Compared with solid ionic compounds, solid molecular compounds generally…
A) have lower melting points
B) are more brittle
C) are harder
D) conduct electricity as liquids
The Correct Answer and Explanation is:
The correct answer is A) have lower melting points.
Explanation:
To understand why solid molecular compounds generally have lower melting points compared to solid ionic compounds, let’s examine the key differences between the two types of compounds:
1. Ionic Compounds:
Ionic compounds consist of positively and negatively charged ions held together by strong electrostatic forces of attraction, known as ionic bonds. These forces are very strong, requiring a significant amount of energy to overcome. Therefore, ionic compounds typically have high melting points because it takes a lot of heat energy to break these strong ionic bonds and separate the ions from their organized crystal lattice structure.
For example, sodium chloride (NaCl), a common ionic compound, has a melting point of about 800°C. This is because the electrostatic attraction between the Na⁺ and Cl⁻ ions in the crystal lattice is strong, requiring considerable energy to break the structure apart.
2. Molecular Compounds:
Molecular compounds, on the other hand, consist of molecules that are held together by weaker forces, such as Van der Waals forces (dipole-dipole interactions or London dispersion forces) or hydrogen bonds (in some cases). These intermolecular forces are much weaker than ionic bonds, so less energy is needed to break the interactions between the molecules. As a result, molecular compounds generally have lower melting points than ionic compounds.
For example, water (H₂O), a molecular compound, has a much lower melting point of 0°C, and methane (CH₄) has a melting point of about -182°C. This is because the intermolecular forces in molecular compounds are weak compared to the strong ionic bonds in ionic compounds.
Why not the other options?
- B) More brittle: Ionic compounds tend to be brittle because when a force is applied, like charges may align, causing the ions to repel each other and fracture the structure. Molecular compounds are often less brittle due to their weaker bonding.
- C) Harder: Ionic compounds are usually harder because the ions are tightly packed in a rigid lattice. Molecular compounds are softer due to weaker intermolecular forces.
- D) Conduct electricity as liquids: Ionic compounds conduct electricity in liquid form because the ions are free to move. Most molecular compounds do not conduct electricity in any phase since they do not have charged particles.
Thus, solid molecular compounds generally have lower melting points compared to ionic compounds due to the weaker forces holding their molecules together.