Considering the given elements, which one has the lowest melting point: xenon, iodine, barium, or magnesium?

Introduction / Context:
Melting point trends in the periodic table are influenced by the type and strength of bonding in the solid state. Metals, nonmetal solids, and noble gases all show very different melting behaviours. This question asks you to compare the melting points of four elements from different groups: xenon, iodine, barium, and magnesium. Recognising which has the lowest melting point helps reinforce the idea that weak intermolecular forces in noble gases lead to very low melting temperatures.

Given Data / Assumptions:

• Xenon is a noble gas in group 18.
• Iodine is a halogen in group 17, existing as a molecular solid I2 at room conditions.
• Barium and magnesium are metals in groups 2 (alkaline earth metals).
• Metals have metallic bonding, molecular solids have van der Waals forces, and noble gases are held together only by very weak dispersion forces in the solid state.

Concept / Approach:
The melting point of a substance depends on how strongly its particles are held together in the solid. Metals like barium and magnesium have metallic bonding with a sea of delocalised electrons, which is strong and gives relatively high melting points. Iodine exists as I2 molecules in a solid lattice, held by intermolecular forces stronger than those in noble gases. Xenon, as a noble gas, forms a molecular solid held together only by very weak London dispersion forces, so it melts at a very low temperature. Therefore, xenon has the lowest melting point among the four elements listed.

Step-by-Step Solution:
Step 1: Classify xenon as a noble gas, iodine as a nonmetal molecular solid, and barium and magnesium as metals. Step 2: Recall that metallic bonding in barium and magnesium is strong, so their melting points are relatively high. Step 3: Note that iodine has discrete I2 molecules but still forms a solid at room temperature, indicating moderate intermolecular forces. Step 4: Recognise that xenon is gaseous at room temperature and only condenses at very low temperatures, meaning its solid is held together by the weakest forces. Step 5: Conclude that xenon has the lowest melting point among xenon, iodine, barium, and magnesium.

Verification / Alternative check:
If approximate data are consulted, xenon melts at about minus 112 degrees Celsius, iodine melts near 114 degrees Celsius above zero, magnesium melts above 600 degrees Celsius, and barium melts above 700 degrees Celsius. These values confirm that xenon has by far the lowest melting point. The order matches the bonding strength: weak van der Waals forces for xenon, stronger molecular forces for iodine, and strongest metallic bonding for magnesium and barium.

Why Other Options Are Wrong:
Iodine has a significantly higher melting point than xenon because the I2 molecules have more electrons and stronger dispersion forces and are arranged in a molecular crystal. Barium and magnesium are metals with strong metallic bonds, so their melting points are much higher than those of molecular solids and noble gases. The suggestion that all have approximately the same melting point is clearly incorrect because their bonding types and phase at room temperature are quite different.

Common Pitfalls:
A common mistake is to focus on atomic mass alone and assume that heavier elements must have higher or lower melting points without considering bonding. Another error is to forget that noble gases are weakly bound and tend to liquefy and solidify only at very low temperatures. To avoid these issues, always link melting point primarily to bonding type: metallic, ionic, covalent network, molecular, or noble gas solids, with noble gas solids having among the lowest melting points.

Final Answer:
Among xenon, iodine, barium, and magnesium, the element with the lowest melting point is xenon.