A melting point is the specific temperature at which a substance transitions from a solid state to a liquid state. This phase change reflects the balance between thermal energy and the cohesive forces holding atoms or molecules together. The substance with the lowest known melting point is the noble gas Helium. The common isotope Helium-4 does not solidify at normal atmospheric pressure, even when cooled to temperatures approaching absolute zero.
The Unique Case of Helium
Helium-4 remains a liquid at standard pressures all the way down to absolute zero (0 Kelvin or approximately -273.15 degrees Celsius). This unusual behavior is caused by the quantum mechanical phenomenon of zero-point energy. Even at the lowest possible temperatures, a helium atom’s position is not perfectly fixed, as dictated by the Heisenberg Uncertainty Principle.
This minimum energy of motion, the zero-point energy, is significant enough to overcome the extremely weak attractive forces between helium atoms. This energy is about seven times larger than the binding energy required to hold the atoms in a solid crystalline structure. This energetic restlessness prevents the atoms from settling into a rigid lattice, keeping the substance liquid.
To force helium to freeze, substantial external pressure must be applied to compress the atoms closer together. Under this compression, the zero-point energy is no longer sufficient to disrupt the structure, and the helium will solidify. Helium-4 requires a pressure of at least 25 atmospheres (atm) to form a solid, even at 0 Kelvin. For example, at 1.463 Kelvin (-271.687 degrees Celsius), the melting point is achieved at a pressure of 26.036 atm.
The Role of Intermolecular Forces in Melting
The melting point of any substance is determined by the strength of the forces between its atoms or molecules. These attractive forces, called intermolecular forces (IMFs), must be overcome by thermal energy for the substance to transition from solid to liquid. Substances with strong bonds, such as ionic compounds like salt, require high energy and thus have high melting points.
Conversely, substances with very weak IMFs require little energy to break the solid structure, resulting in low melting points. Among these weak forces are Van der Waals forces, including London Dispersion Forces (LDFs). LDFs are temporary, induced attractions caused by momentary shifts in electron distribution around an atom.
These forces are minimal in small, non-polar, and monoatomic substances, such as the noble gases. Since noble gases exist as single, unbonded atoms, their only attractive mechanism is the weak LDF. The minimal energy required to separate these weakly attracted atoms explains why they solidify only near absolute zero.
Other Substances Near Absolute Zero
While helium is unique in requiring external pressure to solidify, other elements also have melting points very close to absolute zero, typically due to the weak LDFs. The substance with the next lowest melting point is the gas Hydrogen (\(H_2\)), which is a diatomic molecule. Hydrogen melts at approximately 13.99 Kelvin (about -259.16 degrees Celsius) at standard atmospheric pressure.
Following hydrogen is the noble gas Neon (\(Ne\)), which melts at about 24.55 Kelvin (-248.45 degrees Celsius). Unlike helium, both hydrogen and neon possess enough interatomic attraction to form a stable solid lattice structure at ambient pressure. The slightly stronger forces in neon mean its zero-point energy is not sufficient to prevent it from freezing.
Common Low-Melting Substances
The most notable low-melting point elements are those that melt near or below typical room temperature. The metal Mercury (\(Hg\)) is the only metal that is a liquid at standard room temperature, with a melting point of -38.83 degrees Celsius. Its low melting point made it historically useful in applications such as thermometers.
Another metal known for its low melting point is Gallium (\(Ga\)), which melts at approximately 29.76 degrees Celsius. This temperature is only slightly above typical room temperature, allowing it to melt easily if held in a person’s hand. The low melting point of gallium makes its alloys useful as a non-toxic alternative to mercury.