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

In the Thomson's model of atom,which of the following statements are correct?
$(i)$ The mass of the atom is assumed to be uniformly distributed over the atom.
$(ii)$ The positive charge is assumed to be uniformly distributed over the atom.
$(iii)$ The electrons are uniformly distributed in the positively charged sphere.
$(iv)$ The electrons attract each other to stabilize the atom.

State whether the following statement is true or false:
Each isotope of an element is an impure substance.

The isotope of uranium used as a fuel in nuclear reactors and carbon used in radiocarbon dating are

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.

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