A Level Chemistry Revision "Shapes of Molecules".
Summary
TLDRThis 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
🔬 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.
📐 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.
Mindmap
Keywords
💡Molecular Shape
💡Electron Pair Repulsion Theory
💡Central Atom
💡Lone Pairs
💡Bonding Pairs
💡Linear Structure
💡Trigonal Planar
💡Tetrahedral
💡Trigonal Bipyramidal
💡Octahedral
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]
تصفح المزيد من مقاطع الفيديو ذات الصلة
5.0 / 5 (0 votes)