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Recap – Electron Configuration. Shells contain sub-shells n = 1 has 1 s n = 2 has 2 s and 2 p n = 3 has 3 s , 3 p and 3 d n = 4 has 4 s , 4 p , 4 d and 4 f Sub-shells fill along diagonal. Rules for Lewis Structures.

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recap electron configuration

Recap – Electron Configuration

Shells contain sub-shells

n = 1 has 1s

n = 2 has 2s and 2p

n = 3 has 3s, 3p and 3d

n = 4 has 4s, 4p, 4d and 4f

Sub-shells fill along diagonal

rules for lewis structures
Rules for Lewis Structures

In Lecture 6 & 7 we looked at covalent bonding. This is formalised with the drawing of Lewis Structures.

  • Arrange the atoms.
    • Place the least electronegative atom (not H) in the centre.
  • Count the total number of valence electrons.
    • Remember to add or subtract e- for anions and cations.
  • Allocate two electrons between each pair of atoms which are assumed to be covalently bonded.
  • Use remaining valence electrons to form lone pairs.
    • Start with the surrounding atoms (centre atom last).
  • Check if the central atom has an octet (or more).
    • If not, move lone pairs from the (least electronegative) surrounding atoms into the bonding region (make double bonds).
hydrides ch 4
Hydrides – CH4
  • C is at the centre
  • Total number of valence electrons =

4 (C) + 4×1 (H) = 8

  • Four C-H bonds require 4×2 electrons:
  • Electrons remaining = 8 (valence) – 8 (bonding) = 0
  • Carbon has octet:

4×2 electrons (in bonds): octet

hydrides nh 3
Hydrides – NH3
  • N is at the centre
  • Total number of valence electrons
  • 5 (N) + 3×1 (H) = 8
  • Three N-H bonds require 3×2 electrons:
  • Electrons remaining = 8 (valence) – 6 (bonding) = 2
  • Place lone pair on nitrogen
  • Nitrogen has octet:
  • 3×2 electrons (in bonds) + 2 electrons (lone pair)
hydrides nh 4
Hydrides – NH4+
  • N is at the centre
  • Total number of valence electrons
  • 5 (N) + 4×1 (H) -1 (positive charge) = 8
  • Four N-H bonds require 4×2 electrons:
  • Electrons remaining = 8 (valence) – 8 (bonding) = 0
  • Nitrogen has octet:
  • 4×2 electrons (in bonds)
organic molecules
Organic Molecules
  • Carbon needs to make 4 bonds to achieve its octet.
  • There are very many carbon hydrides and these can contain C-C, C=C and C C bonds
organic molecules1
Organic Molecules
  • Organic molecules commonly also contain other elements such as oxygen and nitrogen
  • Oxygen (valency = 2) and nitrogen (valency = 3) can also make single or multiple bonds
organic molecules2
Organic Molecules
  • Organic molecules commonly also contain other elements such as oxygen and nitrogen
  • Oxygen (valency = 2) and nitrogen (valency = 3) can also make single or multiple bonds
bond lengths and energies
Bond Lengths and Energies
  • The length of a bond and energy is takes to break a bond depends on the type of bond
    • single bonds are longer and weaker than double bonds
    • double bonds are longer and weaker than triple bonds
electrons in bonds
Electrons in Bonds
  • In single bonds:
    • The pair of electrons orbit directly between the two atoms.
    • This called a σ (“sigma”) bond

σbond

electrons in bonds1
Electrons in Bonds
  • In double bonds:
    • The first bond is a σ bond
    • The second pair of electrons orbit above and below the σ bond
    • The second bond is called a p bond

a p bond

a σ bond

  • In triple bonds:
    • The first bond is a σ bond
    • The second and third bonds are p bonds
slide12

Learning Outcomes:

By the end of this lecture, you should:

be able to draw Lewis structures for hydrides, including ones with charges

be able to draw and understand Lewis structures for organic molecules containing C, H, N and O

be able to explain the relationship between the type of bonds (single, double and triple) and bond strength and length

be able to describe bonds as combinations of s and p bonds

be able to complete the worksheet (if you haven’t already done so…)

questions to complete for next lecture
Questions to complete for next lecture:
  • Draw the Lewis structure of the following hydrides:
    • SiH4
    • H2S
    • HCl
    • H3O+
    • BH3
  • Draw the Lewis structure of the following organic molecules:
    • CH3NH2
    • (CH3)2NH
    • (CH3)3N
    • (CH3)4N+