Isotopes

What would be the reason for an element to have atoms with different mass numbers?

Complete the above table relating to the atomic structure of the elements.

Chlorine is an element of atomic number $17$. It is a mixture of isotopes having mass numbers of $35$ and $37$.

Complete the given table:

What are isotopes?

According to Dalton’s Atomic Theory, atoms of the same element are identical in all aspects. But according to the Modern Atomic theory, this postulate is proved wrong. Explain.

${}_{12}{}^{24}\mathrm{Mg}$ and ${}_{12}{}^{26}\mathrm{Mg}$ are symbols of two isotopes of magnesium.
Give reasons why the two isotopes of magnesium have different mass numbers.

${}_{12}{}^{24}\mathrm{Mg}$ and ${}_{12}{}^{26}\mathrm{Mg}$ are symbols of two isotopes of magnesium.
Compare the atoms of these isotopes with respect to:
(i) the composition of their nuclei.
(ii) their electronic configurations.