# What are intermolecular forces?

Jul 5, 2018

Intermolecular forces (IMFs) are attractive interactions between molecules. They lead to differences and sometimes trends in various physical properties.

These are typically listed in order of strength:

$\text{Dispersion" < "Dipole-Dipole" < "Hydrogen-bonding" < "Ion-Dipole" < "Ion Pairing}$

Hydrogen-bonding (which is not bonding) is shown below in an example diagram for $\text{H"_2"O}$ and $\text{HF}$.

Other features which are equally important are described below.

IMFs span a spectrum of:

• temporary partial charge-partial charge interactions
• permanent partial charge-partial charge interactions
• permanent partial charge-full charge interactions
• permanent full charge-full charge interactions

Depending on electronegativities of each atom involved, the IMF strength and interaction time changes.

Here are tables organizing this into the known IMFs.

Table 1. Interaction Times

$\underline{\text{IMF"" "" "" "" "" "" "" ""Interaction Time}}$
$\text{ "" "" ""Dispersion"" "color(white)(....)"Temporary}$
$\text{ "" ""Dipole-Dipole"" "" ""Permanent}$
$\text{Hydrogen-bonding"" "color(white)(....)"Permanent}$
$\text{ "" "" ""Ion-Dipole"" "color(white)(....)"Permanent}$
$\text{ "" "" ""Ion Pairing"" "" ""Permanent}$

Table 2. Degree of Charge Interactions

$\underline{\text{IMF"" "" "" "" "" "" "" ""Degree of Interaction}}$
$\text{ "" "" ""Dispersion"" "color(white)(....)"Partial/Partial}$
$\text{ "" ""Dipole-Dipole"" "" ""Partial/Partial}$
$\text{Hydrogen-bonding"" "color(white)(....)"Partial/Partial}$
$\text{ "" "" ""Ion-Dipole"" "color(white)(....)"Full/Partial}$
$\text{ "" "" ""Ion Pairing"" "" ""Full/Full}$

Table 3. Typical Participants

$\underline{\text{IMF"" "" "" "" "" "" "" ""Participants"" "" "" "" "" "" }}$
$\text{ "" "" ""Dispersion"" "" ""Nonpolar Molecules}$
$\text{ "" ""Dipole-Dipole"" "color(white)(..)"Moderately Polar Molecules}$
$\text{Hydrogen-bonding"" "color(white)(..)"Molecules with XH bonds"^"*}$
$\text{ "" "" ""Ion-Dipole"" "" ""Ions in Polar Solvent}$
$\text{ "" "" ""Ion Pairing"" "color(white)(..)"Ions with large charges}$

$\text{*}$ $-$ $\text{X}$ indicates a very electronegative atom, usually $\text{N}$, $\text{O}$, or $\text{F}$. Note that these must be bonded DIRECTLY to $\text{H}$, not just be in the chemical formula. Note also that there are exceptions, like chloroform interacting with acetone, which do NOT seem to contain sufficiently electronegative atoms.