Shapes of Molecules Flashcards
What are bond angles?
- The angle between any two bonds on a molecule (not between lone pairs.)
Why do molecules have specific shapes with specific bond angles?
- Bonds repel each other equally (without any lone pairs) because they all contain negative electrons that repel one another (want to be as far apart as possible.)
Why do lone pairs of electrons repel more strongly than bonding pairs of electrons?
- Because they are more compact.
Give the order of repulsion between bonding pairs/ lone pairs in a molecule.
lp- lp > bp -lp > bp -bp
Bonding pairs and lone pairs are …… …….. that repel each other.
- Charge.
- Clouds.
How do pairs of electrons in outer shells of atoms arrange themselves?
- Pairs of electrons in outer shells of atoms arrange themselves as far apart as possible to minimise repulsion.
What is the shape of a molecule determined by?
- Determined by the number of electron pairs on the central atom (central atom TENDS to be first atom in the formula of molecule - but, this isn’t always the case.)
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What do lone pairs do to the shape of molecules?
- Lone pairs push bonding pairs closer together (repel the bonding pairs away from itself strongly.)
- GENERALLY (not all the time), for every lone pair you add to molecule, you reduce the remaining bond angles by 2.5 degrees.
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If you are asked to determine shape of molecule that is an ion, what would you do?
- Add/ remove electrons FROM THE CENTRAL ATOM and then continue with your drawing.
What is the shape of molecule that has 2bp, no lone pairs. What is the bond angle? Give example of molecule with this shape.
- SHAPE: Linear.
- Bond angle: 180⁰
- BeCl₂: any central atom that has 2 electrons on outer shell (no lone pairs!!)/ bonded to group 7 atoms/ hydrogen (to donate 1 electron.)
What is the shape of molecule that has 3bp, no lone pairs. What is the bond angle? Give example of molecule with this shape?
- Trigonal planar (flat molecule.)
- 120⁰
- Example: BF₃/ any central atom that has 3 electrons on outer shell (no lone pairs!) / bonded to group 7 atoms/ hydrogen (to donate one electron.)
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What is the shape of molecule that has 4bp, no lone pairs. What is the bond angle? Give an example of molecule with this shape?
- Tetrahedral: 3D shape.
- 109.5⁰
- Example: CH₄/ any central atom that has 4 electrons on outer shell (no lone pairs!) - bonded to either group 7 atoms/ hydrogen (to donate 1 electron.)
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What is structure of tetrehedral molecule?
- 3D structure.
- 1 bond coming towards you and one bond going away from you.
- Atom/ bond coming towards you represented by wedges.
- Atom/ bond going away from you represented by dotted line.
If we see a wedge/ dotted line on a shapes drawing, what does this mean?
- Molecule is 3D.
- Wedge: part of molecule coming towards you.
- Dotted line: part of molecule going away from you.
What is the shape of molecule that has 5bp, no lone pairs. What are the bond angles?
- Trigonal bypyrimidal - one wedged line and one dotted line.
- 90⁰ and 120⁰ (trigonal planar set up is in horizontal plane with vertical axis in vertical plane. Any vertical bond will hit ANY horizontal bond at 90⁰/ angle between bonds in trigonal planar will be 120.)
What is the shape of molecule that has 6bp, no lone pairs. What is the bond angle?
- Octahedral (two wedged lines/ two dotted lines.)
- Bond angle = 90⁰ between ALL bonds.
What is shape of molecule with 2bps and 1lp of electrons? What is the bond angle? What shape is this shape BASED ON? Give example of molecule with this shape?
- Based on trigonal planar (because, we have 3 electron pairs.)
- BOND ANGLE: 117.5⁰
- SHAPE: V- shaped.
- Example: CH₂/ molecule that has central atom with 4 electrons on outer shell w/2 electrons bonded to halogen/ hydrogen atoms (to donate 1 electron.)
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What is shape of molecule with 3bps and 1lp of electrons? What is the bond angle? What shape is this shape BASED ON?Give example of molecule with this shape?
- Based on tetrehedral (because we have 4 electron pairs.)
- BOND ANGLE: 107⁰
- SHAPE: Pyramidal
- Example: NH₃/ molecule that has central atom with 5 electrons on outer shell w/3 electrons bonded to halogen/ hydrogen atoms (to donate 1 electron.)
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What is shape of molecule with 2bps and 2lps of electrons? What is the bond angle? What shape is this molecule based on? Give example of molecule with this shape?
- SHAPE: V- shaped (based on tetrehedral.)
- Bond angle: 104.5⁰
- Example: H₂O molecule that has central atom with 6 electrons on outer shell w/2 electrons bonded to halogen/ hydrogen atoms (to donate 1 electron.)
What is shape of molecule with 3bp and 2lps of electrons? What is this shape based on? What is the bond angle? Why does the bond angle remain unchanged?
