Shapes of Molecules

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Working out the shape

  • Number of electrons in outershell(group number) + Number of atoms bonded to central atom
  • If there is a charge..

Negative charges (e.g. -1 = Add electrons (e.g. +2)

Positive charge  (e.g. +2) = Minus the charge (e.g. -2)

  • Divde by 2 
  • Minus the group number by the number of bonded pairs (the last 2/3 steps)
  • If the answer is not 0, there are lone pairs

e.g. 4-3=1 So there is 3 bonding pairs and 1 lone pair

The electron pairs then have to be arranged in a way which allows the least repulsion.

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Working out the shape- Example

Shape of ClF3

Group number of central atom (Cl)=7

Number of bonded atoms(F)=3

7+3=10

10 divided by 2= 5

7-5=2

3 bonded atoms

2 lone pairs

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Linear

(http://www.chemguide.co.uk/atoms/bonding/shapebecl2.GIF)

2 Bonded atoms

Angle= 180

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Trigonal Planar

(http://www.chemguide.co.uk/atoms/bonding/shapebf3.GIF)

3 Bonded pairs

Bond Angle= 120

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Tetrahedral

(http://www.chemguide.co.uk/atoms/bonding/shapech4.GIF)

4 Bonded pairs

Bond Angle= 109.5

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Trigonal Bipyramidal

(http://www.chemguide.co.uk/atoms/bonding/shapepf5.GIF)

5 Bonded pairs

Bond angles= 90 & 120

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Octahedral

(http://www.chemguide.co.uk/atoms/bonding/shapesf6.GIF)

6 bonded pairs

Bond angle= 90

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Ammonia

(http://www.chemguide.co.uk/atoms/bonding/shapenh3.GIF)

Ammonia has 3 bonded atoms and a lone pair of electrons. Although it takes the shape of a tetrahdral, it's bond angles are slightly different.

The lone pair repels the hydrogens more than the hydrogens repel each other,

This causes the shape of the molecule to be "squeezed" inwards.

Due to the lone pairs repulsion,

the bond angle in ammonia is 107.

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Water

(http://www.chemguide.co.uk/atoms/bonding/shapeh2o.GIF)

Water also has a different shape due to its lone pairs.

As it has 4 bonded pairs (2 atoms, 2 lone pairs) it is in the shape of a tetrahedral

However, the bond angle is reduced due to the lone pairs repulsion

The bond angle in H2O is 104

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Ammonium Ion

(http://www.chemguide.co.uk/atoms/bonding/shapenh4.GIF)

Nitrogen has 5 outer electrons + 4 from the hydrogens making 9.

However, the ammoniums positive charge shows an electron has been lost so the electron number is 8. Divided by 2 gets the answer of 4 as there are 4 bonded atoms.

This makes the ammonium ion tetrahedral (so has a bond angle of 109.5)

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The Hydroxonium Ion

(http://www.chemguide.co.uk/atoms/bonding/shapeh3o.GIF)

Oxygen has 6 outer electrons, plus 3 from the hydrogens, minus 1 due to the postive charge,

This gives 8, divided by 2= 4.

So there are 3 bonded atoms and one lone pair.

As there are 4 bonded pairs of electron,

the shape is a tetrahedral with a bond angle of 107.

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ZeF4

(http://www.chemguide.co.uk/atoms/bonding/shapexef4.GIF)

Square Planar

with bond angles of 90

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