Final answer:
3) ba metal, ca metal, mg metal. The standard molar entropy of the alkaline earth metals decreases as you go up the group, with Ba metal having the highest entropy followed by Ca metal and then Mg metal.
Step-by-step explanation:
To arrange the group of elements magnesium (Mg), calcium (Ca), and barium (Ba) in order of decreasing standard molar entropy (S°) without doing a calculation, we should consider periodic trends related to entropy. Standard molar entropy is often associated with the amount of disorder in the system.
Elements further down a group in the periodic table generally exhibit higher molar entropy due to increased atomic size, complexity, and number of available microstates. Thus, barium metal, being the heaviest and located further down the group, will have the highest standard molar entropy, followed by calcium, and then magnesium, which is the lightest and located highest in the group.
The correct order of the elements in terms of decreasing standard molar entropy is:
- Ba metal
- Ca metal
- Mg metal