Which property of an element can be used for its identification?

$A, B$ and $C$ are three metals with atomic numbers $4, 11$ and $13$ respectively. Arrange the metals in the increasing order of their valency.

Give reasons:
$(a)$ Mass number of an atom excludes the mass of an electron.
$(b)$ The nucleus of an atom is charged.
$(c)$ The alpha particle scattering experiment was possible by using gold foil only and not by foil of any other metal.

$(a)$ Why are anode rays called canal rays?
$(b)$ Mention two postulates of $J. J.$ Thomson's model.
$(c)$ Compare the properties of protons and electrons.

What was Bohr's postulate on the revolution of electrons in the orbits of an atom?

$(a)$ Explain the Bohr$-$Bury rules for the distribution of electrons into different shells.
$(b)$ Draw the electronic structure of element $X$ with atomic number $17$ and element $Y$ with atomic number $16$.