$(a) (i)$ Name the scientist who proposed this model of the atom.
$(ii)$ Write the three postulates of this model.
$(iii)$ How many maximum electrons can be accommodated in the $M$ orbit?
$(b)$ What are canal rays? Give the characteristics of canal rays.

(N/A) $(a) (i)$ This is Neils Bohr's model of the atom.
$(ii)$ Postulates of Bohr's model:
$1$. $A$ positively charged nucleus is present in the centre of the atom.
$2$. Electrons revolve around the nucleus in fixed circular paths called shells or orbits,which are named as $K, L, M, N, \dots$ or $1, 2, 3, 4$.
$3$. Shells are also called energy levels as each shell is associated with a fixed amount of energy. Electrons do not lose or gain energy while revolving in a specific shell.
$(iii)$ The maximum number of electrons in a shell is given by the formula $2n^2$. For the $M$ shell,$n = 3$,so the maximum number of electrons is $2(3)^2 = 2 \times 9 = 18$.
$(b)$ Canal rays are positively charged radiations which consist of positively charged particles. They were discovered by $E. \text{ Goldstein}$.
Characteristics of canal rays:
$1$. They consist of positively charged particles.
$2$. They travel in a straight line.
$3$. They have a charge equal in magnitude but opposite in sign to that of an electron.