The number of electrons in an element $X$ is $15$ and the number of neutrons is $16$. Which of the following is the correct representation of the element?

Answer the following in one line or one word:
$(a)$ Who discovered the neutron?
$(b)$ On the basis of Thomson's model of an atom,explain how the atom is neutral as a whole.
$(c)$ What is the maximum number of electrons that can be accommodated in the outermost shell of an atom?
$(d)$ What term is assigned to the atoms of different elements having the same atomic mass but different atomic numbers?
$(e)$ How many neutrons are present in a hydrogen atom?

What do you understand by an octet of electrons in the valence shell?

Explain the following with one example of each:
$(i)$ Atomic number
$(ii)$ Mass number

State three features of the nuclear model of an atom put forward by Rutherford.

$(a)$ Define the following terms with one example each:
$(i)$ Isotope
$(ii)$ Isobar
$(b)$ Name the elements whose isotopes are used in:
$(i)$ Nuclear Reactor
$(ii)$ Treatment of cancer