Chord Scale analysis of the Ionian mode
The Ionian Mode is one of two purely major modes in the major scale (the mixolydian contains a major triad, but is referred to as a dominant mode, due to the flat seven, just in case you wanted to know). It generally takes either a Major Seven chord, or a Major Six chord. (more on that later) There is also an avoid note, the fourth.
Analysis of C Ionian
D (Major Nine)
E (Major Third)
F (Avoid note, or Scale four, or Scale Perfect fourth)
G (Perfect Fifth)
A (Major Sixth, or Major Thirteenth)
B (Major Seventh)
Major Six usage over Ionian
Okay, now go over to a piano and play B and C right next to each other. Go ahead. Okay, stop. Doesn't that sound bad? Okay, now bump the C up and octave. Okay, now stop that. I said STOP!. Now the reason for this exercise in sonic brutality: intervals of a minor second, specifically up an octave (the dreaded Flat nine) sound really bad unless used right, and unless you know what your doing, you shouldn't use them. Now, say you want more then a triad, but you have the root as melody note. My friend, you have overlooked the useful "Seventh Chord substitute, the Major Six chord! Okay, now play an A and a C together. Doesn't that sound much nicer, much more at rest?
The avoid note of the Ionian mode, the four, should be avoided (only in the voicing of chords, feel free to place them in your melody if you want, i mean, who am i to tell you what to do) for Two reasons.
1: The Four is half step away from the Three, and if you voice it as an eleven (dun dun duuuunnn) Flat Nine
2: The Ionian mode is a tonic mode, and by placing the four in the chord voicing, you change the sound of the chord from a tonic chord (a chord that is at rest, or not trying to resolve to anything) to that of a subdominant chord, which usually heads to dominant territory. Basically, by putting the fourth in the chord, you change the overall feel from Ionian to Lydian, and since the Ionian has a much different use then the Lydian, you lose in the harmony game.