Ne, HF, C2H6, CH4 15. Which of the substances listed above has the highest boiling point, and why? — Bonding Chemistry Question

STEPS:

1. Understand what boiling point measures: The boiling point is the temperature at which a liquid transitions into a gas. This phase change requires overcoming the intermolecular forces (IMFs) that hold the individual molecules or atoms together in the liquid phase. Stronger IMFs require more thermal energy to disrupt, resulting in a higher boiling point.
2. Identify the type of intermolecular forces present in each substance:
* $\text{Ne}$ (Neon): A noble gas consisting of individual atoms. It is completely nonpolar and is held together only by very weak London dispersion forces.
* $\text{HF}$ (Hydrogen fluoride): A highly polar molecule where a hydrogen atom is covalently bound to a highly electronegative fluorine atom. This structure allows $\text{HF}$ molecules to form extremely strong hydrogen bonds (a highly concentrated, strong subclass of dipole-dipole interactions).
* $\text{C}_2\text{H}_6$ (Ethane): A symmetrical hydrocarbon. It is nonpolar and experiences only London dispersion forces.
* $\text{CH}_4$ (Methane): A symmetrical tetrahedral hydrocarbon. It is nonpolar and experiences only London dispersion forces.
3. Compare the relative strengths of the IMFs:
* Hydrogen bonding is much stronger than London dispersion forces for molecules of comparable size.
* Although $\text{C}_2\text{H}_6$ has a slightly larger, more polarizable electron cloud than $\text{HF}$ (18 electrons vs. 10 electrons) which gives it stronger dispersion forces, the network of hydrogen bonding in $\text{HF}$ is far stronger and more difficult to break than the dispersion forces of ethane.
4. Determine the substance with the highest boiling point:
* Because $\text{HF}$ is the only substance in this list capable of forming hydrogen bonds, its molecules are held together by the strongest intermolecular forces.
* Therefore, $\text{HF}$ requires the most thermal energy to boil and has the highest boiling point, confirming Option B is correct.

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WHY_OTHERS_WRONG:

• Option A is incorrect: Neon is a small monatomic element with a very small atomic radius and only 10 electrons. Its London dispersion forces are extremely weak, giving it an incredibly low boiling point (approx. $-246^\circ\text{C}$).
• Option C is incorrect: Ethane ($\text{C}_2\text{H}_6$) is a nonpolar hydrocarbon. It cannot form hydrogen bonds because it does not contain a hydrogen atom covalently bonded to a highly electronegative atom with lone pairs ($\text{N}$, $\text{O}$, or $\text{F}$).
• Option D is incorrect: Methane ($\text{CH}_4$) is a small, nonpolar molecule. It has very weak London dispersion forces—weaker even than those of ethane because methane has fewer electrons (10 electrons vs. 18 electrons in ethane) and a smaller, less polarizable electron cloud.