Why does MgO have a higher melting point than CaS? They are both ionic compounds with the same charges, but CaS is heavier so shouldn't it have a higher melting point?
The charge of a large ion is less concentrated because of its larger radius and surface area. That makes the attraction between Ca^2+ and S^2- ions weaker than the attraction between the smaller Mg^2+ and O^2- ions.
To check the which has higher melting point among the two ionic compounds, check the charge and ionic radius. The charge should be higher and size should be less for both cation and anion. So they will have higher ionic character and melting point
While both MgO and CaS are ionic compounds with similar charges, their differences in melting points can be attributed to various factors, including the differences in their crystal structures, ion sizes, and lattice energies.
1. Crystal Structure: The crystal structure of a compound affects its melting point. MgO adopts a face-centered cubic (FCC) structure, also known as rock salt structure, whereas CaS adopts a different crystal structure, such as fluorite structure. The FCC structure has a more compact arrangement, leading to stronger bonding between the ions. This results in a higher melting point for MgO.
2. Ion Sizes: The size of the ions in the compound also affects its melting point. In MgO, the magnesium ion (Mg2+) has a smaller size compared to the calcium ion (Ca2+) in CaS. Smaller ions can pack more closely together, resulting in stronger electrostatic attractions between the ions. Thus, the smaller size of Mg2+ leads to stronger bonding and a higher melting point for MgO.
3. Lattice Energies: The lattice energy is the energy required to separate one mole of a solid compound into its gaseous ions. It is influenced by the charge and size of the ions. In MgO, both the magnesium and oxygen ions have a 2+ charge, while in CaS, the calcium ion has a 2+ charge and the sulfur ion has a 2- charge. The higher charge on the ions in MgO results in stronger ionic bonds and higher lattice energies, contributing to its higher melting point.
Although CaS is indeed heavier due to the higher atomic mass of calcium compared to magnesium, the factors mentioned above (crystal structure, ion sizes, and lattice energies) have a more significant impact on the melting point. Therefore, MgO has a higher melting point compared to CaS.
The melting point of a compound is primarily determined by the strength of the forces holding its particles together. In the case of ionic compounds like MgO and CaS, the melting point is influenced by the strength of the electrostatic attraction between the positively charged metal ions and the negatively charged non-metal ions.
To understand why MgO has a higher melting point than CaS, we need to consider the following factors:
1. Charge of the ions: Both MgO and CaS have ionic bonds between metal cations (Mg²⁺ or Ca²⁺) and non-metal anions (O²⁻ or S²⁻). The charges on the ions are the same in both compounds.
2. Size of the ions: Although Ca is heavier than Mg, it also has a larger atomic radius compared to Mg. As a result, the Ca²⁺ ion is larger than the Mg²⁺ ion. Similarly, the S²⁻ ion is larger than the O²⁻ ion. The larger ions tend to have lower charge density, meaning that the charge is spread out over a larger surface area.
Based on these factors, we can understand why MgO has a higher melting point than CaS:
The smaller size of the Mg²⁺ and O²⁻ ions in MgO allows for stronger electrostatic interactions between them, leading to a greater attraction between the positive and negative ions. This stronger attraction requires more energy to overcome, resulting in a higher melting point.
On the other hand, the larger size of the Ca²⁺ and S²⁻ ions in CaS results in weaker electrostatic interactions due to the lower charge density. As a consequence, the attraction between the positive and negative ions is not as strong, requiring less energy to overcome, leading to a lower melting point.
Therefore, the difference in ionic radii and charge density is the primary reason why MgO has a higher melting point than CaS, even though CaS is heavier than MgO.