Examines the nature of radical theory change, as radical theory changes pose a great threat to many forms of scientific realism. Draws on the account of theory changed developed by Kuhn late in his life, where a radical change of theory involves the replacement of a scientific lexicon with an incommensurable lexicon. The two lexicons cut nature at different joints. Examines a hitherto unstudied revolutionary change in the history of 20th Century chemistry, when atomic number was discovered, and became the way in which chemical elements were clasified. Argues that changes such as this one threaten the realists' claim that our successful theories are likely approximately true.