h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Very, very simple answers, please.

1) What makes us (and other things) stick together in a recognizable fashion, if we're mostly space, and atoms are mostly space, with their components whizzing around all over the place? What defines the boundaries?

2) Do the atoms which make things up stay a part of the same thing, or do they travel into other things but get replaced by similar atoms? Is that why objects are more-or-less reliable on the short term?

MoG-temporarily banished from her ancestral home. Again. *sigh*

I'll give this a go.

atoms are made up of nuclei (containing positive charge) surrounded by electrons (negative charge). Chemistry dictates how and when atoms stick together to form molecules, and these molecules are what "we" and many other things are made of.

So, you take your finger (ultimately made of molecules, which are made of atoms) and you push on a piece of wood (same structure). Ultimately what you are doing is trying to force the electrons in the atoms of your finger to overlap with the electrons in the piece of wood - and neither one is having any of it! The wood and or your finger will bend/deform - that's because it's easier to re-arrange your finger or the woods large-scale structure than it is to cause those electrons to overlap.

So at the root, it's the "like repels like" of electrons. That's the explanation of the boundaries. Now, the sticking together part, that's chemistry, physics, materials science, biology...

things stick together by forming bonds between electrons.

an atom as shells of electrons surrounding it. it is more stable when this outer shell is full. most arent full though. take chlorine for example, it has 7 electrons in its outer shell and it would prefer 8 to have a full shell. in order to get this full shell it will share an electron with another atom, say another chlorine atom. even though one electron is being shared each chlorine effectively has a full shell. this forms a bond (covalent)

most bonds are formed in this way though there is different ways but you will need to know more about the arrangement of electrons. just think of them in circles around the nucleus for now.

HN is nearly correct in what she says. However, the bonding is actually between atoms rather than bonds between electrons. The elecetrons are used to form the bonds. Subtle difference but an important one. In covalent bonding electrons are shared between two atoms in ionic bonding one atom gives up its electron(s) to the other to form a stable molecule. These are called intermolecular forces that are present within a molecule to hold it together.

The other forces that holds things together are intramolecular forces and hold groups of individual molecules together. These are called Van Der Waals forces which are present in graphite. Electrons are mobile within a molecule and as they move dipole moments are produced which give a polarity to a molecule (a positive and and a negative end) Think of magnets with north and south ends. Throw a load of magnets at each other and they will attract and repel each other to get north and south ends together. When you have lots of "dipole" molecules together they will arrange in such away that the positive and negative ends are together to form a lattice just like in graphite and is one of the reasons a 'lead' pencil works. The individual carbon atoms are held together with covalent bonds into a sheet and each sheet is held to another sheet by Van der Waals forces with electrons smeared over the surface making the sheets mobile.

So, the electrons are pushing away unsimilar electron groups, kinda like magnets do when pointed at each other?

I might well be mixed up on this, but I thought I read that the mental picture of a nucleus surrounded by shells of electrons in a nice, tight, stable orbit is not what's actually occurring, though it makes it easier for people to imagine. Are the electrons actually whizzing off all over the place, or is that some other kinda bit that's jumbled in my mind?

Also, if these atoms want to group up with like atoms, what prevents them from sucking all the similar atoms they come into contact with into the object they're a part of? For example, when you break a piece of wood in half, why doesn't it rejoin if you put the two pieces up against each other?

Alfster, just read your post and it's MoG in here!

Can you hear the *swooooosh* from that flying over my head?

Complete idiots version, please.

Hi MoG,

So you have had a Red Arrows moment.

I am afraid that I cannot give a simpler version of what I have written. Mainly due to the fact that I have forgotten the intracacies of it all and also I would probably have to go from a more detailed level of atomic structure and bonding even more than HN did.

I just hope I proper chemist can give you a potted explanation that you will understand.

'What makes us (and other things) stick together in a recognizable fashion'?

Love?

Yalson

MoG - I will do my best for simple explanation.

