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 diagonal1

Shell (orbit)

s-sub-shell

p-sub-shell

d-sub-shell

Total max electrons

n = 1 Max 2 e- 2

n = 2 Max 2 e- Max 6 e- 8

n = 3 Max 2 e- Max 6 e- Max 10 e- 18

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f etc

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Rules for Lewis Structures

1. Arrange the atoms. – Place the least electronegative atom (not H) in the

centre.

2. Count the total number of valence electrons. – Remember to add or subtract e- for anions and

cations.

3. Allocate two electrons between each pair of atoms which are assumed to be covalently bonded.

4. Use remaining valence electrons to form lone pairs.– Start with the surrounding atoms (centre atom last).

5. 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).

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

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Hydrides – CH4

1. C is at the centre2. Total number of valence electrons =

4 (C) + 4×1 (H) = 8

3. Four C-H bonds require 4×2 electrons:

4. Electrons remaining = 8 (valence) – 8 (bonding) = 05. Carbon has octet:

4×2 electrons (in bonds): octet

H

CH H

H

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Hydrides – NH3

1. N is at the centre2. Total number of valence electrons

• 5 (N) + 3×1 (H) = 8

3. Three N-H bonds require 3×2 electrons:

4. Electrons remaining = 8 (valence) – 6 (bonding) = 2• Place lone pair on nitrogen

5. Nitrogen has octet:• 3×2 electrons (in bonds) + 2 electrons (lone pair)

H N

H

H

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Hydrides – NH4+

1. N is at the centre2. Total number of valence electrons

• 5 (N) + 4×1 (H) -1 (positive charge) = 8

3. Four N-H bonds require 4×2 electrons:

4. Electrons remaining = 8 (valence) – 8 (bonding) = 05. Nitrogen has octet:

• 4×2 electrons (in bonds)

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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

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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

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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

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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

bond length / pm bond energy / kJ mol-

1

C-C 150 376

C=C 133 720

C C 120 962

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Electrons in Bonds• In single bonds:

– The pair of electrons orbit directly between the two atoms.– This called a σ (“sigma”) bond

σ bond

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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 σ bond

a p bond

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

• 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…)

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Learning Outcomes:

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Questions to complete for next lecture:

1. Draw the Lewis structure of the following hydrides:

(a) SiH4

(b) H2S

(c) HCl

(d) H3O+

(e) BH3

2. Draw the Lewis structure of the following organic molecules:

(a) CH3NH2

(b) (CH3)2NH

(c) (CH3)3N

(d) (CH3)4N+