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Recap Lewis Structures

Recap Lewis Structures. Arrange the atoms. Count the total number of valence electrons. Allocate two electrons between each pair of atoms which are assumed to be covalently bonded. Use remaining valence electrons to form lone pairs. Check if the central atom has an octet (or more).

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Recap Lewis Structures

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  1. Recap Lewis Structures • Arrange the atoms. • Count the total number of valence electrons. • Allocate two electrons between each pair of atoms which are assumed to be covalently bonded. • Use remaining valence electrons to form lone pairs. • Check if the central atom has an octet (or more). Example: OH-

  2. Molecules with multiple bonds O2 Total number of valence electrons: 2 x 6 = 12 Each oxygen has 8 electrons. N2Total number of valence electrons: 2 x 5 = 10 Each nitrogen has 8 electrons.

  3. Ozone O3 Total number of valence electrons: 3 x 6 = 18 Small rings are highly ‘strained’ and do not normally form. Try a different arrangement of oxygen atoms Another valid Lewis structure would be

  4. Resonance • When two or more Lewis structures are possible for a molecule it is said to exhibit resonance. • This is indicated by double headed arrows: 

  5. Resonance • This does not mean the molecule flips back and forth between the possible structures – the true structure is actually an average of the possible resonance structures. • Bondsare neither single nor double – they are intermediate between the two. They have intermediate length and strength.

  6. Resonance • Benzene: C6H6 Benzene is best described as being a resonance hybrid of these two bonding arrangements.

  7. Resonance • Benzene: C6H6 • Benzene is also about 140 kJ mol-1 more stable than predicted for 1,3,5-cyclohexatriene (the structure with localised double bonds).

  8. Resonance – polyatomic ions • Nitrate, NO3- Total number of valence electrons: 5 + (3 x 6) + 1 = 24

  9. NO3- Nitrate ion • All N-O bond lengths the same and intermediate between N-O and N=O • Negative charge distributed evenly across the three oxygen atoms (1/3 on each oxygen)

  10. Resonance If resonance occurs… • The actual structure is an average of all possible Lewis structures. • The actual structure is a single structure. • The actual structure is more stable than predicted from the Lewis structure. • There is no easy way of drawing the actual structure!

  11. Learning Outcomes: By the end of this lecture, you should: be able to construct Lewis structures of molecules and polyatomic ions containing C, N, O or F recognise situations where resonance occurs appreciate that resonance results in a structure that is an average of Lewis structures appreciate that resonance results in a more stable molecule then predicted by a single Lewis structure be able to complete the worksheet (if you haven’t already done so…)

  12. Questions to complete for next lecture: • Draw the Lewis structures of the following molecules and ions, including resonance structures where appropriate. • CO2 • CO • HNCO • NO2- • Which pair of structures are resonance structures?

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