Who discovered the neutron?

$(a)$ Describe the main features of Bohr's model of an atom. Draw a neat and labelled diagram of energy levels.
$(b)$ Which of the following pairs are isotopes and which are isobars?
$(i)$ $_{26}^{58} A, _{28}^{58} B$
$(ii)$ $_{35}^{79} X, _{35}^{80} Y$
$(c)$ Elements $A$ and $B$ have atomic numbers $18$ and $16$ respectively. Which of these two would be more reactive and why?

In a sample of ethyl ethanoate $(CH_3COOC_2H_5)$,the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?

An ion $X^{2+}$ contains $18$ electrons and $20$ neutrons. Calculate the atomic number and mass number of element $X$. Name the element $X$.

State whether the following statement is true or false:
Atomic radius is measured in nanometres. $1 \, m = 10^9 \, nm$.

The ion of an element has $3$ positive charges. The mass number of the atom is $27$ and the number of neutrons is $14$. What is the number of electrons in the ion?