hello friends, i have some doubts... While conducting electrical discharge through gas in cathode ray tube,it is required that pressure of gas should be low. What is the connection between the flow of charge and pressure? I mean at normal pressures,there is no electrical discharge through gas but reducing the pressure helps to observe electrical discharge. So what is the connection between pressure and conduction of electricity? Another doubt: according to a principle in chemistry,electron filling in shells takes place in increasing order of energy levels.(aufbau's principle). I have learned that electrons have higher potential energy as distance from the nuclues increases.so energy level increases when we go away from nuclues. So shells nearer to the nuclues should have lower energy level than the one that is far away from it. But the 4s shell is the lower energy level than 3d shell.but 4s shell is actually far and 3d shell is nearer. As a result,4s is the one that should have higher energy level than 3d. But why opposite over here?