atomic numberBrE/əˌtɒmɪk ˈnʌmbə(r)/NAmE/əˌtɑːmɪk ˈnʌmbər/noun (chemistry化学) thenumberofprotonsinthenucleus of an atom, which is characteristic of a chemical element. Elements are placed in the periodic table(= centre)ofanatom, whichischaracteristicofachemicalelement. Elementsareplacedintheperiodic tableaccordingtotheiratomicnumbers原子序数(指原子在元素周期表中的序号)