Final answer:
C6H12O6 (glucose) has the lowest melting point due to its molecular structure held together by weaker intermolecular forces, in contrast to ionic bonds in KCl, CaCl2, and Na2O.
Step-by-step explanation:
The compound with the lowest melting point among KCl, CaCl2, Na2O, and C6H12O6 is C6H12O6 (glucose). The melting points of ionic compounds such as KCl, CaCl2, and Na2O are generally much higher due to strong ionic bonds between cations and anions in the crystal lattice. In contrast, glucose is a molecular solid held together by weaker intermolecular forces, such as hydrogen bonds and van der Waals forces, leading to a much lower melting point compared to ionic compounds.
It should be noted that factors like ionic charge and the size of ions influence the melting point of ionic compounds, as higher charge and smaller size generally increase the lattice energy and, consequently, the melting point. For example, CaCl2 has a higher melting point than KCl because the Ca2+ ion has a higher charge than the K+ ion, leading to stronger ionic bonding and higher lattice energy. However, the molecular nature of C6H12O6 makes it different from these ionic compounds, resulting in a lower melting point.