ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

Thomson proposed the Plum Pudding model of atomic structure in the early 20th century, which was groundbreaking at the time. This model suggested that atoms are composed of a positively charged 'pudding' with negatively charged electrons embedded throughout, much like plums in a pudding. The model was a significant advancement in understanding atomic structure and aimed to explain the existence of electrons within an atom.

Thomson's concept arose from his experiments with cathode rays, which led him to identify the electron as a component of the atom. According to the Plum Pudding model, the overall charge of the atom is neutral because the positive charge (the 'pudding') balances the negative charges (the 'plums'). While this model was later replaced by more accurate representations such as the nuclear model proposed by Rutherford, it was essential in the historical development of atomic theory, highlighting the importance of electrons as constituents of the atom.

The other models listed are associated with different scientists and concepts that emerged later. The planetary model, for instance, describes electrons orbiting a nucleus, similar to planets around the sun, which differed significantly from Thomson's initial ideas. The quantum mechanical model and the electron cloud model incorporate principles of quantum mechanics and describe electron behavior in terms of