What happens when the positively charged alpha particle hits the positively charged nucleus?

Answer 1

Alpha particles interact with atomic nuclei by what is called scattering. An atomic nucleus will recoil from the scattering event somewhat, but the heavier the nucleus being "hit" by the alpha particle, the more the alpha particle will "bounce off" that nucleus. These interactions are not as "physical" as some we are familiar with, like billiard balls acting on each other, but the analogy is a good one. We actually say the alpha particle is scattered and comes away with less energy and the nucleus comes away having taken a bit from the alpha particle. With very small nuclei, the results are much more dramatic, particularly when we look at those elements with the lowest atomic numbers. As you can imagine, the smaller the atom, the more it reacts in the scattering process. A hydrogen-1 nucleus will "go flying" if it is involved in a scattering event with an alpha particle. And you can easily visualize that, too. Use the links below to related questions to learn more.

Answer 2

The nucleus loses an alpha particle, which takes two protons and two neutrons out of the nucleus. This reduces the isotope number by four, and the atomic number by two. An example is the alpha decay of thorium-232 (atomic number 90) to radium-228 (atomic number 88).