History of the Atom

At the start of the 19th century, John Dalton described atoms as solid spheres and said different spheres made different elements. But in 1897 JJ Thomson said that atoms weren't solid spheres and said that atoms must contain smaller negatively charged particles- electrons. The new theory was known as the 'Plum pudding model' This model showed the atom as a ball of positive charge with electrons in it. In 1909, Ernest Rutherford and his student Ernest Marsden conducted alpha particle scattering experiments where they fired positively charged alpha particles at a very thin sheet of gold. They were expecting the particles to pass through, however, while most of the particles went straight through, some were unexpectedly deflected, which meant the plum pudding model couldn't be right. Rutherford came up with the nuclear model of the atom in which there was a very small positively charged nucleus where most of the mass was and there was a 'cloud' of negative electrons surrounding it.But scientists realized that if the layout of an atom was how Rutherford described, it would cause the atom to collapse. So Niel Bohr created a revised Nuclear Model where the electrons were all contained in shells. He also proposed that they orbited the nucleus in fixed shells and distances. His model was supported by many experiments and helped to explain many scientists' observations at the time. Further experiments done showed the nuclear could be divided into smaller particles called protons. Then, 20 years later James Chadwick provided evidence for neutral particles which we now call neutrons and this resulted in the model of the atom being very close to the modern day version which is known as the Nuclear Model.