Recap – Electron Configuration

1. 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

2. 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).

3. 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

4. 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)

5. 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)

6. 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

7. 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

8. 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

9. 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

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

11. 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

12. 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…)

13. 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+