Explanation:

Spin-Orbit Coupling: Spin-orbit coupling is the interaction between the magnetic field generated by the motion of the electrons and the magnetic field generated by their spin. It is responsible for the fine structure of atomic spectral lines. The spin-orbit coupling leads to the splitting of energy levels, which results in the formation of different energy levels.

Magnetic Moment: The magnetic moment is a measure of the strength of a magnetic field generated by an object. It is the product of the magnetic field strength and the area of the object.

Measured and Calculated Magnetic Moment: The measured magnetic moment is the magnetic moment that is observed experimentally. On the other hand, the calculated magnetic moment is the magnetic moment that is predicted by theory, based on the spin-orbit coupling.

Difference in Measured and Calculated Magnetic Moment: The difference in the measured and calculated magnetic moment is observed for some elements, which is due to the strong spin-orbit coupling. The spin-orbit coupling is strong when the atomic number is large, and the electrons are confined to the innermost shells.

Europium (Eu): Europium is a chemical element with the symbol Eu and atomic number 63. It is a member of the lanthanide series of elements. Europium has a large atomic number, and the electrons are confined to the innermost shells. Therefore, the spin-orbit coupling is strong in europium.

Conclusion: The difference in the measured and calculated magnetic moment is observed for some elements, which is due to the strong spin-orbit coupling. Europium is one of these elements, which has a large atomic number and strong spin-orbit coupling, leading to the observed difference in magnetic moment.