The 'smallest' unit of which anything is made up of is the atom. We get taught this at around age 10-11.

At age 14-15, we get taught that atoms are not actually the smallest thing going. There are smaller things of which atoms are made up of. The most important of these are electrons.

Confusingly, electrons are also the things that make electricity - without which this conversation would be tricky - by flowing down wires.

Atoms are more or less stable depending on how many electrons they possess, and a law of thermodynamics (for which, read 'law of how things happen, and accept it') states that atoms always want to be as stable as possible. Therefore, there exists a compromise situation whereby atoms donate or share electrons in order to be as stable as possible.

Say you're an atom. If you are sharing an electron with another atom, you've got to stick close to it, obviously. If you donate an electron to another atom, you get a positive charge (because electrons have a negative charge). The 'other' atom now has a positive charge, and - like in magnetism - opposites attract. Hence, in both cases, you get sticking-together.

B

Magnolia

Accept as an axiom that there are molecules, which are held together by some forces, and that molecules are held to each other by different forces.

If you break a piece of wood apart you are both breaking apart individual molecules (that got caught in the gap) and breaking molecules away from each other. 2 things occur which prevent you from sticking it back together.

The first and most immediate is that the arrangement of molecules (internal arrangment, and arrangment with respect to each other) at a surface can be wildly different from those in the interior. So when you break a piece of wood in half, you create 2 new surfaces - and the molecules at those surfaces quickly re-arrange so that they form the surface structure present at a wood surface.

The 2nd thing that happens is that the wood reacts with air. Most surfaces we interact with are not the actual material - it's a layer of the material which has been oxidized/corroded by being in contact with the air. You've probably never actually touched a piece of pure aluminum because it reacts almost instantly with air to form a thin, surface layer of aluminum oxide. The same is true with "wood", although I'm not sure to what extent.

But anyway, once the broken surfaces have reacted with the air, they are then unreactive towards each other.


In theory, if you cooled your piece of wood down to -296 C, and then broke it an a vacuum chamber (with pressure 1 trillionth of atmospheric), you might be able to stick it back together. The cold temeprature *might* prevent the surface re-arrangement, and the vacuum would prevent reaction with air.

hope this helps.

*whew*
I was worried that I'd already broken the boundaries of explainable *to me* with my very first SEx question.

Yalson, your answer comes pretty close to the way *I* think things work, but there *are* other kinds of explanations.

And with what the rest of you have said, followed up by Dealer's oxidizing/surface explanation for why things don't just keep sucking similar atoms into a huge clump, I feel a bit more enlightened, though not especially *happy* as I was hoping to find a really good excuse for weight gain that has nothing to do with input and burnoff of calories.

The other bit about all the space and movement of those electrons-- are they actually making a shell or are they zinging all over the place while the atom itself maintains stability?

the electrons are *not* in well defined orbits. They do have well defined probability distributions as to where they can be found at any one instant in time. They also have well defined probability distributions for their momentum (e.g. speed/velocity).

So I think you're statement of them zipping around randomly is the closest to the truth. Just with the caveat that it's not completely random - it does have some specific probability distribution.

Cool!
Now, are the electrons the lil fellas that are popping in and out of places all of a sudden, or are they staying present, kinda traveling in a path of some sort?

Also, how big a scale is that probability distribution? Is it on the teeny-tiny level, or will thumb electrons stay within the thumb, or can they show up all over a person, or do they really get around?

Electrons can pop in and out of places all of a sudden ("tunnel"). They aren't really travelling in any sort of path.

The length scale of the probability distribution is dependent upon the material. In a metal, the electrons are highly delocalized. This is equivalent to saying that their probability distribution is spread out over a wide area. I would roughly guestimate that in a typical piece of metal you might encounter in everyday life, they might be spread out over a micron (= 1 millionth of a meter).

