What prompted rutherford to do the alpha particle experiment

( Discovery of Nucleus in an atom )

Ernest Rutherford was interested in knowing the arrangement of electrons in an atom. For this, he conducted a unique experiment of scattering of positive charged Alpha particles by a gold foil.

Infact, alpha particles are nuclei of helium atom, with two units of positive charge and four units of mass. These particles are emitted by radio active elements like Radium and potassium. Fast moving alpha particles have considerable amount of energy and they can penetrate through matter to some extent.

He used a gold foil which is very malleable element and can be stretched to extreme thin sheet.He bombarded the alpha particles over the gold foil which was about 1000 atoms thick.

The mass has escaped into a gas which you can not see so the law stays true.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.