Intermolecular forces are attractions between atoms or molecules. The strength of these attractions determines the physical properties of the substance at a given temperature. The stronger the intermolecular forces, the more tightly the particles will be held together, so substances with strong intermolecular forces tend to have higher melting and boiling temperatures. Neon is a gas at room temperature and has a very low boiling temperature of -246 degrees Celsius--just 27 Kelvin.
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Types of Intermolecular Force
There are three main types of intermolecular force that exist between entities in different chemicals. The strongest type of intermolecular force is the hydrogen bond. Chemicals exhibiting hydrogen bonding tend to have much higher melting and boiling points than similar chemicals that do not partake in hydrogen bonding. Dipole-dipole attractions are weaker than hydrogen bonds, but stronger than the third type of intermolecular force: dispersion forces.
Hydrogen bonds occur when a hydrogen atom covalently bonded to an electronegative atom, such as oxygen, nitrogen or fluorine, interacts with another electronegative atom on a neighbouring molecule. The strength of hydrogen bonds is high, at around 10% of the strength of a normal covalent bond. However, neon is an element and does not contain any atoms of hydrogen, therefore hydrogen bonding cannot take place in neon.
Dipole-dipole attractions occur in molecules exhibiting permanent dipoles. A permanent dipole results when the electrons in a molecule are unevenly distributed such that one part of the molecule has a permanent partial negative charge, and another part has a permanent partial positive charge. Substances in which the particles have permanent dipoles have intermolecular forces slightly higher than substances without. Neon particles are single atoms, therefore they have no permanent dipole; so this type of intermolecular force is not present in neon.
All substances including neon demonstrate dispersion forces. They are the weakest type of intermolecular force since they are only transient, but even so their overall effect is sufficient to form a significant attraction between particles. Dispersion forces occur due to the random motion of electrons within the atom. At any one time, it is likely that there will be more electrons on one side of the atom than the other, which is referred to as a temporary dipole. When an atom experiences a temporary dipole, it can have an effect on neighbouring atoms. For example, if the more negative side of the atom came close to a second atom, it would repel the electrons, inducing another temporary dipole in the nearby atom. The two atoms would then experience a transient electrostatic attraction.
Strength of Dispersion Forces
The strength of dispersion forces depends on the number of electrons in the particle, since if there are more electrons, there is a chance any temporary dipole will be much more significant. Neon is a relatively small atom with only 10 electrons, so its dispersion forces are only weak. Even so, the dispersion forces of neon are sufficient to facilitate a boiling temperature -5 degrees C higher than helium, which only has two electrons. Thus significantly more energy is required to overcome the dispersion forces sufficiently to allow the atoms to separate and become gaseous.
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