An atom with atomic number $9$ and mass number $19$ will have the following constituents:

Find out the valency of the atoms represented by the Fig. $(a)$ and $(b)$.

Dalton's atomic theory successfully explained which of the following laws?
$(i)$ Law of conservation of mass
$(ii)$ Law of constant composition
$(iii)$ Law of radioactivity
$(iv)$ Law of multiple proportion

Give a brief account of the development of the presently accepted model of the atom with the contributions made by different scientists. Write two features of this atomic model.

$(a) (i)$ 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.

Which of the two elements given below would be chemically more reactive: $X$ of atomic number $18$ or element $Z$ of atomic number $16$? And why?