What is the lanthanide contraction, and why does it occur?

Video transcript

Hi, everybody. Let's take a look at our next problem. It says true or false. The lanthanide contraction results from the excessive shielding of nuclear charge. So we need to first think about what is the lanthanide contraction. And that refers to the smaller than expected atomic radii of the elements, the transition elements in the lampen series. So we're referring to the fact that when we look at the transition elements in row six, so the row six transition elements have almost identical atomic radii to the row five transition elements. There's been a contraction, they're smaller than we'd expect. This is contrary to the fact that in general, as we go down the periodic table, atomic radii increase due to shielding from inner electrons. So as you go down, you're adding more shells, the valence electrons in those outer shells are at a greater and greater distance from the nucleus. So they're farther and farther away from the positively charged nucleus. So they feel less and less attractive force from those protons. So that's we talk about the Z effective or Z subscript eff referring to the actual attractive force those outer electrons feel as opposed to the actual force that the protons exert on the electrons in general. So we'd say ZF is or Z effective is lower than Z. So if shielding from inner electrons causes an increase in atomic radius, would we expect to see the lanthanide contraction resulting from an excessive shielding? And the answer would be no, because excessive shielding would cause an increase in atomic radius since it would mean the electrons are feeling less of that attractive force from the nucleus. So we can focus on our answer expect that we expect it to be false. And indeed, the reason for this contraction is that in the lanthanide series, the four F orbitals are filled in the beginning of our lenite series and they have ineffective shielding when we think about our different orbitals, SPD and then f each new orbital has when you look at its, you know, probability, its shape. So the probability where you'd find the electron, the F electrons are not going to be in by the nucleus nearly as often as are other orbital types. So they don't do a lot of shielding in terms of preventing the outer electrons from feeling that attractive force. So the lanthanide contraction results from not excessive shielding but ineffective shielding from the nuclear charge of the four F electrons. So our answer will indeed be false. The lanthanide contraction does not result from the excessive shielding of nuclear charge. See you in the next video