Relating to atoms or composed of atoms; another form of the word 'atomic.'
From atom + -ical (suffix forming adjectives, from Latin -icalis). This version competed with 'atomic' in earlier scientific writing but became archaic as 'atomic' was standardized.
Languages often have multiple ways to say similar things—'atomical' vs 'atomic' shows how scientific terminology gets simplified over time, with the shorter, simpler word usually winning.
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