In the previous video we learnt about how the structure of atom developed from Dalton's Solar Sphere model to Thomsen's Splump Putting Model, who modified Thomsen's Splump Putting Model and how? On the basis of Thomsen's Splump Putting Model to verify the internal structure, Ruderford designed an experiment which is famous as Ruderford's Gold Foils Scattering Experiment. He took a thin gold foil. This gold foil was bombarded with alpha particles which are emitted by a radioactive element. Alpha particles are positively charged. Now we also know them as helium nuclei. A fluorescent screen of zinc sulphide was placed around the gold foil to observe the path of alpha particles. Alpha particles cannot be seen with naked eye, but whenever an alpha particle hits the fluorescent screen, we observe a flash of light. Can you guess what Ruderford should have observed? Ruderford expected positively charged alpha particles to bounce back or change their path after hitting the gold foil as positive charge is evenly distributed inside the gold atom according to Thomsen's Splump Putting Model. But then, what were the observations and conclusions of Ruderford's Gold Foils Scattering Experiment? Ruderford's Gold Foils Scattering Experiment contrary to the expectation that many alpha particles to get reflected unexpectedly most of the alpha particles went straight through the foil without any change in path. This suggests there is tremendous empty space in the atom. A small number of alpha particles get deflected from the original path through a small angle. This suggests that positively charged particles few times come near a central core which later was named nucleus by Ruderford. As the alpha particles are not attracted but deviated shows that this centrally located nucleus is positively charged. Very few alpha particles got deflected through larger angle and surprisingly one alpha particle out of 20,000 bounced back in 180 degrees means in the direction opposite to original path. This suggests that nucleus is very much concentrated in small space as the positively charged protons are 1800 times heavier than electrons. The nucleus becomes very dense or massive. With this experimentation and study, Ruderford put forth a nuclear model for atom as follows. There is a positively charged nucleus at the center of an atom. The entire mass of the atom is concentrated at the center in the nucleus. Negatively charged particles called electrons revolve around the massive positively charged nucleus. The total negative charge on all electrons is equal to the total positive charge on the nucleus. This way atom continue to be neutral. There is a lot of empty space between the revolving electrons and the atomic nucleus. As this was closer to our solar system with sun as nucleus. This Ruderford's model is also called planetary model. Ruderford's model had a shot coming. The electron revolves around the nucleus but according to law of physics an electrically charged body revolving in a circular orbit loses its energy and will finally spiral into nucleus making the atom unstable. But in reality all atoms are stable except radioactive atoms. This shot coming was removed by Neel's Bohr about which we will study in the next video. In this video we study Ruderford's nuclear model. The total negative charge on all electrons is equal to the total positive charge on the nucleus. This way atom continue to be neutral. This Ruderford's model is also called planetary model. Now answer the following question. And Ruderford's gold foil scattering experiment which observation led to the discovery of nucleus and why?