State whether the following statement is true or false:
An atom of each element has a definite combining capacity,called its valency.

$(a)$ $_{12}^{24} Mg$ 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.

Which of the two elements given below would be chemically more reactive: $X$ of atomic number $18$ or element $Z$ of atomic number $16$? And why?

Which of the following in the figure do not represent Bohr's model of an atom correctly?

Which of the $Na^+$ and $He$ has completely filled $K$ and $L$ shells? Give a reason to support your answer.

Find out the valency of the atoms represented by the Fig. $(a)$ and $(b)$.