Think about the electrons in the valence shell. The fact that it forms a liquid it means that something is holding it together. It’s an inert gas, right? But if you cool it to –186 ☌, you can actually condense it into liquid argon. These represent the attraction between instantaneous dipoles in a molecule. The weakest intermolecular forces of all are called dispersion forces or London forces. Van der Waals Dispersion Forces (“London forces”) So on average these forces tend to be weaker than in hydrogen bonding. However since the electronegativity difference between carbon (electronegativity = 2.5) and the electronegative atom (such as oxygen or nitrogen) is not as large as it is for hydrogen (electronegativity = 2.2), the polar interaction is not as strong. These dipoles can interact with each other in an attractive fashion, which will also increase the boiling point. For instance, each of these molecules contains a dipole: Other groups beside hydrogen can be involved in polar covalent bonding with strongly electronegative atoms. So in a sense, HO, and NH are “sticky” – molecules containing these functional groups will tend to have higher boiling points than you would expect based on their molecular weight. As you might expect, the strength of the bond increases as the electronegativity of the group bound to hydrogen is increased. Owing to rapid molecular motion in solution, these bonds are transient (short-lived) but have significant bond strengths ranging from (9 kJ/mol (2 kcal/mol) (for NH) to about 30 kJ/mol (7 kcal) and higher for HF. The dipole of one molecule can align with the dipole from another molecule, leading to an attractive interaction that we call hydrogen bonding. However, the bond to hydrogen will still be polarized and possess a dipole. Since H has an electronegativity of 2.2 (compare to 0.9 for Na and 0.8 for K) these bonds are not as polarized as purely ionic bonds and possess some covalent character. Hydrogen bonding occurs in molecules containing the highly electronegative elements F, O, or N directly bound to hydrogen. The highly polarized (charged) nature of ionic molecules is reflected in their high melting points (NaCl has a melting point of 801 ☌) 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) 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. Positively charged ions, such as Na(+), Li(+), and Ca(2+), are termed cations. Ionic are interactions between charged atoms or molecules (“ions”). Van der Waals Dispersion Forces (“London forces”).Van Der Waals Dipole-Dipole Interactions.Let’s look at them individually, from strongest to weakest. Ionic bonds > Hydrogen bonding > Van der Waals dipole-dipole interactions > Van der Waals dispersion forces. The four key intermolecular forces are as follows: With 10 examples of solved problems! (Also contains all the key points discussed in this post) Now available – Download this awesome (free) 3-page handout on how to solve common boiling point problems. There are four major classes of interactions between molecules and they are all different manifestations of “opposite charges attract”. 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. (We call these intermolecular forces – forces between molecules, as opposed to intramolecular forces – forces within a molecule. Properties like melting and boiling points are a measure of how strong the attractive forces are between individual atoms or molecules.
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