A Level Chemistry Revision "Effect of Lone Pairs on the Shape of Molecules".

Summary

This educational video delves into the molecular shapes of ions and molecules, focusing on electron pair repulsion theory. It explains how the shape of a molecule is influenced by the electron pairs around the central atom, with lone pairs exerting a stronger repulsion than bonding pairs. The video illustrates the trigonal planar and tetrahedral structures of ions like CO3^2- and SO4^2-, and how lone pairs alter the bond angles in molecules like ammonia (NH3) and water (H2O), leading to pyramidal and V-shaped geometries, respectively.

Takeaways

• 🔬 Electron pair repulsion theory is the basis for understanding molecular shapes, suggesting that electron pairs repel each other and arrange to minimize this repulsion.
• 🧲 Multiple bonds, such as double bonds, are treated as single bonding areas when determining molecular geometry.
• 📐 The carbonate ion (CO3^2-) has a trigonal planar shape with a bond angle of 120 degrees due to three bonding areas around the central carbon atom.
• 📐 The nitrate ion (NO3^-) also exhibits a trigonal planar shape with a 120-degree bond angle, similar to the carbonate ion.
• 📐 The sulfate ion (SO4^2-) has a tetrahedral structure with a bond angle of 109.5 degrees, influenced by four bonding areas around the central sulfur atom.
• 🔑 Lone pairs of electrons repel more strongly than bonding pairs, which affects the bond angles in molecules.
• 💧 In ammonia (NH3), the presence of a lone pair on the nitrogen atom results in a pyramidal shape with bond angles of approximately 107 degrees.
• 💧 The ammonium ion (NH4^+), formed by the reaction of ammonia with a hydrogen ion, has a tetrahedral shape with bond angles returning to 109.5 degrees due to the conversion of the lone pair into a dative bond.
• 💧 Water (H2O) has a V-shaped or non-linear structure because of the two lone pairs on the oxygen atom, which reduces the bond angle to 104.5 degrees.
• 🌟 Understanding the impact of lone pairs on molecular geometry is crucial for predicting the shapes of molecules and ions.

Q & A

What is the main focus of the video script?

The main focus of the video script is to teach viewers how to describe the shapes of ions and molecules, particularly how lone pairs of electrons affect these shapes.

What theory is discussed in the video to explain the shape of molecules?

The video discusses the Electron Pair Repulsion Theory, which states that the shape of a molecule is determined by the electron pairs surrounding the central atom.

How does the video script define the term 'bonding area'?

In the context of the video script, a 'bonding area' refers to the presence of a single covalent bond or a multiple bond (like a double bond), which is treated as a single entity when determining molecular shape.

What is the significance of the bond angle in a trigonal planar structure as described in the script?

The script mentions that a trigonal planar structure, like the carbonate ion (CO3^2-), has a bond angle of 120 degrees, which is a key characteristic of this molecular geometry.

How does the presence of a lone pair affect the bond angle in a molecule?

The script explains that lone pairs repel more strongly than bonding pairs, which decreases other bond angles by 2.5 degrees, thus affecting the overall shape of the molecule.

What is the bond angle in the ammonia molecule due to the presence of a lone pair?

The script states that the bond angle in the ammonia molecule (NH3) is 107 degrees due to the presence of a lone pair, which is less than the typical tetrahedral bond angle of 109.5 degrees.

How does the ammonium ion (NH4^+) differ in shape from the ammonia molecule?

The ammonium ion (NH4^+) has a bond angle of 109.5 degrees, which is the same as a regular tetrahedron, because the lone pair in ammonia has formed a dative covalent bond, reducing the repulsion and returning the bond angle to the tetrahedral angle.

What shape is the water molecule, and how does the presence of lone pairs influence this?

The water molecule is described as having a non-linear or V-shaped structure due to the presence of two lone pairs on the oxygen atom, which reduces the bond angle to 104.5 degrees.

What is the role of dative bonds in the context of molecular shape as discussed in the script?

The script mentions that dative bonds behave similarly to regular covalent bonds in terms of their effect on molecular shape, contributing to the overall geometry without altering the repulsion dynamics significantly.

How does the sulfate ion (SO4^2-) differ in structure from the carbonate ion (CO3^2-)?

The sulfate ion has a central sulfur atom surrounded by two single bonds and two double bonds, resulting in a tetrahedral structure with a bond angle of 109.5 degrees, unlike the carbonate ion which has a trigonal planar structure.

Outlines

00:00

🔬 Electron Pair Repulsion Theory and Ion Shapes

This paragraph introduces the concept of electron pair repulsion theory, which dictates that the shape of a molecule is determined by the electron pairs in the outer shell of the central atom. The theory is based on the principle that electron pairs repel each other and arrange themselves to minimize this repulsion. The video discusses how to apply this theory to predict the shapes of ions, starting with the carbonate ion (CO3^2-), which has a trigonal planar shape due to three bonding areas around the central carbon atom. The nitrate ion (NO3^-) is also mentioned, which despite having a dative bond, maintains a trigonal planar shape. The sulfate ion (SO4^2-) is highlighted as an example of a tetrahedral structure with four bonding areas. The paragraph emphasizes that lone pairs are not present in these ions, setting the stage for the next section on how lone pairs affect molecular shapes.

