Inner (core) electrons are those seen in the previous noble gas and any completed transition series. They fill all the lower energy levels of an atom.
Outer electrons are those in the highest energy level (highest n value). They spend most of their time farthest from the nucleus.
Valence electrons are those involved in forming compounds. Among the main group elements, the valence electrons are the outer electrons. Among the transition elements, the (n - 1)d electrons are counted among the valence electrons because some or all of them are often involved in bonding.
