Which of the following statements is always correct?

$(a)$ $_{12}^{24} Mg$ and $_{12}^{26} Mg$ are symbols of two isotopes of magnesium. Compare atoms of these isotopes with respect to:
$(i)$ composition of their nuclei.
$(ii)$ electronic configuration and valency.
$(b)$ Give the reason why two isotopes of magnesium have different mass number.

According to the Bohr-Bury scheme,the maximum number of electrons that can be accommodated in a given shell is given by the formula:

The isotope of cobalt used in the treatment of cancer and iodine used in the treatment of goitre respectively are

Rutherford's alpha $(\alpha)$ particles scattering experiment resulted in the discovery of:

$(a)$ Explain Bohr$-$Bury rules for the distribution of electrons into different shells.
$(b)$ Write the electronic configuration of an atom of sulphur. Also,draw a schematic diagram of its atom showing the distribution of electrons in its shells.