# lanthanide contraction

"The Lanthanide Contraction refers to the fact that the 5s and 5p orbitals penetrate the 4f sub-shell so the 4f orbital is not shielded from the increasing nuclear change."

First, 5s and 5p are filled before 4f, so if the atom wants to give an electron out, which energy shell does the electron come out of? How 5s and 5p penetrate 4f? Isn't 4f outer than 5s and 5p?

• Have a look here. This should give you an idea why 5s is so contracted that it "penetrates" 4f (I'm not completely sure whether it is right to say that 5p penetrates 4f too, but if so the principle is the same). Mar 11, 2014 at 14:44