Classify the types of intermolecular forces present in each of the following fluorides of the second period elements.
LiF, melting point 845 oC:
a. ionic
b. hydrogen bonding
c. dipole-dipole
d. dispersion
BeF2, melting point 800 oC:
a. ionic
b. hydrogen bonding
c. dipole-dipole
d. dispersion
The Correct Answer and Explanation is:
Correct Answers:
- LiF: a. ionic
- BeF₂: a. ionic
Detailed Explanation (300+ words):
Intermolecular forces are the interactions between molecules that affect a substance’s physical properties, such as melting point, boiling point, and solubility. In the case of fluorides of second-period elements, the nature of bonding depends heavily on the electronegativity difference and the type of elements involved (metal vs. non-metal).
1. Lithium Fluoride (LiF)
Correct answer: a. ionic
- LiF consists of a metal (Li⁺) and a highly electronegative non-metal (F⁻), which form ionic bonds.
- The electronegativity difference between lithium (0.98) and fluorine (3.98) is significant (Δ ≈ 3.0), indicating complete transfer of electrons from lithium to fluorine.
- This transfer leads to the formation of oppositely charged ions that are held together by strong electrostatic forces, known as ionic bonds.
- These ionic forces are much stronger than any typical intermolecular force such as hydrogen bonding, dipole-dipole, or dispersion.
- This strong bonding is reflected in LiF’s very high melting point of 845°C, which is characteristic of ionic compounds.
2. Beryllium Fluoride (BeF₂)
Correct answer: a. ionic
- BeF₂ is composed of beryllium (Be²⁺) and fluoride ions (F⁻).
- While Be has a smaller size and higher charge density compared to Li, its interaction with F still leads to predominantly ionic bonding.
- The electronegativity difference between Be (1.57) and F (3.98) is substantial (Δ ≈ 2.4), justifying its ionic character.
- BeF₂ has a high melting point of ~800°C, further indicating the presence of strong ionic bonds.
- Although Be²⁺ has a high polarizing ability, which might impart some covalent character to the bond, the overall bonding is best described as ionic in nature.
Other Options Explained:
- Hydrogen bonding (b) requires H to be directly bonded to N, O, or F — which does not occur in either LiF or BeF₂.
- Dipole-dipole (c) interactions are weaker and occur between polar molecules, not in ionic salts.
- Dispersion forces (d) are present in all molecules but are negligible compared to the strong ionic bonds in these salts.
Thus, both LiF and BeF₂ are best classified as having ionic bonding.