Your thumb is made up of molecules, which are made up of atoms. Generally, atoms in molecules contain 2 types of electrons - "core" and "valence". The "core" electrons are just that - the core of the atom. They are highly localized near the nucleus of the atom. Their probability distribution is spread out only near the nucleus. Depending on the atom, the core electrons would be confined to a region from 0.5 to maybe 3 Angstroms (1 Angstrom = 1 part in 10 trillions of a meter).

"Valence" electrons are those that are shared by atoms when they form chemical bonds. Chemical bonds are what link atoms together to form molecules. Valence electrons can be relatively localized (e.g. only occupying the space between 2 atoms) or they can be spread out over the entire molecule. The relatively localized valence electrons are spread out over a space of maybe 2 to 5 angstroms. The more delocalized could be spread out over a range of typically 8-1000 Angstroms. (1000 Angstroms = 100 nanometers = 100 billionths of a meter).

To understand the chemical bond, imagine you had 2 atoms near each other. These atoms start out neutral - they still have all their electrons. When a chemical bond forms, some of the electrons from each of the atoms begin to take up residence in the space directly in between the 2 atoms. This means that effectively, each of the atoms has become slightly positively charged. And now there is a negative "cloud" of charge directly in between them. The "cloud" of negative charge draws the slightly positively charged atoms in, until the force of their mutual repulsion balances it out. This is then a chemical bond - it is when these two atoms share some electrons with each other. Technically this is called a "covalent bond" - there are other types of chemical bonds as well. But generally it is shortened to "bond" and calling it a chemical bond is also legit, if slightly incorrect.

So there are both valence and core electrons in your thumb. It is the valence ones that are going to come into contact with other objects first when you press your thumb against something. The molecules in your thumb are such these valence electrons are the kind that are relatively localized kind - so they're only spread out by about 2-5 Angstroms. So they're not all over you, they generally don't get around.

That makes sense.

So what's going on with the atoms when someone dies and starts to turn to mush? Why do they lose their stickyness?

Living cells are constantly building new molecules (proteins, carbohydrates, etc.) that they then use to repair/maintain their cellular structures (cell walls, cell membranes, various apparatus within the cell).

So when something dies, that stops. So for example, a protein in the cell membrane gets damaged. Well, instead of being removed and replaced, it falls apart. This would affect a whole region of the membrane. Now the membrane isn't functioning effectively. Stuff is getting into and out of the cell that shouldn't. This, I would imagine would accelerate the destruction of the initial failed protein, and let in stuff that would also destroy other molecules in the cell, or other molecules in the membrane.

So fundamentally, it's not that the physics/chemistry changes. It just that the cell is no longer carrying out its chemistry to repair and maintain its structure & function anymore, so it falls apart.

This isn't a great explanation, but I know practically nothing about biology. So, if there are any biologists who know about death in the house...

Well the answer is that they don't lose their stickiness. Most electrons and molecules in the universe are very tightly stuck to their host materials. For life to occur we need fluxional molecules - one's that can transformed fairly easily into others and back again at the mild temperatures that occur on earth. This makes most biological molecules highly fragile and most are destroyed quite quickly only to be replaced by new ones (why we have to eat and drink - fuel for this).
So when we transcend to the choir invisible this process ceases and these vulnerable softy molecules just get annihalated by other life forms. Effectively we are eaten by bacteria, moulds and other microscopic organisms (even higher ones like pigs if you're killed by an East London gangster ).

So to summarise, our molecules are relatively 'unsticky' compared to most matter in fact and when we die their stickiness is simply transferred to other forms of life.
Life feeds off life.

So the atoms in dead things stay the same, but the molecules break up.

I was thinking about the surface molecules and the way things oxidize when broken last night as I went to sleep (weird dreams last night!) -- fluids, mercury and things that are heated up til they become fluid-like came to mind. They rejoin when they're put next to each other, and they seem to 'like' making group efforts. Is their surface-molecule structure more like the inner-molecule structure than 'hard' objects? Do their properties resist oxidizing? Or is something totally different going on with them?