What is the valency of $(a)$ aluminium in aluminium sulphate,and $(b)$ sodium in sodium sulphate?

The presence of protons and neutrons at the center of the atom was suggested by:

An atom of an element has full $K$ and $L$ shells and has $5$ electrons in its $M$ shell. What is its atomic number?

$(a)$ An ion $M^{2+}$ contains $10$ electrons and $12$ neutrons. What is the atomic number and mass number of the element $M$?
$(b)$ Is it possible in an atom to have $12$ protons and $13$ electrons? Explain.
$(c)$ Why is helium gas inert?

$(a) (i)$ 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.

Differentiate between atomic mass and mass number.