OCR AS Chemistry F331: Shapes of Molecules

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Shapes of Molecules
The arrangement of the electrons in methane can be represented by a dot-cross diagram.
There are four groups of electrons around the central carbon atom ­ the four
covalent bonding pairs.
Because similar charges repel, these group of electrons arrange themselves so that they
are as far apart as possible.
Chemists have developed a system for drawing three-dimensional structures on two-
dimensional paper as shown below.
Bonds which lie in the plane of the paper are drawn as solid lines in the normal
way
Bonds which come forwards, towards you, are represented as wedge-shaped
lines
Bonds which go backwards, away from you, are represented as dashed lines.
A methane molecule
The type of electron pair affects how much it repels other electrons
- Lone-pair / lone-pair bond angles are the biggest
- Lone-pair / bonding-pair bond angles are the second biggest
- Bonding-pair / bonding-pair bond angles are the smallest
The term for this is: electron-pair repulsion principal

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Electron Pairs around a Central Atom
Linear molecules have two electron pairs surrounding a central atom.
Because there are fewer groups of electrons they can get further apart.
The furthest apart they can get is at an angle of 180.
Number of electron pairs Angle Shape
2 180: Linear
Cl Be Cl
The central atom of BeCl2 is Be.
It is surrounded by 2 bonding pairs of electrons.
They repel each other and move as far apart as possible.…read more

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Electron Pairs around a Central Atom
Planar molecules have three electron pairs surrounding a central atom.
The furthest apart they can get is at an angle of 120°.
Number of electron pairs Angle Shape
3 120: Planar
F F
B
F
The central atom of BF3 is B.
It is surrounded by three bonding pairs of electrons.
They repel each other and move as far apart as possible.
They form a planar shape with bond angles of 120°.…read more

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Electron Pairs around a Central Atom
Tetrahedral molecules have four electron pairs surrounding a central atom.
The furthest apart they can get is at an angle of 109°.
Number of electron pairs Angle Shape
4 109: Tetrahedral
H
C
H H
H
The central atom of CH4 is C.
It is surrounded by four pairs of bonding electrons.
They repel each other and move as far apart as possible.
They form a tetrahedral shape with bond angles of 109°.…read more

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Electron Pairs around a Central Atom
Bipyramidal molecules have five electron pairs surrounding a central atom.
The furthest apart they can get is at an angle of 120° or 90°.
Number of electron pairs Angle Shape
4 120: or 90° Bipyramidal
Cl
Cl
Cl P Cl
Cl
The central atom of PCl3 is P.
It is surrounded by four bonding pairs of electrons.
They repel each other and move as far apart as possible.…read more

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Electron Pairs around a Central Atom
Octahedral molecules have six electron pairs surrounding a central atom.
The furthest apart they can get is at an angle of 90°.
Number of electron pairs Angle Shape
6 90° Octahedral
F
F F
I
F F
F
The central atom of IF6 is I.
It is surrounded by six bonding pairs of electrons.
They repel each other and move as far apart as possible.
They form an octahedral shape with bond angles of 90°.…read more

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