Hello , about the band theory I was confused some how seeing the attached image I ask :- 1- when the atoms are far away we take a single atom alone as a system , but when atoms get closer we identify them all as the system , right ?! 2- why do we use the word " splitting" , why don't we say that every atom contributes with its _ for example_ 2s level and so the overall 2s can carry more electrons ?! 3- does the whole band have the same energy ?! if so why do we represent the increase of electron's energy by farther " rising" in conduction band ?! and if not why is there difference in energy ?! 4- I have to say that i'm studying this as an introduction to semi-conductor devices not physics course .