An alpha particle is a helium nucleus, consisting of two protons and two neutrons bound together. When it comes sufficiently close to the nucleus of an atom, it experiences a strong electrostatic repulsion from the positively charged protons in the nucleus. This repulsive force is much stronger than the attractive nuclear force that binds the alpha particle together.

As a result, the alpha particle is deflected away from the nucleus, and its path is significantly altered. The exact nature of the deflection depends on the energy of the alpha particle and the size of the nucleus. In some cases, the alpha particle may be completely deflected and scattered away from the nucleus, while in other cases it may be able to penetrate the nucleus and cause a nuclear reaction.

For an isolated nucleus or a nucleus with no orbital electrons, the deflections can be calculated with relatively simple physics. However, in realistic atoms and molecules, the effect of the electrons, as well as potential quantum effects, makes a detailed analysis much more complex.