$(a)$ 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?

(N/A) Features of Bohr's Model of an Atom:
$(i)$ Electrons revolve in discrete orbits,which are associated with a fixed amount of energy. These are called energy levels or shells,designated as $K, L, M, N, ...$
$(ii)$ While revolving in these discrete orbits,electrons do not radiate energy.
$(iii)$ The energy of an orbit increases as its distance from the nucleus increases. The orbit closest to the nucleus has the lowest energy.
$(iv)$ When energy is supplied to an electron,it jumps to a higher energy level. When it returns to a lower energy level,it emits energy in the form of radiation.
$(b)$ $(i)$ $_{26}^{58} A$ and $_{28}^{58} B$ are isobars because they have the same mass number $(58)$ but different atomic numbers ($26$ and $28$).
$(ii)$ $_{35}^{79} X$ and $_{35}^{80} Y$ are isotopes because they have the same atomic number $(35)$ but different mass numbers ($79$ and $80$).
$(c)$ Element $B$ (atomic number $16$) is more reactive than element $A$ (atomic number $18$). The electronic configuration of $A$ is $2, 8, 8$,which is a stable octet (noble gas configuration). The electronic configuration of $B$ is $2, 8, 6$,which requires two more electrons to complete its octet,making it chemically reactive.