Atoms of the same element with different mass numbers are known as $\qquad$ .

Discuss the experimental results of Geiger-Marsden's $\alpha$-particle scattering.

Who discovered the spin quantum number?

$A$ beam of fast-moving alpha particles was directed towards a thin gold foil. The parts $A'$,$B'$,and $C'$ of the transmitted and reflected beams corresponding to the incident parts $A$,$B$,and $C$ of the beam are shown in the adjoining diagram. The number of alpha particles in:

If in Rutherford's experiment,the number of particles scattered at $90^o$ angle are $28$ per min,then the number of scattered particles at an angle $60^o$ and $120^o$ will be:

What radioactive source did Geiger and Marsden use in the scattering experiment?