LDF, Dipole - Dipole, H-Bonding, Ion-ion?

Bonding happens because some atoms have a stronger pull on electrons, a property called electronegativity.

Consider HCl. Cl is more electronegative than H, so the electrons in the molecule are pulled closer to the Cl and that end of the molecule becomes slightly negative compared to the positive end. The molecule is said to have a 'dipole', which literally means, 'two ends', as in a slightly positive and slightly negative end.

(ANALOGY: Imagine a shopping mall where boys and girls are evenly spread out. Suddenly, an announcement comes over the PA saying that Justin Beiber is in the south end of the mall. A lot of the girls suddenly run to the south end, which becomes 'more female', while the north end of the mall ends up 'more male.')

If you have a whole bunch of HCl molecules, then all the partially positive ends will be attracted to all the partially negative ends and the molecules will be held together. Since this is a result of the dipoles, we call these attractions 'dipole-dipole forces.'

Hydrogen bonding is just a special case of dipole-dipole forces which we distinguish because hydrogen is so common in molecules. When hydrogen is bonded to a very electronegative element (fluorine, oxygen, or nitrogen), there is a fairly large pull of electrons toward the electronegative element. That end becomes even more partially negative than in normal dipole-dipole bonds and you get very strong dipole-dipole forces which we then call 'hydrogen bonds.' You can remember which elements form hydrogen bonds (rather than ordinary dipole-dipole bonds) by thinking that, "Hydrogen bonding is FON!" (fluorine, oxygen, nitrogen)

If you have two elements involved that have an even greater difference in electronegativity (a difference of more than 1.7), then we consider that the more electronegative element pulls hard enough on the electrons to actually take them from the other atom. It becomes a negative ion while the other atom, which loses electrons, becomes a positive ion and you get 'ionic bonding'. In this case the particles stick together not because they have partial positive and negative charges, but they have full charges. This makes ionic bonds stronger than either dipole-dipole or hydrogen bonds.

What about LDF? In this case, the shape of the molecule is important.

Consider CH4. Carbon is more electronegative than hydrogen, so if you had 1 C bonding to one H, the electrons would be pulled towards the carbon and you'd expect a dipole. But carbon has 4 hydrogens around, not just one and they are arranged symmetrically. So any pull on electrons in one direction involving one of the hydrogens is cancelled out by a pull from the opposite direction by another one of the hydrogens. The net result is that the molecule overall has no dipole, no partial positives and negatives to attract each other.

The only thing left to hold methane together is the LDF, which result from only temporary dipoles coming from the random movement of electrons around the molecule. At any given instant, more electrons will be found on one end than the other so there will be dipole. But an instant later the electrons shift around again so that this dipole disappears. Another one may form on another part of the molecule, but this one will also be temporary and disappear an instant later. And so on.

To summarize the different types of forces:

LDF - electrons shift to make small temporary dipoles. Only requirement is to have electrons that can shift so ALL molecules have LDF, even ones that also have other forces. Temporary nature and weakness of partial dipoles make LDF weakest type of intermolecular force. Molecules that are made up of only one type of atom (eg; H2) will not have differences in electronegativity, so they will only have LDF. Molecules that are symmetrical will have dipoles cancel out so they, too, will have only LDF.

Dipole-dipole: Difference in electronegativity between atoms create permanent dipoles. Requirement is difference in electronegativity (so that electrons are pulled more strongly to one end of the molecule) and asymmetry (so that dipoles don't cancel). Permanent dipoles make these bonds stronger than LDF.

H-bonding: Special case of dipole-dipole due to large electronegativity difference between H and F, O, N. Stronger than ordinary dipole-dipole because of stronger dipoles.

Ionic: Electronegativity difference great enough that, instead of shifting electrons about the molecule, one atom actually takes the electrons and becomes a negative ion, causing the other atom to become a positive ion. Requirement is electronegativity difference of more than 1.7. Attraction between fully rather than partially-charged particles makes ionic bonds strongest.

only most electronegative elements(O,N,F) have h-bonding but other elements are not that electronegative to form a strong h-bond.