Please note: there is a slight mistake at 2:13 - delocalised pi electrons are not always spread out over all the atoms in a molecule as they are in the carbonate ion. The text I have linked to below has a more detailed description on the delocalised pi electrons in the carbonate ion and benzene on page 366 and 367.
Source:
[ Ссылка ]
Link to worksheet:
[ Ссылка ]
