Which scientist's model lacked the concept of a nucleus?

The model proposed by Thomson, known as the "plum pudding" model, did not include a nucleus. Instead, it depicted the atom as a diffuse cloud of positive charge with negatively charged electrons embedded in it, similar to plums in a pudding. This model was based on Thompson's discovery of the electron and represented a significant step in atomic theory; however, it failed to account for a concentrated nucleus at the center of the atom.

In contrast, scientists like Rutherford later provided evidence for the existence of the nucleus through his gold foil experiment, which demonstrated that atoms are mostly empty space with a dense, positively charged nucleus at the center. Bohr further refined the concept of the atom by introducing quantized energy levels around the nucleus, while Dalton's model focused primarily on indivisible atoms without any internal structure. Thompson's lack of a nucleus reflects the understanding of atomic structure prior to the pivotal experiments conducted by Rutherford and others that shaped modern atomic theory.