Answer:
a) As I remember "the atomic spectra" of bohr
1- Existence of a positively charged nuclei in the centre of the atom
2- Number of negatively charged electrons = number of positive charges ported by the atom.
3- Electrons navigate in fixed energy levels , with a high speed without gain or loose energy
4- centrifugal force is in equilibrium with attraction force.
5- The empty space between energy levels is forbidden for electrons.
