For whom was atonality an important compositional technique?

Atonality, also know as serialism (and the 12-tone row), was an extremely important compositional technique for Austrian-American composer Arnold Schoenberg (spelled Schönberg in German). So much so, that he is considered to have been the composer and teacher with the most significant contributions during the 1900s. Specifically, with regards to developing and spreading atonality to other musicians and composers. He lived from 1874 to 1951.

Well if some words are like big you could say enormous .just make them sound more egxarreted

When an orchestra plays at a concert they might play a symphony (a piece for orchestra) and they might play a concerto (with a soloist). if the solo instrument is a violin the piece is called a “violin concerto”, if it is a piano it is called a “piano concerto”, etc.