the smallest unit of any chemical element, consisting of a positive nucleus surrounded by negative electrons. Atoms can combine to form a molecule

A molecule of carbon dioxide (CO₂) has one carbon atom and two oxygen atoms. 二氧化碳（CO2）分子是由一个碳原子和两个氧原子构成的。

figurative He hasn't an atom of sense (= he has no sense), that boy. 那个孩子啊，一点头脑也没有。