This is the electron configuration for Ag (Silver) found on the wikipedia page: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s1 I used the Aufbau Principle and got this instead: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2 4d9 1. I can see that Ag is an exception to the rule, but are there any generalizations I can make so that I don't write down the wrong configurations just because of an exception? 2. Also, is it traditional to write these in order of increasing quantum number (just like it's written above), because the Aufbau Principle is meant for the orbitals to be filled in increasing energy? For example, Br is written, on wikipedia, as 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5, but written using Aufbau Principle, it's actually 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5. Which one is more correct?