che00001che00005che00022 lecture 9

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Lewis Structures Dr Graeme Jones
LECTURE 9
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Contents of lecture

• What are Lewis structures?
• General rules for writing Lewis structures
• Examples
• Molecules
• Molecular cations and anions

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What are Lewis structures?

• Lewis structures are diagrams that show how
  electron pairs are shared between atoms in a
  molecule. They can explain the bonding in a
  molecule but not its shape.
• In lecture 8 we saw how to construct the Lewis
  structures of some diatomic molecules. When there
  are more than two atoms in a molecule, it is
  useful to have some rules, which help us to
  proceed. Jones and Atkins (4th ed.) pages 333-337
  give a useful account of this, from which some of
  the examples given in the lecture will be taken.

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General rules

• Count the valence electrons on each atom, add
  them up and find the number of electron pairs.
• Decide how the atoms are to be bonded, and use a
  pair of electrons for each bond.
• Distribute the remaining electron pairs so that
  each atom is surrounded by 4 pairs of electrons
  (the octet rule).

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Examples (a)

• NH3
• (i) valence electrons 5 3 x 1 8 gt 4 pairs.
• (ii) 3 pairs are used to link the atoms.
• (iii) 1 pair completes Nitrogens octet (a lone
  pair).
• HOBr
• (i) valence electrons 1 6 7 14.
• (ii) 2 pairs are used to link the atoms.
• (iii) Distribute the remaining pairs around O and
  Br.

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Examples (b)

• HCN
• Step (i) count the valence electrons on each
  atom 1 4 5 10 gt 5 electron pairs.
• Step (ii) use 2 pairs to form bonds between the
  atoms, leaving 3 pairs.
• Step (iii) arrange these in such a way that
  both C and N have an octet of electrons (this has
  to be achieved using multiple bonds).

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How to deal with ions

• If ions are charged, the number of electrons
  is/are increased or reduced accordingly
• H3O
• (i) 3 x 1 6 1 8 gt 4 pairs
• (ii) 3 pairs are used to link the atoms
• (iii) the 4th pair completes the O octet
• NH2-
• (i) 5 2 x 1 1 8 gt 4 pairs
• (ii) 2 pairs link the atoms
• (iii) 2 remaining pairs complete the N octet

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More complex examples

• The summary sheet mentions HCOOH,
  formic/methanoic acid, which is also in Jones and
  Atkins (ex. 8.4, p 335).
• In examples like HCOOH it helps to know the
  structure, at least at the level of which atom is
  joined to which, in advance!
• However it can be done entirely by following the
  rules as there will only ever be one structure
  which obeys them in full.

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HCOOH (i)

• (i) Count the valence electrons
• 2 x 1 4 2 x 6 18 gt 9 pairs
• (ii) Decide on possible ways in which the atoms
  could be joined up.
• (iii) Put pairs of electrons between the atoms
  and ensure that all the atoms have the correct
  number of electrons round them.

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HCOOH (ii)

• Having tried a number of possibilities, we
  finally arrive at the correct structure
• This structure has the correct number of pairs of
  electrons around each atom.
• Another example, SO3, will be done (if time
  permits)

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Valency

• Knowing the position of an element in the
  periodic table helps you to predict its valency
  the number of bonds it requires to make to ensure
  a full octet. This is useful to know when
  predicting the bonding in organic molecules.
• Valency of 1 H, F, Cl, Br
• Valency of 2 O, S
• Valency of 3 N, P
• Valency of 4 C, Si

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Examples

• Predict the bonding of the following molecules
  and draw Lewis structures
• CO2 N2H4 H2O2
• CCl4 Cl2CO POBr3
• O3 HNO2 ClO3-

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Useful websites but there are lots more! (type
Lewis structures into Google)

• (i) http//www.stolaf.edu/depts/chemistry/courses/
  toolkits/121/js/lewis/
• This site enables you to choose a molecule or ion
  from a drop-down menu and it draws the Lewis
  structure for you.
• (ii) http//www.ausetute.com.au/lewisstr.html
• A site explaining how to draw the Lewis
  structures of atoms and simple molecules.
• (iii) http//library.thinkquest.org/3659/structure
  s/lewis-drawing.html
• A site which explains the general rules with
  reference to a SO3 molecule.