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