Who discovered the neutron?

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.

The first model of an atom was given by

$(a)$ How many neutrons are present in $C-14$ isotope of carbon?
$(b)$ How many protons does $He^{2+}$ ion possess?
$(c)$ How many maximum electrons can be filled in the $3^{rd}$ orbit of an atom?
$(d)$ How many electrons does $He^{2+}$ ion possess?

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?

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?