Shapes of Molecules + Ions - Help!

Hello! I've always had trouble with shapes of molecules in chemistry, and i am resitting OCR Atoms Bonds and Groups, so i would really appreciate it if someone could give me a hand with this :biggrin:

BF3 has the bond angle 120 and is a 2D shape (trigonal planar)
CH4 has the bond angle 109.5 and is a 3D shape (tetrahedral)

I was just wondering how you knew if the shape of a molecule is 2D or 3D. Is it because Boron can not form an octet in its outer shell, so it will always be 2D?

Also, this may be a silly question, what does it mean when it says

Bonded pair/Lone pair

I'm not sure why there's a slash, and how you can get a bonded/lone pair?

Thanks :smile:

Reply 1

Pride

21

well don't say they are 2D or 3D shapes because that'll confuse everything...

I see what you're trying to get at though. You're right, when you draw BF₃, all atoms are in the same plane, because that's the furthest the bonds can repel from each other, 120 degrees away, in a trigonal planar shape. When you look at CH₄, the molecule is tetrahedral, there are 4 covalent bonds, no lone pairs of electrons. The bonds repel each other as far as possible, 2 bonds in the plane, one behind the plane, one in front of it. The angle between each bond is 109.5 degrees.

Now if you want to talk about lone pairs of electrons. A lone pair is a pair of electrons in a molecule, not involved in bonding. A bonded pair of electrons is the pair of electrons that are in a covalent bond. Remember, all a covalent bond is, is a pair of electrons. People tend to forget this, and a dative covalent bond, is a pair of electrons in a covalent bond, where the electrons are supplied from only one of the bonded atoms, not one from each like a normal covalent bond.

Think about a water molecule, H₂O. The O is bonded to 2 hydrogen atoms, 2 covalent bonds. Then there are 2 lone pairs of electrons, these are unbonded. So the number of regions of electron density is actually 4. They repel each other as far as they can, but because a lone pair of electrons have a stronger repulsive effect than a normal bonded pair, they increase the bond angle by 2.5 degrees each.

So the bond angle instead of being 109.5 degrees like a tetrahedral molecule, is 109.5 - 5 or 104.5 degrees.

Hope that helps, feel free to ask more questions.

Reply 2

thegodofgod

19

Original post by BigWilfo

Hello! I've always had trouble with shapes of molecules in chemistry, and i am resitting OCR Atoms Bonds and Groups, so i would really appreciate it if someone could give me a hand with this :biggrin:

BF3 has the bond angle 120 and is a 2D shape (trigonal planar)
CH4 has the bond angle 109.5 and is a 3D shape (tetrahedral)

I was just wondering how you knew if the shape of a molecule is 2D or 3D. Is it because Boron can not form an octet in its outer shell, so it will always be 2D?

Also, this may be a silly question, what does it mean when it says

Bonded pair/Lone pair

I'm not sure why there's a slash, and how you can get a bonded/lone pair?

Thanks :smile:

You shouldn't really use the terms 2D and 3D as you're talking about 3D molecular shapes, so they're all 3D, but I see what you mean by those.

Essentially, most compounds where there are 3 or fewer atoms attached to the central atom are planar, e.g.

BeCl₂ = 180 degrees, Linear.
CO₂ = 180 degrees, Linear.

BF₃ = 120 degrees, Trigonal planar.

...O
...||
R-C-R bond in ketones, aldehydes, carboxylic acids, esters etc. = 120 degrees, Trigonal planar, even though there are 4 bonds (as there are only 3 atoms attached to the central atom).

Central atoms with lone pairs on them, e.g. Nitrogen don't fit this pattern. This is due to the lone pair-bond pair repulsion of the like charged electrons in the structure, so another shape is formed: Trigonal Pyramidal, with bond angles of 107 degrees, e.g. in :NH₃.

Oxygen has 2 lone pairs on it, so the amount of repulsion is double. Hence in H-O-H (water), the bond angle is 105 degrees, but that is planar.

Reply 3

BigWilfo

OP

Original post by Pride

well don't say they are 2D or 3D shapes because that'll confuse everything...

I see what you're trying to get at though. You're right, when you draw BF₃, all atoms are in the same plane, because that's the furthest the bonds can repel from each other, 120 degrees away, in a trigonal planar shape. When you look at CH₄, the molecule is tetrahedral, there are 4 covalent bonds, no lone pairs of electrons. The bonds repel each other as far as possible, 2 bonds in the plane, one behind the plane, one in front of it. The angle between each bond is 109.5 degrees.

Now if you want to talk about lone pairs of electrons. A lone pair is a pair of electrons in a molecule, not involved in bonding. A bonded pair of electrons is the pair of electrons that are in a covalent bond. Remember, all a covalent bond is, is a pair of electrons. People tend to forget this, and a dative covalent bond, is a pair of electrons in a covalent bond, where the electrons are supplied from only one of the bonded atoms, not one from each like a normal covalent bond.

Think about a water molecule, H₂O. The O is bonded to 2 hydrogen atoms, 2 covalent bonds. Then there are 2 lone pairs of electrons, these are unbonded. So the number of regions of electron density is actually 4. They repel each other as far as they can, but because a lone pair of electrons have a stronger repulsive effect than a normal bonded pair, they increase the bond angle by 2.5 degrees each.

So the bond angle instead of being 109.5 degrees like a tetrahedral molecule, is 109.5 - 5 or 104.5 degrees.

Hope that helps, feel free to ask more questions.

Thanks!

So, if an atom can make more than 3 electron pairs, such as Oxygen or Carbon, will the shape always be "3D" (for use of a better word), and if an atom cannot fill its outer shell at all, will it always be "2D" or trigonal planar?

Reply 4

thegodofgod

19

Original post by BigWilfo

Thanks!

So, if an atom can make more than 3 electron pairs, such as Oxygen or Carbon, will the shape always be "3D" (for use of a better word), and if an atom cannot fill its outer shell at all, will it always be "2D" or trigonal planar?

Read my post about R-CO-R group. :smile:

Reply 5

Pride

21

Original post by BigWilfo

Thanks!

So, if an atom can make more than 3 electron pairs, such as Oxygen or Carbon, will the shape always be "3D" (for use of a better word), and if an atom cannot fill its outer shell at all, will it always be "2D" or trigonal planar?

don't confuse it, you don't need to come up with trends. Just remember the shapes and angles, and why the angles are what they are. You'll certainly be asked questions on these in the test. Use the words bonded pairs, lone pairs, repelling from each other as far as possible, extra repulsive effect.

You'll end up being able to guess what's in the mark scheme, because it's the same buzz words they require.

Reply 6

Medicina

Original post by BigWilfo

Hello! I've always had trouble with shapes of molecules in chemistry, and i am resitting OCR Atoms Bonds and Groups, so i would really appreciate it if someone could give me a hand with this :biggrin:

BF3 has the bond angle 120 and is a 2D shape (trigonal planar)
CH4 has the bond angle 109.5 and is a 3D shape (tetrahedral)

I was just wondering how you knew if the shape of a molecule is 2D or 3D. Is it because Boron can not form an octet in its outer shell, so it will always be 2D?

Also, this may be a silly question, what does it mean when it says

Bonded pair/Lone pair

I'm not sure why there's a slash, and how you can get a bonded/lone pair?

Thanks :smile:

It's 3D if it's 4 or more bonds. So with CH4 it's tetrahedral and has 4 bonds so one of the bonds go behind the molecule...

I would recommend using knockhardy notes, they have some great PowerPoints on shapes of molecules. Good luck! :smile:

Shapes of Molecules + Ions - Help!

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