1. Lewis acids and bases

2. Draw the bonding in a hydronium ion
Notice that another name for a dative bond is “coordinate bond”

3. In this reaction the H+ ion acts as an acid
The H2O acts as a base (it accepts a proton)
A dative bond is formed
It could be considered that the base (water) donates an electron pair to the acid (proton)
This is the definition that is used for a Lewis acid or base

4. Lewis acid = a substance that accepts an electron pair
Lewis base = a substance that donates an electron pair
This definition actually applies to ALL acids and bases.
Arrhenius and Bronsted- Lowry are just special cases!

6. The oxygen donates an electron pair to the proton of the hydrochloric acid

7. The big advantage of this definition of acids and bases is that we can extend our understanding to molecules which don’t contain a proton!
The classic example of a Lewis acid is boron trifluoride, BF3
Draw a Lewis diagram for BF3
The boron only has 6 electrons in its outer shell so it can accept a pair of electrons

8. The boron trifluoride can react with ammonia.

9. A dative bond is formed when the lone pair of electrons on the nitrogen atom form a dative bond by completing the outer shell of the boron

10. Show the bonding that occurs when BF3 reacts with F- Show the shape of the resulting molecule

11. Another common example is AlCl3
This behaves in a similar way to BF3, but in addition it forms a dimer

12. Other good examples involve metal ions.
HL students may remember this from transition metal complexes!

13. Lewis theory can also explain the AMPHOTERIC nature of some metal hydroxides.
Al(OH)3(s) + 3 H+ --> Al H2O
Here Al(OH)3 is a Brønsted-Lowry base.
Al(OH)3(s) OH- --> Al(OH)4-
Here Al(OH)3 is a Lewis acid. (it accepts a pair of protons from the hydroxide ion)

14. Question
For each of the following species, state whether it is more likely to behave as a Lewis acid or a Lewis base. Explain your answer

15. PH3
BCl3
H2S
SF4
Cu2+

16. PH3
P has 5 electrons in its outer shell (2,8,5)
3 of these are used in making bonds to Hydrogen
The remaining 2 electrons are a lone pair
Thus it is a Lewis base

17. BCl3
B has 3 electrons in its outer shell (2,3)
All of these are used in bonding
It still only has 6 electrons in the outer shell
So it has room to accept another pair of electrons
It is a Lewis acid

18. H2S
S has 6 electrons in outer shell
Uses 2 of these in bonding
Still has lone pair electrons left
It is a Lewis base

19. SF4
S still has a lone pair of electrons
It is still a base

20. Cu2+
Easy!
It can accept a pair of electrons
So it is a Lewis acid