Why do we need to calculate the average atomic mass of isotopes of an element for computation?

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

Why is the atomic mass of chlorine taken as $35.5\, u$ and not a whole number like $35\, u$ or $36\, u$? Explain.

Give reasons for the following:
$(a)$ In potassium and calcium,the filling of electrons in the $N$-shell starts after the $M$-shell has acquired $8$ electrons,although the $M$-shell can accommodate up to $18$ electrons. Why?
$(b)$ There is a difference between mass number and atomic mass.

Fill in the blanks in the following statements:
$(a)$ Rutherford's $\alpha$-particle scattering experiment led to the discovery of the ............
$(b)$ Isotopes have same ............ but different .............
$(c)$ Neon and chlorine have atomic numbers $10$ and $17$ respectively. Their valencies will be ............ and ............. respectively.
$(d)$ The electronic configuration of silicon is .......... and that of sulphur is ...........

$(a)$ Write the names of three isotopes of hydrogen.
$(b)$ State one use of an isotope of uranium.
$(c)$ Isotopes of an element have similar chemical properties. Why?