Why do some shells in an some atoms, like Copper have more than 8 electrons?

Question: Why do some shells in an some atoms, like Copper have more than 8 electrons?
- Keywords:
  chemistry,
  subatomic
Asked by Maestro to Arthur, Clare, Daniel, David, Tora on 10 Nov 2014. This question was also asked by ikoniic.
- Daniel Parsons answered on 10 Nov 2014:
  
  When I first learnt chemistry I was told that electron shells had 2, then 8, then 8, then 8 electrons in them as you moved out. But I found out doing my A-levels that it is a bit more complicated that that!
  
  Copper for example has valence electrons. These are free electrons that can buzz about and essentially is the reason why copper is such a great conductor of electricity! Copper is amazing – due to these incredible conduction properties, it is still used in pretty much every wire we use.

Question: Why do some shells in an some atoms, like Copper have more than 8 electrons?