05:05

🌌 Lone Pairs and Their Impact on Molecular Geometry

The second paragraph delves into the influence of lone pairs on molecular shapes. It explains that lone pairs repel more strongly than bonding pairs, which results in a decrease of bond angles by 2.5 degrees. The example of ammonia (NH3) is used to illustrate this concept, where the presence of a lone pair on the nitrogen atom leads to a pyramidal shape with bond angles of 107 degrees, as opposed to the typical tetrahedral angle of 109.5 degrees. The ammonium ion (NH4^+) is contrasted to show that when the lone pair forms a dative bond, the bond angle returns to the tetrahedral angle of 109.5 degrees. The paragraph also discusses water (H2O), where the oxygen atom's two lone pairs result in a non-linear or V-shaped molecule with bond angles of 104.5 degrees. The video concludes by encouraging viewers to apply these principles to describe the shapes of ions and molecules with lone pairs.

Keywords

💡Electron Pair Repulsion Theory

Electron Pair Repulsion Theory is a fundamental concept in chemistry that explains the three-dimensional shapes of molecules. It states that the shape of a molecule is determined by the arrangement of electron pairs around a central atom, with these pairs repelling each other and arranging themselves to be as far apart as possible. This theory is central to the video's theme as it is used to predict the shapes of various ions and molecules. For example, the video explains that in the carbonate ion (CO3^2-), the central carbon atom is bonded to three oxygen atoms, forming a trigonal planar structure due to the electron pair repulsion.

💡Trigonal Planar

Trigonal planar is a molecular geometry where the central atom is surrounded by three bonding areas, typically forming a flat, equilateral triangle shape. This term is directly related to the video's theme as it describes the shape of ions like the carbonate ion (CO3^2-) and the nitrate ion (NO3^-), where the central atom is bonded to three other atoms, resulting in a bond angle of 120 degrees.

💡Bond Angle

A bond angle is the angle between any two bonds that are connected to the same central atom. It is a key aspect of understanding molecular geometry. In the video, bond angles are used to describe the shapes of molecules and ions, such as the 120-degree bond angle in trigonal planar molecules and the 109.5-degree bond angle in tetrahedral molecules.

💡Tetrahedral

Tetrahedral is a geometric shape that describes the arrangement of four bonding areas around a central atom, forming the corners of a pyramid. The term is used in the video to describe the shape of the sulfate ion (SO4^2-), where the central sulfur atom is surrounded by four bonding areas, each at a bond angle of 109.5 degrees.

💡Lone Pair

A lone pair refers to a pair of non-bonding electrons in a molecule or ion. The video emphasizes the impact of lone pairs on molecular geometry, as they repel bonding pairs more strongly than bonding pairs repel each other. Lone pairs are crucial in determining the shape of molecules like ammonia (NH3) and water (H2O), where their presence alters the bond angles from the ideal tetrahedral angle.

💡Ammonia

Ammonia is a molecule composed of one nitrogen atom bonded to three hydrogen atoms, with one lone pair on the nitrogen. The video uses ammonia to illustrate how lone pairs affect molecular shape, resulting in a pyramidal shape with bond angles of approximately 107 degrees due to the repulsion of the lone pair.

💡Ammonium Ion

The ammonium ion (NH4^+) is formed when ammonia reacts with a hydrogen ion. In the video, it is used to contrast the effect of a lone pair in ammonia with the absence of lone pairs in ammonium, where the bond angle returns to the tetrahedral angle of 109.5 degrees, demonstrating how the presence or absence of lone pairs influences molecular geometry.

💡Water

Water is a molecule consisting of one oxygen atom bonded to two hydrogen atoms, with two lone pairs on the oxygen. The video explains how the presence of these lone pairs results in a non-linear or V-shaped molecular geometry, with bond angles reduced to approximately 104.5 degrees due to the repulsion of the lone pairs.

💡Dative Bond

A dative bond is a type of covalent bond where both electrons come from one of the atoms involved in the bond. In the context of the video, the dative bond in the nitrate ion (NO3^-) is mentioned, and it is explained that it does not affect the molecular shape, behaving similarly to a regular covalent bond.

💡Sulfate Ion

The sulfate ion (SO4^2-) is used in the video to demonstrate a tetrahedral molecular geometry. It consists of a central sulfur atom surrounded by four oxygen atoms, each connected by a single bond, resulting in a tetrahedral shape with bond angles of 109.5 degrees.

💡Covalent Bond

A covalent bond is a chemical bond formed by the sharing of electron pairs between atoms. The video discusses how covalent bonds, including multiple bonds like double bonds, are treated as single bonding areas when determining molecular geometry, which is essential for understanding the shapes of ions and molecules.

Highlights

Introduction to the video on describing the shapes of ions and the effect of lone pairs on molecular shapes.

Review of electron pair repulsion theory and its influence on molecular shape.

