Dear Student , A diatonic scale is one which moves through pitch letter names in sequence, without skipping any letters, applying accidentals to certain pitches so as to create a pattern of whole steps and half steps.
Ex.)
A-B-C-D-E-F-G-(A) is the diatonic scale known as A minor.
C-D-E-F-G-A-B-(C) is the diatonic scale known as C major.
Note that no letter names are skipped in the scale sequences of diatonic scale. If we had to make every note in the A minor scale flat, it would then become the A-flat minor scale; we again had to sharp every note in the C major scale, it would become the scale of C-sharp major. Thus you see that, using accidentals, more than one sort of diatonic scale can be constructed from any given sequence of pitch letters. A purely diatonic scale contains two half-step intervals, and the rest are whole-step intervals. The position of these half steps within a diatonic scale determines its quality (major, minor, a certain mode, etc.). Diatonic scale is not a same meaning for major scale. A major scale is one of several possible types of diatonic scale.