Why did Thomson change Dalton's model of the atom? What did he propose as an alternative?
JJ Thomson discovered the electron to be a fundamental constituent of all matter. Thus he came to the conclusion that there are positive and negative charges in the atom (as postulated by Lorentz).
Dalton's atomic theory regarded the atom to be indivisible while after the discovery of more fundamental particles it was clear that the atom must have an internal structure - how are these charges distributed ? What is the shape of the atom ? What explains stability of matter ? What explains chemical bonding ?
Therefore, atomic models were proposed, Thomson's model being one of the earliest.
Thomson proposed that the electrons are embedded into a positively charged sphere so that the atom as a whole is electrically neutral.
That could explain charge neutrality, and chemical bond formation to some extent.