Explanation that multiple bonds are treated as single bonding areas in molecular geometry.

Shape of the carbonate ion (CO3^2-) is trigonal planar with a bond angle of 120 degrees.

Nitrate ion (NO3^-) also has a trigonal planar shape with a 120-degree bond angle, despite the presence of a dative bond.

Sulfate ion (SO4^2-) has a tetrahedral structure with a bond angle of 109.5 degrees due to four bonding areas.

Introduction to the effect of lone pairs on molecular geometry.

Lone pairs repel more strongly than bonding pairs, affecting bond angles.

Ammonia (NH3) has a pyramidal shape with bond angles of 107 degrees due to the presence of a lone pair.

Ammonium ion (NH4^+) returns to a tetrahedral bond angle of 109.5 degrees as the lone pair forms a dative bond.

Water (H2O) molecule has a V-shaped structure with bond angles of 104.5 degrees due to two lone pairs on the oxygen atom.

Linear shape that water would have if the oxygen atom did not have any lone pairs.

The tetrahedral bond angle is normally 109.5 degrees, but lone pairs reduce this angle.

Each lone pair reduces the bond angle by 2.5 degrees, affecting the overall molecular shape.

Summary of how to describe the shapes of ions and the impact of lone pairs on molecular shapes.

Transcripts

[Music]

hi and welcome back to free science

lessons

by the end of this video you should be

able to describe the shapes of ions

you should then be able to describe the

effect of lone pairs on the shapes of

molecules

this is the second part of a two-part

video looking at the shapes of molecules

and if you haven't watched the first

part of this video then you should watch

it now

in the last video we looked 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

remember that we're looking at electrons

in the outer shell

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

as we said before this refers to pairs

of electrons but you need to remember

that a covalent bond is a pair of

electrons

we also saw that we treat multiple bonds

such as double bonds

as a single bonding area in other words

we treat them in the same way that we

treat a single bond

in this video i'm going to start by

looking at the shapes of some ions

and in all of these ions the central

atom does not have a lone pair of

electrons

okay this shows the carbonate ion co32

minus

in this ion the central carbon atom is

bonded to three atoms of oxygen

we've got two single bonds and one

double bond

and remember that we treat the double

bond as a single bonding area

because there are three bonding areas

around the central atom

this ion forms a trigonal planar

structure with a bond angle of 120

degrees

here's another ion with a similar

structure this is the nitrate ion no3

minus again in this ion the central atom

is surrounded by two single bonds

and one double bond and again this ion

has a trigonal

planar shape with a 120 degree bond

angle

now you'll notice that in this ion one

of the single bond is actually a dative

bond

but that has no effect at all on the

shape of the ion

that's because data bonds behave in the

same way as regular covalent bonds

okay here's a sulfate ion so4 two minus

now in this case the central sulfur atom

is surrounded by two single bonds and

two double bonds

treating the double bonds as single

bonding areas this means that there are

four bonding areas around the central

atom

so in this case we've got a tetrahedral

structure with a bond angle of 109.5

degrees

okay in the next section we'll see how

to deal with lone pairs

[Music]

okay now in all of the molecules we've

seen so far there were no lone pairs of

electrons on the central atom

so how do we deal with lone pairs well

the key fact you need to learn is that

lone pairs repel more strongly than

bonding pairs

and this extra repulsion decreases other

bond angles by 2.5 degrees

i'm showing you here the structure of

ammonia as you can see the nitrogen atom

has a lone pair

so we've got three bonding pairs and one

lone pair

as we saw in the last video four pairs

of electrons form a tetrahedral

structure

so the shape of the ammonia molecule is

based on a tetrahedron

normally the tetrahedral bond angle is

109.5 degrees

but as i said before a lone pair repels

more strongly than a bonding pair

and this extra repulsion reduces the

bond angle by 2.5 degrees

so that means that the bond angle in the

ammonia molecule is 107 degrees

scientists call this shape pyramidal as

it looks like a pyramid

now in the video on covalent bonding we

saw that ammonia can react with a

hydrogen ion

to form the ammonium ion nh4 plus

in this case the lone pair forms a

dative covalent bond

a dative covalent bond has the same

level of repulsion as a regular covalent

bond

so this means that the bond angle in the

ammonium ion returns back to the

tetrahedral angle

of 109.5 degrees

okay here's another example i'm showing

you here a molecule of water

the oxygen atom has got two single

covalent bonds to hydrogen atoms but the

oxygen also has two lone pairs of

electrons

now if the oxygen atom did not have any

lone pairs then

water would have a linear shape and we

saw examples of linear molecules in the

last video

however as we said the oxygen atom has

got two lone pairs

so this means that we've got four

bonding areas in total

and the shape will be based on the

tetrahedron

now remember that the tetrahedral bond

angle is 109.5 degrees

however one lone pair reduces this angle

by 2.5 degrees

because there are two lone pairs the

angle is now reduced to 104.5 degrees

scientists call this either a non-linear

or a v-shaped molecule

okay so hopefully now you can describe

the shapes of ions and describe the

effect of lone pairs on the shapes of

[Music]

molecules

you