What is a diatonic arpeggio?
Let’s first look at what a diatonic arpeggio is. If you have a scale like the C major scale: C D E F G A B C, you can build the diatonic 7th chords by stacking 3 thirds on top of each other. A diatonic third is essentially the 2nd note from the note you are on so for C the third above it is E, for D it is F etc.