigonal because the three atoms on the

central plane are forming a triangle

and by pyramidal because these form two

pyramid shapes with the two other atoms

so here's the pyramid above the plane

and here's a pyramid below

now i should just point out that if

you're following the ocr spec then

you're not expected to know the trigonal

bipyramidal

shape however i'd recommend that you're

aware of it in case it appears in a

question where you have to apply your

knowledge

okay i'm showing you here the compound

sulfur hexafluoride

as you can see this molecule has got six

bonding pairs around the central atom

scientists call this shape octahedral

again we have a bonding pair above and

below the central plane

and four bonding pairs lying on the

central plane

just like before the bonds pointing up

and down are at 90 degrees to the

central plane

however in this case the angle between

the bonds lying on the central plane is

also 90 degrees

in the next video we're going to look at

the shapes of ions and how to deal with

lone pairs of electrons

[Music]