Recap  Electron Configuration

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# Recap  Electron Configuration - PowerPoint PPT Presentation

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

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

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

σbond

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

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:
• 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+