As seen in the following chart, tonality gets "ruled out" as more notes are added.
Atonal music is nearly or completely chromatic. Scales, and the idea of scales, become irrelevant.
We seem to veering from the fact that we are talking about atonal music, which is music that is highly and continuously chromatic, and thus creates no sense of definite, sustained tonality.
Howard Hanson's Harmonic Materials of Modern Music shows:
If we begin with one note, and begin adding notes by fifths, we get the following:
2 notes (C-G): 1 fifth
3 notes (C-G-D): 2 fifths, 1 major second
4 notes (C-G-D-A): 3 fifths, 1 minor third, 2 major seconds
5 notes (C-G-D-A-E): 4 fifths, 1 major third, 2 minor thirds, 3 major seconds
6 notes (C-G-D-A-E-B): 5 fifths, 2 major thirds, 3 minor thirds, 4 major seconds, 1 minor second
7 notes (C-G-D-A-E-B-F#): 6 fifths, 3 major thirds, 4 minor thirds, 5 major seconds, 2 minor seconds, 1 tritone
8 notes (C-G-D-A-E-B-F#-C#): 7 fifths, 4 major thirds, 5 minor thirds, 6 major seconds, 4 minor seconds, 2 tritones
9 notes (C-G-D-A-E-B-F#-C#-G#): 8 fifths, 6 major thirds, 6 minor thirds, 7 major seconds, 6 minor seconds, 3 tritones
10 notes (C-G-D-A-E-B-F#-C#-G#-D#): 9 fifths, 8 major thirds, 8 minor thirds,8 major seconds, 8 minor seconds, 4 tritones
11 notes (C-G-D-A-E-B-F#-C#-G#-D#-A#): 10 fifths, 10 major thirds, 10 minor thirds, 10 major seconds, 10 minor seconds, 5 tritones
12 notes (C-G-D-A-E-B-F#-C#-G#-D#-A#-E#): 12 fifths, 12 major thirds, 12 minor thirds, 12 major seconds, 12 minor seconds, 6 tritones
Each new note adds one new interval, plus adding one more to those already present; but beyond seven tones, no new intervals can be added. In addition to this loss of new material, there is also a gradual decrease in the difference of the quantitative formation; i. e., redundancy begins to set in.
The sound of a sonority, whether it be harmony or melody, depends on what is present, but also on what is not present. The pentatonic sounds as it does because it contains mainly perfect fifths, and also maj seconds, minor thirds, and one major third, but also because it does not contain the minor second or tritone.
As sonorities get projected beyond the six-range, they tend to lose their individuality.
In other words, a sustained sense of tonality cannot be maintained. This does not mean that atonal, 12-tone, or serial music cannot have sonorities and harmonic meaning and color. It simply means that, when using all 12 notes continuously, no overall "gestalt" of harmony or sonority will be dominant. There will be no sustained harmonic consistency which will be continuously exerting influence and gravity towards one note, or even a larger tonal area. The effect is "spread out" evenly among the 12 notes, and any sense of tonality which one thinks one perceives is fleeting, and must be grasped from moment-to-moment, which is not really the way real Western tonality was intended to function. Here, with atonality, we have entered a world of "moment time" which is instantaneous, and is really more "Eastern" and vertical by nature.
That is, if you are looking for tonality. If you're not, tonality does not matter. It doesn't matter if it is fleeting or not.
I have the feeling that a lot of listeners are pre-conditioned by listening to tonal music, and to be fair, I can't blame them, because tonality is music based on principles which are sensual, harmonic, and ear-based. This is only natural for the ear to search out a central note.
My point is, if you are listening to Webern and are habitually hearing snatches of "tonality," or are consciously trying to hear it in tonal terms, you are not really accepting the music on its own terms, with a more severe, objective, intellect-based stance.
Serial and modern music is quite different, in that it is generated from mathematical and geometric principles of symmetry. This is not to say that it cannot sound good, at the same time.