- SHAPE: Trigonal Planar (with one wedged/ one dashed line.)
- Based on trigonal bipyrimidal.
- BOND ANGLE: 120⁰
- Bond angles remain same because the two lone pairs repel equally from opposite sides (same repulsion upwards as downwards) so, no bps are brought closer together.
What is shape of molecule with 4bps and 2lp of electrons? What is the bond angle? What shape is this shape BASED ON? Why do the bond angles remain the same?
- Based on octahedral (due to having 6 pairs of electrons.)
- SHAPE: Square planar
- ANGLE: 90⁰
- Bond angles remain same because the two lone pairs repel equally from opposite sides (same repulsion upwards as downwards) so, no bps are brought closer together.
Give an example of molecule with trigonal bipyrimidal shape (what properties do molecules with this shape have to have?)
- Example: PCl₅/ any central atom that has 5 electrons on outer shell (no lone pairs!) - bonded to either group 7 atoms/ hydrogen (to donate 1 electron.)
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Give an example of molecule with octehedral shape (what properties do molecules with this shape have to have?)
-Example: SF₆ / any central atom that has 6 electrons on outer shell (no lone pairs!) - bonded to either group 7 atoms/ hydrogen (to donate 1 electron.)
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Give an example of molecule with 3bp 2lp (trigonal planar)?
Example: ClF₃: any central atom that has 7 electrons on outer shell (no lone pairs!) - w/ 3 electrons bonded to either group 7 atoms/ hydrogen (to donate 1 electron.)
Give example of molecule with square planar shape.
- XeF₄
- Any central atom that has 8 electrons on outer shell (no lone pairs!) - bonded to either group 7 atoms/ hydrogen (to donate 1 electron.)
What is shape of molecule with 4bps and 1lp of electrons? What is the bond angle? What shape is this shape BASED ON?
- SHAPE: see- saw.
- 117.5⁰ and 90⁰.
- Wherever bond in horizontal plane meets bond in vertical plane angle = 90⁰. The angle between the bonds in same plane as lone pair = 117.5⁰.
- BASED ON: Trigonal bipyrimidal.
Give example of molecule with see-saw shape.
Example: SF₄/ any central atom that has 6 electrons on outer shell w/ 4 electrons bonded to either hydrogen/ halogen atoms.
What is shape of molecule with 5bps and 1lp of electrons? What is the bond angle? What shape is this shape BASED ON?
- SHAPE: Square pyrimidal.
- Bond angle: 87.5⁰ and 90⁰.
- Wherever bonds in vertical plane hit bond in horizontal plane, angle will always be 87.5⁰
- Two angles in the horizontal plane: 90⁰.
- Shape is based on octahedral.
(refer to diagram on sheet of paper in pink folder/ on wall if needed)
Give example of molecule with square pyrimidal shape.
Example: BrF₅/ any central atom that has 7 electrons on outer shell w/ 5 electrons bonded to either hydrogen/ halogen atoms
What are the steps to doing difficult dot- cross diagrams (where the central atom MUST have full outer shell ie. CO₂/ SO₂/ SO₃?)
- Fill outer atoms with how many electrons need to be added to them.
- Then fill central atom will how many electrons it needs added.
- Ensure that each atom has 8 electrons.
If you have a double bond in a molecule, does this act as 2 bonds when deducing the shape of the molecule? How about dative covalent bonds?
Give example of where concept of double bonds applies.
- No, acts as a single bond when deducing the shape (same with dative covalent bonds.)
- Ie. CO₂ has two double bonds but CO₂ is linear because the double bonds act as single bonds.
What are 3 molecules that must have full outer shell of electrons on central atom when doing the dot- cross diagram?
- SO₃
- SO₂
- CO₂
What is the shape of SO₂ molecule? Bond angle?
- First, draw dot-cross diagram (refer to Gautum notes for correct diagram.)
- SO₂ has one double bond/ one dative covalent bond/ there is lone pair on sulphur atom.
- 2bp + 1lp = V- shaped BOND ANGLE: 117.5⁰
What is the shape of CO₂ molecule? Bond angle?
- First, draw dot-cross diagram (refer to Gautum notes for correct diagram.)
- CO₂ has 2 double bonds (no lone pairs.)
- 2bp = linear 180⁰
What is the shape of SO₃ molecule? Bond angle?
- First, draw dot-cross diagram (refer to Gautum notes for correct diagram.)
- SO₃ has one double bond and 2 dative covalent bonds (no lone pairs.)
- 3bps: Trigonal planar. 120⁰
What is shape of molecule with 2bp 3lp’s/ why? What is the bond angle?
- Linear. The lone pairs are symmetrical so all cancel out leaving two bonds (linear.)
- Bond angle = 180 degrees.
If you have 1 bonding pair (regardless of how many lone pairs you have) what must the shape be?
- Shape must be linear, regardless of no of lone pairs you have.
