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

Elements with valency $1$ are

Given that a natural sample of iron $(Fe)$ has isotopes $_{26}^{54}Fe$,$_{26}^{56}Fe$,and $_{26}^{57}Fe$ in the ratios of $5\%$,$90\%$,and $5\%$ respectively. What will be the average atomic mass of iron $(Fe)$ (in $u$)?

Atomic models have been improved over the years. Arrange the following atomic models in their chronological order:
$(i)$ Rutherford's atomic model
$(ii)$ Thomson's atomic model
$(iii)$ Bohr's atomic model

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

Find the number of electrons,protons,and neutrons possessed by the alpha particles $\left({ }_{2}^{4} He ^{2+}\right)$ used in the gold leaf experiment.