Something is confusing me again. In one of my text books it clearly states that: generally there are only a certain number of electrons which are allowed in a shell. 1st:2 2nd:8 3rd:8 etc … I believe this is known as the octet rule. If I move on to my other text book it states that the 3rd shell can now hold 18 electrons. Are they trying to confuse me or is there a reasonable explanation to this?