You can find the atoms lesson here: https://cognitoedu.org/coursesubtopic...
The Cognito platform just launched. It's totally free, and has been built from ground up to make learning and revision as easy as possible. Some of the main features include:
Automatic progress tracking of which lessons you've completed, and how well you did.
Quiz generation on any combination of subtopics you like.
Signature Cognito videos to teach you new concepts in the shortest possible time.
An extensive bank of past papers, sorted both by year, but also by topic.