Ka'idar atomist ita ce ka'idar kimiyya da ke bayyana yanayin kwayoyin halitta ta hanyar bayyana cewa dukkan kwayoyin halitta sun hada da kanana, barbashi marasa ganuwa da ake kira atoms. A bisa wannan ka'idar, atoms sune tushen ginin kwayoyin halitta, kuma sun yi kadan da ba za a iya gani da ido ba. Ka'idar atomist kuma ta nuna cewa atoms suna cikin motsi akai-akai, kuma suna iya haɗuwa da sake haɗuwa da juna don samar da abubuwa daban-daban. Wannan ka'idar ta kasance wani muhimmin bangare na ilimin kimiyyar lissafi da ilmin sinadarai na zamani, kuma ya ba da gudummawa wajen fahimtar tsari da halayen kwayoyin halitta a matakin atomic da subatomic.