A Level Chemistry Revision "Effect of Lone Pairs on the Shape of Molecules".
Summary
TLDRThis 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
- đŹ Electron pair repulsion theory is the basis for understanding molecular shapes, suggesting that electron pairs repel each other and arrange to minimize this repulsion.
- đ§Č Multiple bonds, such as double bonds, are treated as single bonding areas when determining molecular geometry.
- đ 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.
- đ The nitrate ion (NO3^-) also exhibits a trigonal planar shape with a 120-degree bond angle, similar to the carbonate ion.
- đ 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.
- đ Lone pairs of electrons repel more strongly than bonding pairs, which affects the bond angles in molecules.
- đ§ 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.
- đ§ 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.
- đ§ 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.
- đ 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
đŹ 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.
đ 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.
Mindmap
Keywords
đĄElectron Pair Repulsion Theory
đĄTrigonal Planar
đĄBond Angle
đĄTetrahedral
đĄLone Pair
đĄAmmonia
đĄAmmonium Ion
đĄWater
đĄDative Bond
đĄSulfate Ion
đĄCovalent Bond
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
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