Properties like melting and boiling points are a measure of how strong the attractive forces are between individual atoms or molecules. (We call these intermolecular forces – forces between molecules, as opposed to intramolecular forces – forces within a molecule. )
It all flows from this general principle: as bonds become more polarized, the charges on the atoms become greater, which leads to greater intermolecular attractions, which leads to higher boiling points.
There are four major classes of interactions between molecules and they are all different manifestations of “opposite charges attract”.
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The four key intermolecular forces are as follows:
Ionic bonds > Hydrogen bonding > Van der Waals dipole-dipole interactions > Van der Waals dispersion forces.
Let’s look at them individually, from strongest to weakest.
Table of Contents
1. Ionic forces
Ionic are interactions between charged atoms or molecules (“ions”). Positively charged ions, such as Na(+) , Li(+), and Ca(2+), are termed cations. Negatively charged ions, such as Cl(–), Br(–), HO(–) are called anions (I always got this straight through remembering that the “N” in “Anion” stood for “Negative”) The attractive forces between oppositely charged ions is described by Coulomb’s Law, in which the force increases with charge and decreases as the distance between these ions is increased. The highly polarized (charged) nature of ionic molecules is reflected in their high melting points (NaCl has a melting point of 801 °C) as well as in their high water solubility (for the alkali metal salts, anyway; metals that form multiple charges like to leave residues on your bathtub)
5. Bottom Line
Boiling points are a measure of intermolecular forces.The intermolecular forces increase with increasing polarization of bonds.The strength of intermolecular forces (and therefore impact on boiling points) is ionic > hydrogen bonding > dipole dipole > dispersionBoiling point increases with molecular weight, and with surface area.
For another discussion of these principles see Chemguide
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