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 one point of similarity and one point of difference between isotopes of ${ }_{6}^{14} C$ and ${ }_{6}^{12} C$.

The ratio of the radii of a hydrogen atom and its nucleus is $\sim 10^5$. Assuming the atom and the nucleus to be spherical,
$(i)$ What will be the ratio of their sizes (volumes)?
$(ii)$ If the atom is represented by the planet Earth $(R_e = 6.4 \times 10^6 \, m)$,estimate the size of the nucleus.

The electron distribution in an aluminium atom is

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.