Final answer:
The predicted order of melting points from highest to lowest for calcium, strontium, and rubidium is calcium, strontium, and then rubidium, based on their positions in the periodic table and the strength of their metallic bonds.
Step-by-step explanation:
The melting points of calcium (Ca), strontium (Sr), and rubidium (Rb) can be predicted based on their positions in the periodic table and the nature of metallic bonding. In the alkali and alkaline earth metals, the melting point generally decreases as we move down the group because the metallic bonds get weaker due to the increased distance between the nucleus and the outermost electrons. This larger distance makes the metallic bond easier to break. Therefore, the predicted order of melting points from highest to lowest is calcium, strontium, and then rubidium.
Calcium and strontium are both group 2 elements, with calcium being above strontium in the periodic table. Strontium's outer electrons are further from the nucleus compared to calcium, making Sr's metallic bonds weaker and its melting point lower than Ca. Rubidium, on the other hand, is a group 1 element and is even farther down the periodic table than strontium, which means it has even weaker metallic bonding and a lower melting point still.