What scale is defined as one that ascends or descends in only half steps and has 12 notes?

The correct answer is the chromatic scale, which consists of all 12 notes within an octave, each separated by a half step. This scale includes every possible pitch within that range, meaning that it embraces both the pitch classes of the natural notes and their sharps or flats.

By ascending or descending exclusively in half steps, the chromatic scale differs significantly from the major scale and both the natural and harmonic minor scales, which follow a specific pattern of whole and half steps, resulting in fewer notes being played in a single octave. The major scale, for example, features a specific sequence of tones and semitones that create its distinct sound. Similarly, the natural minor scale has a distinct pattern that defines its structure, and the harmonic minor scale introduces a raised seventh to enhance the harmonic aspect, leading to a different series of intervals.

In summary, the chromatic scale is unique due to its comprehensive inclusion of all twelve pitches, which ascend and descend strictly in half steps, making it foundational in understanding music theory and the relationships between different notes.