The first model of an atom was a simple, neutral sphere.

explain why J. J Thomson had to come up with a new model of the atom when he discovered the electron?

Because he found that electrons are negatively charged particles, seperate from the positively charged nucleus, although his model was still incorrect

Thomson proposed in 1898 a new model, according to which the atom would be a massive sphere whose mass corresponded almost to the total mass of the atom itself. In this sphere would be the negatively charged electrons. As the atom was electrically neutral, Thomson stated that the matter constituting the sphere was positively charged.