Write any two observations which support the fact that atoms are divisible.

An atom of an element $X$ may be written as ${ }_{4}^{9} X$.
$(a)$ What does the figure $9$ indicate?
$(b)$ How many neutrons does this element $X$ contain?

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?

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 ...........

What was Bohr's postulate on the revolution of electrons in the orbits of an atom?

An atom with $3$ protons and $4$ neutrons will have a valency of: