Full Idea
The two big ideas in chemistry are chemical periodicity and chemical bonding, and they are deeply interconnected.
Clarification
'Periodicity' is recurrent patterns across the periodic table
Gist of Idea
Periodicity and bonding are the two big ideas in chemistry
Source
Eric R. Scerri (The Periodic Table [2007], Intro 'Per')
Book Reference
Scerri,Eric R.: 'The Periodic Table' [OUP 2007], p.-9