My chemistry teacher assigns online homework through a site called Web Assign. For this question I originally said it was [Kr] 5s2 4d9, but I got it wrong. It showed me which parts of the question I got wrong, so I guessed and changed it to the answer below. Why does this work? I thought all orbitals in a sublevel had to be filled before it could move on to the next sublevel, and Ag clearly isn't the last element in the d section, so why is it full? Is Web Assign just wrong?

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Ask your teacher, I don't think it's a good idea to ask people who have no idea.

Silver is one of the exceptions to Aufbau's principle. There is reasoning behind it, but it may be better to just remember that it doesn't follow the rule.

The common exceptions are: Chromium, Gold, Silver, and Copper.

EDIT: Here's a list from wikipedia containing all of the exceptions if you are interested: http://en.wikipedia.org/wiki/Electro...elung.27s_rule

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Whenever the d orbital is about to be filled OR half-filled, it grabs the an electron from the s orbital above it. So if you had 5s2 4d4 it would actually become 5s1 4d5. The d orbitals are more stable when they have 5 or 10 electron so they tend towards that.