VSEPR Theory and molecular geometry Flashcards
(25 cards)
VSEPR theory - Valence Shell Electron Pair Repulsion
* The idea is that electrons are negatively charged, and whether we have bonding or non bonding electrons around some central atom, they want to spread out as far apart as you can because they’re repelled by eachother. The further they are apart they less repelled by eachother they are, which keeps their energy down. Which is part of why they do this. Lower energy again is assocaited w/ stability
We look at groups of electrons around an atom (called electron domains)
The below are electron domain geometries
So with 2 electron domains they are 180 degrees apart, and try to spread out as far away from eachother as possible
* The “bond angle” (from bond to bond) is 180 degrees
* This is considered linear
* so with 2 electron domains its going to be linear
3 electron domains:
* Planar structure (its like in the plane of the board, not 3D)
* 120 degrees apart
* bond angle = 120 degrees
* You could draw a triangle around it, which is why its called Trigonal, and its planar because its in a single plane
* So this is triginal planar
4 electron domains:
* This is where it gets a little tricky because its not 2D, its 3D
* if it was 2D they’d be 90 degrees apart, however, when making it 3D we can make the electrons be even further apart, meaning they’ll be less repelled by eachother, meaning they can be in a lower E state.
* wedge bond = filled in = atom is coming out of the board towards you
* Dashed bond = something going into the board away from you
* So this basically forms a pyramid w/ the A being at the top of the pyramid.
* Its called tetrahedral, because pyramids are tetrahedrens
* the bond angle is 109.5 degrees (notice how this is more than if we drew it 2D, thats why this must be 3D)
We can also do 5/6 electron domains, but these deal w/ an expanded octet (because that center atom is bound 5-6 times) - not everyone needs to know these
5 electron domains
* basically its 1 pyramid w/ and inverted pyramid under under it. So basically it looks like a diamond
* Called trigonal Bypryamidal - makes since, its two triangle based pryamids (theres a triagnoal in the middle that both branch off of)
* bond angles not all the same
* bond angle in the triangle in the horizontal plane = 120 degrees
* in the vertical plane its 90 degrees
* however picking the two opposite in the vertical plane = 180 degrees
6 electron domains
* so these form a sqaure at their base thats in the horizontal plane - with a pyramid going up and one going down (each pyramid has 4 faces, 8 total)
* Called octahedral
* so don’t get fooled by the word octa, it has 6 electron domaind, but the pryamids have 8 faces
* any 2 that are next to each other = bond angle of 90
* any 2 across from eachother = bond angle of 180
remember all of the above is electrons surrounding a central atom, not other elements
drawing electron domain geometries
The below shows that w/ 2 electron domains you electron geometry will be linear, and also your molecular geometry will be linear (meaning they’re in a straight line)
* this is from the central atoms perspective
* it doesnt matter how many bonds it makes. as long as its on one side its considered 1 electron domain
for molecular geometry its not about the bond # its about the # of atoms its bound to to determine its geometry.
single bonds, double bonds, tripple bonds all count as 1 electron domaind, shown below
so again for molecular geometry its really about the # of atoms its bound to, not the # of bonds its making that determines its moelcular geometry.
this is 3 electron domains = Trigonal planar (because it looks like a triangle and its in 1 plane
all of the below have 3 electron domains
The last one has 2 sets of double bonds and 1 lone pair = 3 electron domains
* so bonds on 1 side = 1 electron domain
* lone pairs = electron domain
For the molecular geometry we find that all 3 are bound to the 3 atoms
* the first 2 = Trigonal planar (same as the electron geometry name) - this is because they have bonds instead of lone pairs
* However, when the central atom has lone pairs (non bonding electron pair)
NOTE: for the middle one below when drawing the molecular geometry that double bond actually repells the other 2 bonds slightly more (because its 4 electrons there instead of 2), meaning instead of having 120 degress between them, the H’s will be slightly closer than 120 degrees and the distance between H and O (on both sides) will be slightly more than 120
Lone pairs lead to greater repulsion than bonding electrons, meaning in the third picture that distance between S and O is greater than 120, and between the two O’s is < 120
So this third picture can’t be called trigonal planar, we don’t even form a triangle in the same way as we did w/ the others.
When you only have 3 atoms in your structure you’ve only got 2 options
* its either going to be all staright in a row (linear)
* Or they’re not going to be in a row, and were going to call it bent (thats what we have below in the third picture)
* NOTE: this only happens in molecular geometry. We look at where the atoms line up, and the 3 atoms line up in a bent pattern. the 3 atoms are def not linear so they’re bent.
4 electron domains
So in the first one we see all bonding electron domains = tetrahedral
because they’re all bonding domains the molecular geometry takes on the same name
In the second picture there are still 4 electron domains, so the electron geometry remains tetrahedral
* however it does change the name for the molecular geometry because its got a lone pair
* so you still have the trigonal on the bottom, however to connect to that lone pair ontop they need to be slightly elevated making a pyramid. So the molecular geometry here is called trigonal pyramidal. Its kind of a flattened pyramid
* remember the electron pair gives more repulsion to the bond electrons, which is why its a flattened pyramid
NOTE: each lone pair counts as 1 electron domain. so water has 4
You might get a question like: Which of these pictures has a bond angle less than 109.5. Well you would have to know tetrahedrals have a bond angle of 109.5. However, when you add lone pairs it will decrease the bond angles between bonding electrons making them have a bond angle of less than 109.5
* when you put lone pairs on the central atom it will slightly lower the bond angles (note the bond angles are the angles between the bonding electrons, not the lone pair)
This is a good representation of a tetrahedral bond angle
5 electron domains
The first one gets the name trigonal bipryamidal because they’re all bonding electrons (mathcing the electron geometry name)
For the first one:
* The ones that point straight up and down are labled as axial
* The two axial have a bond angle of 180 degrees
* the ones that make up the triangle portion are refered to as equatoraial because they go around the equator of the molecule
For the second picture
* Where the lone pairs go actually matter because we have different bond angle sin this molecule
* They go in these equatorial positions (make up that triangle) so we can minimize the repulsions - so they dont adpot the axial positions
* If you turn the molecule sideways, it looks like seesaw or sawhorse
Picture 3
* T shaped because if you turn it on its side it makes the letter T
Picture 4:
* All 3 lone pairs are in equatorial positions. Leaving just 3 total atoms. With 3 atoms they’re either in a linear position or a bent position.However, since those F atoms really are 180 degrees apart in that equatorial position its actaully considered linear
* basically the lone pairs go in the equatorial position so they have fewer neighbors at 90 degrees. The axial ones had 3 90 degree neighbors, while the equatoral ones only have 2 90 degree neighbors and the ones in the same plane as it are 109.5 degrees apart - this is why electrons take over the equatorial portion here.
