Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paulEL on 25/02/2019 09:43:29

Title: How do atoms 'know' what other atoms to bond with?
Post by: paulEL on 25/02/2019 09:43:29
Hope i can explain my question sufficiently...
If two "things" are made of same material, why won't the atoms in the first object try to bond with the atoms in the other object? If i press two aluminum cans together why don't they try and combine or fuse together? they are made of the same material.
If my understanding is correct, basically at the sub-atomic level atoms either repel or attract other atoms. We cannot walk thru solid walls simply because the atoms in our bodies repel the atoms in the wall. How do the atoms "know" that they are not part of that wall?
this question may sound weird or stupid, but i'm not sure how else to word it.
Title: Re: How do atoms 'know' what other atoms to bond with?
Post by: evan_au on 25/02/2019 10:11:17
Quote from: OP
If i press two aluminum cans together why don't they try and combine or fuse together? they are made of the same material.
Actually, aluminium cans are covered by a thin layer of aluminium oxide, caused by the rapid reaction of aluminium with oxygen in the air.

This oxide layer keeps the aluminium in the cans from touching.

It was a surprise to engineers designing early spacecraft (often with aluminium alloys) that in outer space, away from air and oxide layers, metal surfaces do slowly weld together, as atoms migrate into the surface with which they are in contact.
See: https://en.wikipedia.org/wiki/Vacuum_cementing

Quote
why won't the atoms in the first object try to bond with the atoms in the other object?
You could view it as a case of "lonely electrons".

Electrons like to hang around in pairs, and in larger groups like multiples of 8. If you have an atom with a lone outer electron, it tends to pair up with another nearby atom which also has a lonely electron.

Overall, atoms are more stable if they have a full outer shell of electrons, even if they have to:
- steal an electron from a nearby atom, forming a negative ion (mostly seen on the right side of the periodic table, like chlorine)
- donate an electron to a nearby atom, forming a positive ion (mostly seen on the left side of the periodic table, like sodium)
- share an electron with a nearby atom, forming a covalent bond (mostly seen between atoms that are nearer the center-right of the periodic table)
- share a sea of electrons with many surrounding atoms (mostly seen with metals)

Once all the atoms are satisfied with full outer shells, it takes exposure to a new, more reactive compound, and often a spark to trigger a rearrangement into a new, stabler compound; we call this a chemical reaction.

See: https://en.wikipedia.org/wiki/Chemical_bond

Quote
How do the atoms "know" that they are not part of that wall?
Because they are not chemically bonded to the molecules in the wall, and there is a thin film of air between them.

On the other hand, a gecko can form a weak, temporary bond with the wall, by getting the molecules of its setae in very close contact with the molecules of the wall.
See: https://en.wikipedia.org/wiki/Gecko_feet#Structure
Title: Re: How do atoms 'know' what other atoms to bond with?
Post by: paulEL on 25/02/2019 12:17:14
evan_au,
thank you for your information.

As a kind of follow-up, secondary question/example....
if I cut a block of material, let's say plastic (something that does not normally re-attract back to itself like puddy) in half, why does it not try to re-combine when I touch the two parts together?
What happened to the atoms that cause them not to want to fuse back together?
Title: Re: How do atoms 'know' what other atoms to bond with?
Post by: chiralSPO on 25/02/2019 14:51:40
Great questions!

if I cut a block of material, let's say plastic (something that does not normally re-attract back to itself like puddy) in half, why does it not try to re-combine when I touch the two parts together?
What happened to the atoms that cause them not to want to fuse back together?

Liquids (including putty, which is just a very viscous liquid) are held together by relatively weak intermolecular forces--these interactions are easy to break (compared to actual chemical bonds) and easy to form because a) there aren't many restrictions on how the molecules have to be oriented to interact, and b) the molecules are free to move around enough that they can adjust to acommodate the few restrictions there are.

Solids come in many forms, depending on how the atoms are bonded. The main ones are: ionic solids, like table salt--made up of positively and negatively charged species (mono-atomic or polyatomic) that do not share electrons with each other, and are only attracted by Coulombic forces, molecular solids, like sugar--made up of molecular units in which the atoms are (strongly) covalently bonded, and these molecules are then attracted to each other through weak intermolecular forces, polymers, like PVC, made of extremely large molecules, attracted to each other through weak intermolecular forces, and extended solids, like metals, diamond, and semiconductors (the differences being how they share their electrons)--where all of the atoms are covalently bound to each other (these materials can be extremely strong because all of the bonds are very strong.

Molecular solids are the easiest to "put back together" because they are more like liquids in that only weak intermolecular forces are at play. Unlike liquids, however the components don't have much freedom to align properly. Imagine making an enormous solid cube out of lego pieces--ten km in any direction. Then break it in half--and try to put it together again. It will be nearly impossible to reattach because in addition to breaking into two major pieces, there will be several pieces (thousands or millions of them) that also break off individually or in tiny clusters. The jagged edges on the major pieces have to align perfectly to reattach, but this is impossible because there are gaps where the little pieces should be, and there is a layer of the little pieces that gets in the way. Even if you sweep the little pieces out of the way and align the two major pieces perfectly and push them together, the reformed cube will be much weaker along the original break because of the missing chunks.

Network and ionic solids are harder to put back together because the requirements for forming covalent bonds are much more stringent--requiring very specific alignment, and the atoms have to get very close to form a bond--so close that they repel each other first. So great pressure is required (sometimes effectively making it like a liquid).
Title: Re: How do atoms 'know' what other atoms to bond with?
Post by: Bored chemist on 25/02/2019 19:16:42
What happened to the atoms that cause them not to want to fuse back together?
Two things;
this
https://en.wikipedia.org/wiki/Surface_reconstruction
 and air molecules react with the cut surfaces and get in the way of them sticking back together.
Title: Re: How do atoms 'know' what other atoms to bond with?
Post by: yor_on on 28/02/2019 00:18:55
This is pretty cool. Happy to read it.