Both modes have the same notes. But why do they sound different. How to approach while composing a song in a particular mode as opposed to a key. Is it the harmony or the pivoting root? I read few articles about the same, couldn't find a convincing explanation. Mind helping?
Listen to the scales first and use your ears to hear the difference. Play the C major scale and then the same scale starting and ending on the D note.
To imply a modal chord progression, using extensions on the chords is common to hear the modal tonality throughout the progression.
Here is an article on the modes and chord progressions.
As you say both contain the same notes, but the scales are different and this has a huge impact on the resulting experience.
If you review Dorian's scale on D, you will see a minor 3rd (
F) and a minor 7th (
C). On the other hand if you check Ionian (major) scale on
C, you will see a major 3rd (
E) and a major 7th (
B). These are the differences between these 2 scales, the 3rd being most important as it will set a sad mood when using minor 3rd (Dorian) or a happy mood when using a major 3rd (Ionian).
The important thing about scales is not the base note it's placed on, but the intervals from each note to the base note. For example, scales with a minor 2nd (like the Phrygian scale), that is half-tone up from the base note, will sound more aggresive than scales with a mayor 2nd (full tone from the base note). Scales with an augmented 4th (Lydian) or diminished 5th (Locrian) might sound a little "out of the box".
This being said, the chord progressions will mostly set the playground for your key. If your key is
C led with Ionian scale, you will probably set progressions to create tension (subdominants and dominants) and finally release when going back to C or other tonic notes (like Am). These same progressions won't work quite as well if you suddently decide to change your key to D with Dorian scale, because they were designed to revolve around C!
But why do they sound different
The difference is the tonal center. A song in D Dorian contains the same notes as a song in C major, but it's resting note is D (or Dm chord), not C, and this makes is sound totally different, because of what EzLo has already explained about scales and intervals.
How to approach while composing a song in a particular mode as opposed to a key
A general tip to composing in modes would be to avoid resolving to the major or minor corresponding key, because then it gets hard to return to the mode you were on. I've read you should avoid the 7th degree chord of the corresponding major scale too, because it pulls towards the major key and indeed sounds strange in modal scales.
Composing in a non-modal key I think you are more free, because you can modulate several times, borrow lots of chords, and in the end still go back to the original key and resolve. In modes you could easily lose the feel and it wouldn't sound modal anymore, so you must kind of "stick to it".
Is it the harmony or the pivoting root?
I would say.. both? The harmony depends on the pivoting root, as does the scale. Each mode has particular harmonies due to the interval relation to the pivoting root (as EzLo explained).