Which atomic model proposed that electrons are embedded in a positively charged "soup"?

The model you are asking about is the Thomson model, which is often referred to as the "plum pudding model." This atomic model was proposed by J.J. Thomson in 1897, following his discovery of the electron. According to this model, an atom is composed of a diffuse cloud of positive charge (the "soup"), within which negatively charged electrons are embedded like raisins in a pudding.

This model suggests that the positive charge is uniform throughout the atom, balancing out the negative charges of the electrons. This was one of the earliest attempts to describe the internal structure of the atom and provided a significant step forward in atomic theory at the time. Although later models, such as the Rutherford and Bohr models, replaced the plum pudding model due to their more accurate representations of atomic structure, the Thomson model's introduction was crucial in advancing our understanding of atomic particles.