Ernest Rutherford's atomic model describes an atom with a dense central nucleus surrounded by electrons that orbit at varying distances, which was a significant advancement from earlier models. JJ. Thomson introduced the plum pudding model, which depicted the atom as a sphere of positive charge with negatively charged electrons embedded within it, resembling a pudding with plums. Democritus and Dalton contributed to the concept of the atom as an indivisible sphere, suggesting that atoms are the fundamental building blocks of matter, which laid the groundwork for modern atomic theory.