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Recently I have been reading Feynman lectures on physics. In the first chapter ,atoms in motion,Feynman talks about decreasing the temperature of water ,forming ice, and writes the following paragraph:

"The interesting point is that the material has a definite place for every atom, and you can easily appreciate that if somehow or other we were to hold all the atoms at one end of the drop in a certain arrangement, each atom in a certain place, then because of the structure of interconnections, which is rigid, the other end miles away (at our magnified scale) will have a definite location. So if we hold a needle of ice at one end, the other end resists our pushing it aside, unlike the case of water, in which the structure is broken down because of the increased jiggling so that the atoms all move around in different ways".

I cannot understand it.Can anyone please explain it?

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  • $\begingroup$ Some formatting instead of a block of text would help please. $\endgroup$ – user207455 Jun 19 at 18:32
  • $\begingroup$ why the deselection? $\endgroup$ – Árpád Szendrei Jun 20 at 6:35
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Feynman is stressing the main difference between a solid and a liquid. In the former, the permanence of each atom in a well definite position induces long range correlations. In the latter, diffusion hampers the build up of long-range correlations and knowledge about the atomic positions in a region does not say anything about positions at large distances.

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This is a more QM answer, there is a main difference between solids and liquids:

  1. solids do have a lattice structure, where the relative position of atoms/molecules is mostly fixed. This is caused by covalent bonds, meaning that the valence electrons around each separate molecule start existing around multiple molecules/atoms, thus creating a EM bond between the neighboring molecules and thus fixing their relative position

  2. liquids do not have such a lattice structure, the liquid molecules have something called the van der waals force, that bonds them into droplets, and all molecules have the ability to roll over each other thus creating the liquid effect. Now in liquids, there is no covalent bond between molecules, the valence electrons do not exist around both molecules, thus the neighboring molecules do not have a fixed relative position.

Now Feynman is connecting this to temperature. The reasoning is, that the vibrational energies of the molecules (that we can interpret as temperature), decreases as we decrease the temperature of water.

As you keep decreasing the temperature, you reach a point where the lattice structure starts being created, that is, the valence electrons around the molecules start existing around both neighboring molecules, thus creating a covalent bond between the molecules. Thus, the molecules relative position is fixed, and this has to do with the decrease in vibrational energies of the molecules (jiggling around).

As the molecules spend less and less time jiggling around, they allow for the valence electrons to spend more and more time around both molecules. It is QM, and all about probabilities, where the valence electrons do have some probability of existing around both molecules always.

This probability is what is increasing as the molecules position becomes more and more relatively fixed, as the jiggling around decreases. At a certain point, the molecules spend more and more time at a relatively fixed position, and this will allow for the valence electron to exist around both molecules at higher probabilities, thus creating a covalent bond.

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  • $\begingroup$ A contrast between solids/covalent bonded and liquids/van-der-waals bonded is not correct. Whatever is the underlying bond mechanism, matter forms solids at low temperature/high pressure, with the possibility of a liquid pocket in the phase diagram at higher temperatures but not so high to have a gas. In solid argon, bonds do remain based on van der Waals' interactions. $\endgroup$ – GiorgioP Jun 20 at 5:10

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