Question #02f6b

1 Answer
May 11, 2017

Intermolecular forces are the forces between molecules that are close to each other. There are several kinds of intermolecular forces, discussed below.


(Intramolecular forces are the forces between parts within the same molecule.)

All bonding forces are electrostatic in nature: attractions between things with different electrical charges.

An ion has had one or more electrons added (to yield a negative charge) or removed (to yield a positive charge (due to the charge imbalance with the remaining protons)).

An ionic charge is the largest kind of charge that participates in intermolecular forces. If two ions of opposite charge are attracted to each other, that attraction is so strong we call it a chemical bond: an ionic bond .

The next strongest kind of interaction is between an ion and a 'dipole', but I need to explain what a dipole is first.

When a molecule has not had electrons added or removed, the charge may still not be evenly spread. Some atoms are more 'electronegative' - attractive to electrons - than others. If a hydrogen and an oxygen atom are covalently bonded together, for example, the pairs of electrons that form the bond will be attracted more strongly to the oxygen than to the hydrogen. We can think of it as that they 'spend more of their time' at that end of the bond, though it's not really quite like that.

This separation of the charge means the oxygen end of the bond will be a little more negative and the hydrogen end a little more positive. This charge separation is called a 'dipole'. The amount of charge is less than a whole electron charge, and the interactions are correspondingly weaker.

A molecule with a lot of charge separation in it - a large dipole - is called a 'polar molecule', and one with little or no charge separation is called a 'non-polar molecule'.

The negative end of a dipole can be attracted to a positive ion, or the positive end to a negative ion, and we call this attraction an ion-dipole interaction . It is weaker than an ionic bond but stronger than the other interactions we'll discuss below.

A dipole-dipole interaction occurs when the positive end of a dipole in one molecule is attracted to the negative end of a dipole in another molecule, or vice versa. It is weaker than an ion-dipole interaction.

One special class of dipole-dipole interaction occurs when hydrogen is involved. Hydrogen is the smallest atom and can get quite 'naked' of electrons if bonded to something highly electronegative, so that its charge is a significant proportion of an electron charge. That makes these interactions particularly strong, so we call them 'hydrogen bonds ', but they are much weaker than ionic bonds.

Finally, in non-polar molecules with no significant dipole, charge sometimes undergoes fleeting separations, leading to the very weak interactions we describe as 'dispersion forces' or 'London forces'.

I have discussed these kinds on intermolecular forces in order of decreasing charge separation and therefore in order of decreasing strength.