6 electron domains
The first one has 6 bonding pairs so is called octahedral
* the first one doesnt have axial and equatorial because all the positions are equivilent, meaning you could turn it on its side and it would look the exact same
adding lone pairs can be put anywhere because they are the same distance apart
Second picture:
* its a square base pyramid, so the electron geometry is called square pyramidal
* remember those lone pairs experience the greatest repulsion, so you want to make sure you put the lone pairs 180 degrees apart when adding them. Doesnt matter for this picture but matters for the next
* Note you could put those lone pairs anywhere (could choose elft and R) just need to make sure they’re opposite one another and 180 degrees apart
3rd picture
* biggest thing to remember is that those lone paris need to be 180 degrees apart
* Called square planar because for the atoms in this they’re all in a single plane (not the lone pairs though). and they’re in a square
Will need to be able to look at a lewis structure and count the electron domains, which should tell me the electron domain geometry and then I should be able to decide how many are bonding vs nonbonding to get the correct electron goemetry
They could just give me the chemical formula (something like XeF4) and have me draw the lewis structure and then figure out the molecular geometry / electron domain
Its tetrahedral.
* the molecular geomtry name matches the electron geometry name because they’re all bonding electrons (it changes when you have lone pairs)
NOTE: its called tetrahedral because it has 4 faces when you draw it in
Trigonal pyramidal
NOTE: the bond angle (distance between specifically the bonding electrons, not the lone pairs and bonds) is <109.5 because the lone pairs repell more than the bonding electrons, meaning they push all the bonding electrons away from it, and thereby closer to one another.
Called tirgonal pyramidal because it makes a triangle on the bottom, and the lone pairs ontop make it a pyramid when connected
The geometries match because its all bonding electrons
= octahedral
* called this because there are 8 faces total. It makes 2 pyramids that are attached at the base from a square = 8 faces
notice that it does actually matter where you palce the electrons on these
* they want to be in the equatorial ones and not the axis because in the equator ones they only have 2 neighbors 90 degrees away and the rest are 109.5 degrees away. However, the axial ones have 3 neighbors at 90 degrees away, making the more repulsive lone pairs not want to be there
* Remember bonds repell less than lone pairs. Lone pairs want to be away from everyone more than bonds.
remember lewis structures are always planar or 2D
* single plane
Electron/molecualr goemetry = Trigonal bipyramidal. Makes since they make a triangle and it forms 2 pyramids.
* they’re the same because its all bonding electrons and no lone pairs.
Why the lone pairs end up together:
* the molecule cant put the two lone pairs any “farther away” than they already are - they’re aready in the best 3D arrangement to minimize repulsion
* So even though lone pairs repel more strongly, they still occupy a position in the tetrahedral shape.
* Lone pairs push the bonding pairs closer together, because
1) lone pairs spread out more than bonding pairs
2) So they create greater repulsion, squeezing the bonding pairs closer
* however, the bond angle between the lone pairs > than the angle between the bonded electrons
the electrons are slightly further spaced than the bonded ones
remember electrons must go on the equator becuase it only gives them 2 neighbors that are 90 degreees away, opposed to the axial ones that have 3 that are 90 degrees away (closer than the 109.5).
This one is T shaped, when finding the T shape only look at the molecules. The 3 peripheral molecules will make a T shape
Electron geometry = octahedral
* this is because it forms 2 pyramids, sitting base to base. from a square orientation. Meaning each pyramid has 4 faces, for 8 total
Molecular geometry = square planar
* remember, the electrons must be as far away from each other as possible. So you can have them 180 degrees apart here. Doesnt matter where you put the first one, but the second must be opposite it.
* its square because the all the atoms are in a square and planar because all the atoms are in the same plane.
* note becuase the electrons are on opposite sides and repell evenly the bond angles between bonds are 90 degrees, while its 180 degrees between electrons
So across has more electronegativity than down. Meaning that oxygen has mor electornegativity than Sulfur (loses electronegativity faster when you go down than when you go from R –> L)
Its Trigonal pyramidal, however it only has electrons on one side of it meaning it keeps its pyramid formation, and the bottom is a triangle, meaning its trigonal pyramidal
You can see that its Trigonal planar so it must be 120 degrees apart
This is a little tricky. The lewis structure makes this look like 90 degree bond angles. However, its a 3D structure meaning that they’re 109.5 degrees apart
This is tetrahedral
answer was 120, realistically its less than 120 because those elctrons will push the bonds closer to eachother.
slightly less than 109.5 because the electron pair pushes htem all together slightly
so be careful on this one it has 3 bond angles. 180 between axial, 120 between equator ones, and 90 between equator and axial
