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ELECTRON-PAIR REPULSION THEORY
· The shape of a molecule is determined by the number of electrons in the outer shell
of the central atom.
· When bonding occurs, electrons can form bonding pairs or lone pairs can occur.
· Electrons all have a negative charge to each electron pair repels each other.
Electron Pairs in order of decreasing repulsion:
Lone pair -> Lone pair
Lone pair -> Bonded pair
Bonded pair -> Bonded pair…read more

Slide 3

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SHAPE AND BOND ANGLES
Name Example Bonding Pairs Lone Pairs Angles Shape
Linear CO, BeCl2 2 0 180
Non-linear H2O 2 2 104.5
Bent
Trigonal BCl3 3 0 120
Planar
Trigonal NH3 3 2 107
Pyramidal
Tetrahedral CH4 4 0 109.5…read more

Slide 4

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HOW TO WORK OUT THE SHAPE OF A
MOLECULE (XYN)
Step 1 -> Find the number of bonding pairs of electrons in the molecule
(N)
Step 2 -> Find the number of valence (outer shell) electrons in the
central atom ­ X
Step 3 -> Find the number of lone pairs using this method
X-N
2
Step 4 -> Distribute electron pairs around the central atom ­ start with
lone pairs and put them at the top then distribute bonded pairs
around the rest of the atom as lone pairs repel stronger.…read more

Slide 5

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WORKED EXAMPLE FOR PH3
Step 1 -> Find number of bonding pairs of electrons
3
Step 2 -> Find the number of valence electrons
5
Step 3 -> Find number of lone pairs
5-3 1
2
Step 4 -> Draw diagram
Lone pairs first Bonded Pairs…read more

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MOLECULES WITH DOUBLE BONDS
· A double bond has four electrons as two bonded pairs all in the same
bond.
· Each double bond is treated as a bonded region in the same way as a
bonded pair.
For example in CO2
OO
O
O
X X
O O C O O
X X
O O
O 180°
O
OO…read more

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