Why are liquids generally less dense than solids?

[wasteofo2]

I took Honors Chemistry this year with the assumption I might actually get some good questions answered, but alas, no such luck. When we went over different stages of matter, the teacher used those little diagrams I'm sure you've all seen to describe how solids, liquids and gasses differ. The diagrams where solids were little balls connected in a crystal-lattice structure by little rods, liquids were little balls just piled atop one another, and gasses were little balls floating all over the place.

Now, I understand that liquids have more energy than solids, and thus there's more atomic motion, but still, based on these diagrams, I don't understand why that excess energy you need to go from solid to liquid doesn't break the strong bonds that hold solid particles in place, and allow the particles to essentially collapse in on that vacant space that was kept vacant by bonds keeping all the solid particles strictly in place.

Can someone try to explain to me why all substances aren't like water, in that the solid state of the molecule expands from the liquid state?

Thanks a lot,
Jacob

 

[ShawnD]

I don't understand why that excess energy you need to go from solid to liquid doesn't break the strong bonds that hold solid particles in place, and allow the particles to essentially collapse in on that vacant space that was kept vacant by bonds keeping all the solid particles strictly in place.

solid - tape some tennis balls together, put them on your racket, and fling them into the air
liquid - put normal tennis balls on your racket and fling them into the air.

Which balls stayed closer together, the taped or the nontaped? If you can think of a reason for why that happened, you can understand solids and liquids.

 

[wasteofo2]

solid - tape some tennis balls together, put them on your racket, and fling them into the air
liquid - put normal tennis balls on your racket and fling them into the air.

Which balls stayed closer together, the taped or the nontaped? If you can think of a reason for why that happened, you can understand solids and liquids.

That explains why solids are more dense than liquids when flung into the air, but what about when they're just sitting around?

 

[ShawnD]

The only time they'll be "sitting around" is at absolute zero... but at that point everything is a solid.

 

[Gokul43201]

The diagrams where solids were little balls connected in a crystal-lattice structure by little rods, liquids were little balls just piled atop one another, and gasses were little balls floating all over the place.

The problem is from using models like these. Some models can do more damage than good. This one seems to have done some damage at least.

Now, I understand that liquids have more energy than solids, and thus there's more atomic motion, but still, based on these diagrams, I don't understand why that excess energy you need to go from solid to liquid doesn't break the strong bonds that hold solid particles in place, and allow the particles to essentially collapse in on that vacant space that was kept vacant by bonds keeping all the solid particles strictly in place.

The "bonds" do not keep the atoms away from each other. A better model would have the balls (representing the atoms) stuck to each other. If bonding results from sharing or exchanging valence electrons, would this not require that the valence shells actually overlap ?

A slightly better explanation of bonding would be the following:
What the bonding does is it sucks nearby atoms to each other till their valence shells are touching - as long as other requirements of bonding (such as the number of valence electrons making aoctets and so on) are satisfied. Bonding does not hold atoms apart. There is no space created for the atoms to fall into.

Atoms can not come any closer to each other than they get to be in a solid at low temperatures. If you do try to push a pair of atoms closer to each other, they will start to feel a strong repulsion.

Also keep in mind that gravity has a negligible effect on the dynamics of individual atoms. The force due to gravity on an atom/molecule is tiny compared to the electrostatic forces due to neighbouring atoms/molecules.