Among molecular solids, the one with the lowest boiling point is helium, which boils at just 4.2 Kelvin (-269°C or -452°F). If your question comes from a chemistry class comparing specific answer choices, the rule is straightforward: the smallest, least polar molecular substance on the list will have the lowest boiling point. Helium sits at the extreme end of that scale because it has the fewest electrons and the smallest atomic radius of any element, giving it the weakest intermolecular attractions possible.
Why Intermolecular Forces Control Boiling Point
A molecular solid’s boiling point depends entirely on how strongly its molecules attract each other. Stronger attractions mean you need more heat energy to pull molecules apart and send them into the gas phase. Three types of intermolecular forces matter here, listed from weakest to strongest:
- London dispersion forces: Temporary, fleeting attractions that exist between all molecules. These are the only forces acting between nonpolar substances.
- Dipole-dipole interactions: Stronger attractions between molecules that have a permanent positive and negative end, like acetone.
- Hydrogen bonding: The strongest common intermolecular force, found in molecules where hydrogen is bonded to oxygen, nitrogen, or fluorine. This is why water stays liquid up to 100°C.
A molecular solid held together only by London dispersion forces will always have a lower boiling point than a similarly sized molecule with dipole-dipole interactions or hydrogen bonding. Formaldehyde and ethane have nearly identical molecular weights (about 30 g/mol), but formaldehyde is polar and boils at -19.5°C while nonpolar ethane boils much lower at -88°C. The permanent dipole in formaldehyde creates stronger molecule-to-molecule attraction.
How Size and Electron Count Affect Dispersion Forces
Among nonpolar substances where only London dispersion forces operate, two factors determine how strong those forces are: the number of electrons in the molecule and the size of its electron cloud. More electrons mean the cloud can be temporarily distorted more easily, creating stronger fleeting attractions between neighboring molecules.
This is why boiling points climb steadily as you move through a series of similar molecules. In straight-chain hydrocarbons, pentane (5 carbons) boils at 36°C, hexane at 69°C, heptane at 98°C, and octane at 126°C. Each added carbon brings more electrons and more surface area for molecules to interact with each other.
The same pattern appears with noble gases. These are single atoms held together purely by dispersion forces, making them a clean test case. Helium (2 electrons) boils at 4.2 K. Neon (10 electrons) boils at 27.1 K. Argon (18 electrons) boils at 87.3 K. Krypton reaches 120.9 K, xenon 166.1 K, and radon 211.5 K. Each step up the periodic table adds electrons, strengthens dispersion forces, and raises the boiling point.
Identifying the Lowest Boiling Point on an Exam
If you’re looking at a multiple-choice question asking which molecular solid has the lowest boiling point, follow this process. First, eliminate anything with hydrogen bonding (molecules containing O-H, N-H, or F-H bonds). These will have the highest boiling points in the group. Next, eliminate polar molecules, since their dipole-dipole interactions add extra attraction beyond simple dispersion forces.
Among the remaining nonpolar options, pick the one with the fewest electrons and the lowest molecular weight. A small, nonpolar molecule like H₂ (boiling point of -253°C) will always beat a larger nonpolar molecule like methane (-161°C) or carbon tetrafluoride (-128°C). Molecular shape plays a role too: compact, spherical molecules have less surface area for dispersion interactions than long, stretched-out ones, so they tend to boil at lower temperatures. Tetramethylbutane, for example, boils at 106°C compared to 126°C for the more elongated octane, even though both have eight carbons.
Common Comparisons in Chemistry Courses
Certain comparisons show up repeatedly. Here are the patterns worth knowing:
- Noble gases vs. diatomic molecules: Helium (4.2 K) beats molecular hydrogen (20.3 K) because helium has fewer electrons. Both are nonpolar, but hydrogen’s two atoms give it a slightly larger, more polarizable electron cloud.
- Nonpolar vs. polar molecules of similar size: Ethane (-88°C) vs. formaldehyde (-19.5°C). Same ballpark molecular weight, but the polar molecule boils nearly 70 degrees higher.
- Hydrogen bonding vs. everything else: Ethanol (78°C) vs. dimethyl ether (-24°C). Both have the same molecular formula, but ethanol can form hydrogen bonds and stays liquid at room temperature while dimethyl ether is a gas.
- Small nonpolar molecules: F₂ (-188°C), ethylene (-104°C), and carbon tetrafluoride (-130°C) all have low boiling points, but F₂ wins because it has the smallest electron cloud of the three.
The logic always comes back to the same question: how many electrons does the molecule have, and what types of intermolecular forces can it form? The answer with the fewest electrons and only London dispersion forces will have the lowest boiling point.