we can write the electron configuration for an atom by filling atomic orbitals until all the electrons in an atom have a place to stay we will fill up each subshell completely before going up to the next one that's higher in energy and the order we will use is dictated by the aufbau principle in german aufbau means to build up and that's exactly what we are doing when determining the electron configuration of an atom we are starting with the lowest energy orbital the 1s and building up to the higher energy orbitals until all the electrons are assigned to an orbital it may be difficult to remember the order of the orbitals by looking at them in this way so this diagram with each type of orbital together in one column shows us an easy way to remember the correct order to fill up the subshells we just follow an arrow and then jump down to the next one an even more convenient way to follow the aufbau principle is to simply know what sections on the periodic table correspond to which subshells looking at the periodic table now we must understand that each period or row on the table represents a shell or a particular n value the first row is n equals 1 then n equals 2 and so forth then we must know that this section containing groups 1 and 2 is called the s block this section is the p block the transition metals are the d block and the lanthanides and actinides are the f block the only trick is that the d block is always one behind the period number in terms of principle quantum number and the f block is always two behind we can see that in the fourth period it's actually the three d orbitals that follow the 4s and in the sixth period the order is 6s then 4f then 5d then 6p the s and p orbitals will both match the period number which will equal the principal quantum number the d's are one behind and the f's are two behind if we can internalize this way of looking at the periodic table then the aufbau principle reveals itself as we simply read left to right and up to down on the table starting at the top left corner the order would be 1s 2s 2p 3s 3p 4s 3d 4p and so forth which is precisely the order dictated by the aufbau principle this will make it much easier for us to assign electron configurations to any atom you