Shapes of Molecules

The shape of a molecule is related to the

number of outer electron pairs

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shapes of Molecules

 

  • linear
  • No. of outer electrons pairs: 2
  • Bond angle: 1800
  • Example: BeCl2
  • trigonal planar
  • No. of outer electrons pairs: 3
  • Bond angle: 1200
  • Example: BF2, BCl3
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  • tetrahedral
  • No. of outer electrons pairs: 4
  • Bond angle: 1090 and 270
  • Example: CH4
  • trigonla bipyramidal
  • No. of outer electrons pairs: 5
  • Bond angle: 1200 and 900
  • Example: PF5, PCl5
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  • octahedral
  • No. of outer electrons pairs: 6
  • Bond angle: 900
  • Example: SF6
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Distorted shapes due the presence of lone pairs

Ammonia

This molecule has three bonding pairs of electrons and one lone pair. Although, based on the tetrahedral shape, due to the extra repulsion from the presence of a lone pair, the 3 N-H bonds bend further away from the lone pair, in order to minimise the repulsion.

 shape is called pyramidal and has a bond angle of 1070.


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Water

This molecule consists of two lone pairs and two bonding pairs. Again, these repel each other towards the corners of the tetrahedron, due to the extra repulsion from the presence of two lone pairs. This shape is described asbent and has a bond angle of 1040.

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Particle

Relative Charge

Relative Mass

Electron

-1

1/1820 or negligible

Proton

+1

1

Neutron

0

1

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What differences are there between 12C and
14C?

 12C has 6 protons, 6 neutrons and 6 electrons. 14C
has 6 protons, 6 electrons and 8 neutrons. The difference between this pair
is that 14C has two more neutrons than 12C.

 Define the term 'isotope'.

SAME NUMBER OF PROTPNS AND ELOCTRONS DIFFERENT NUMBER OF NUETRONS 

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 The first ionisation energy for lithium is 519 kJmol-1.

a) Li(g) - e- = Li+(g)

b) The second ionisation energy for lithium is 7300 kJmol-1.
Explain the difference in value for the first and second ionisation energy for
lithium.

b) To remove the second electron for the second ionisation energy requires
considerably more energy as this electron comes from the 1s shell which is closer
to the nucleus and not subject to any shielding.

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CHEMICAL BONDING

 

Ionic bonding

This involves the transfer of electrons from metal atoms to a non-metal atom to form charged ions.

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Covalent bonding

A covalent bond is made when atoms share one or more electrons to form a molecule

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Co-ordinate or dative covalent bonding:

In a normal covalent bond, each atom donates one electron to the shared pair. In a co-ordinate bond electrons come from the same atom.

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