(i) The three sub-atomic particles of an atom are electrons, protons and neutrons.
(ii) (a) Since, there are 5 electrons in L-shell, so there must be 2 electrons in K-shell. Thus, the element has a total of 7 electrons, which is equal to the number of protons, hence the atomic number of the element is 7.
(b) L-shell has 8 electrons when completely filled, since it has 3 electrons less hence, the valency of element is 3.
(c) Element having atomic number 7 is nitrogen (N).