Evidence For The Nucleus Elliott

Rutherford Scattering In the early part of the last century, the accepted model of the atom was proposed by J J Thompson in his plum pudding model.  This consisted of a matrix of protons in which were embedded electrons.   Ernest Rutherford (1871 – 1937) used alpha particles to study the nature of atomic structure with the following apparatus: Rutherford was using alpha particles (helium nuclei) as nuclear bullets to smash up the atoms; he wanted to see atoms bursting

Observations Most of the atom was empty space. The positive charge was concentrated in a very small space The radius of the nucleus was in the order of 3 ´ 10-14 m. The alpha particles that were deflected back had to be travelling in a line with the nucleus.  Later research has shown the nuclear radius to be in the order of 1.5 ´ 10-14 m

Check Your Progress What led Rutherford to conclude that the nucleus was very tiny and had a positive charge?

Answer Alpha particles could pass through atoms without deflection so mostly empty space Alpha particle is positively charged Deflection is due to repulsion from positively charged nucleus

Experiment Limitations The distance is the distance of the closest approach. This is determined by the energy of the alpha particle. The higher the energy, the closer it will get.

Check Your Progress A 5 MeV alpha particle approaches a gold nucleus. (a) What is the charge carried by the alpha particle? (b) What is the charge carried by the gold nucleus? (c) What is the energy in joules of the alpha particle? (d) What is the minimum approach distance?

Answer