Final answer:
The trend in decreasing melting points for elements like Sc, Y, La, and Lu as you move down the group is due to increasing atomic radii and correspondingly weaker metallic bonds, not only increased atomic mass.
Step-by-step explanation:
When considering the melting points of elements such as Sc, Y, La, and Lu, it's important to recognize that other factors besides atomic mass play significant roles. Indeed, one might expect that increased atomic mass would lead to stronger Van der Waals forces due to a larger electron cloud and greater surface area, but this doesn't accurately predict the trend in melting points for these elements as you move down the group. The decrease in melting points is largely due to the increasing atomic radii of these elements, which implies that the individual atoms are further apart within the solid lattice structure, weakening the metallic bonds.
As the atomic size increases, the strength of the metallic bonds decreases, making these elements easier to melt. Moreover, the trend observed where melting points decrease down a group while atomic size increases is also seen in other periodic table groups—indicating a general pattern in periodic variations in element properties.
This discussion underscores the complex interplay between atomic structure and physical properties, which involves considerations of effective nuclear charge, electron shell distribution, and metallic character among others. It's not simply a matter of atomic mass, but rather the arrangement and energy of the valence electrons that dictate how an element will behave in its solid state.