Atomic number( Z): the number of protons present in the nucleus( Moseley1913).
Mass Number( A): Sum of the number of protons and neutrons present in thenucleus.
Thomson model of an atom This model proposed that atom is considered as a uniform positively charged sphere and electrons are embedded in it. An important feature of Thomson model of an atom was that mass of atom is considered to be evenly spread over the atom. Thomson model of atom is also called as Plum pudding, raisin pudding or watermelon model Thomson model of atom was discarded because it could not explain certain experimental results like the scattering of α- particles by thin metal foils.
Observations from α- particles scattering experiment by Rutherford a. Most of the α- particles passed through gold foil un deflected b. A small fraction of α- particles got deflected through small angles
c. Very few α- particles did not pass through foil but suffered large deflection nearly180 o
Conclusions Rutherford drew from α- particles scattering experiment:
a. Since most of the α-particles passed through foil undeflected, it means most of the space in atom is empty
b. Since some of the α-particles are deflected to certain angles, it means that there is positively mass present in atom
c. Since only some of the α-particles suffered large deflections, the positively charged mass must be occupying very small space
d. Strong deflections or even bouncing back of α-particles from metal foil were due to direct collision with positively charged mass in atom