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Both diamond and graphite are covalent networks and are both made entirely from carbon, but why does diamond have a three dimensional network of strong covalent bonds which makes it hard, whereas graphite has flat layers of carbon atoms which makes it a weak object and breakable.

This seems strange to me, because they both are made of the same atoms and both are covalent networks.

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    $\begingroup$ Why do USA and Cuba have different governments, when they are both composed of people? $\endgroup$ – Ivan Neretin Mar 19 '18 at 9:18
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    $\begingroup$ @IvanNeretin people are not identical, but carbon atoms are identical, that's why te OP is confused. The answer is because: 1) they were formed in different conditions, 2) the convertion from one to another is very slow.. $\endgroup$ – santimirandarp Mar 19 '18 at 11:51
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    $\begingroup$ @santimirandarp OK, forget people, look at two very different buildings built from similar bricks. Identical parts do not imply an identical whole, not even tentatively; that's my point. $\endgroup$ – Ivan Neretin Mar 19 '18 at 12:11
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    $\begingroup$ Why are you asking this question? When a simple Google search would give you like a thousand answers have you researched this at all? Or maybe you know the answer but your just looking for points. This is far too basic to be on the Chemistry Stack Exchange, surely... $\endgroup$ – Jalapeno Mar 19 '18 at 16:41
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    $\begingroup$ @IvanNeretin : Perhaps LEGO blocks would be an ideal analogy. $\endgroup$ – Eric Towers Mar 19 '18 at 19:02
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Chemical structures are a tradeoff of several factors, including the conditions on how they were formed.

The stability of any given chemical structure depends on the ease with which any specific reaction can turn it into something else. Both graphite and diamond are very stable structures which basically means they are hard to easily convert into something else. Another way to think about this is that to convert either into a different allotrope (different versions of the same element) of carbon requires a lot of bonds to be broken and rearranged and this requires a big input of energy. That's why both are stable.

But why is there more than one stable allotrope of carbon? Note that there are more allotropes than just graphite and diamond: buckminsterfullere is also a stable allotrope of carbon with a completely different structure that involves molecules of 60 carbons not a covalent network solid. The basic answer is that there are a lot of ways to arrange the atoms to give a stable structure (partially because there are a lot of options for stable C-C bonds given the chemistry of carbon). This is related to the same reason that organic chemistry is a big subject: C-C bonds are very "stable" and can be organised to give a huge variety of structures.

Not every arrangement of C-C bonds is equally stable: many will fall apart into other structures because there are mechanisms that make the rearrangement easy. But there are no easy mechanisms for rearranging the bonds in diamond or graphite into something more stable. Buckminsterfullerene is only formed from a plasma of carbon atoms when it is allowed to cool: you can't just make it by simple routes from other allotropes of carbon.

Both graphite and diamond have very similar energies of formation at normal conditions, though graphite is marginally more stable (the particular arrangement of bonds has a slightly lower energy than the arrangement in diamond under normal conditions). But this is not true at all conditions. Under extreme pressures, diamond is more stable. And this is how diamond is formed: it crystallised from solution in hot melted volcanic rocks deep in the earth containing lots of carbon and under high pressure. In these conditions, diamond is the more stable way to arrange carbon atoms and the conditions are such that the less stable form will covert to the more stable form because there is enough energy around to break the C-C bonds until the most stable structure is created.

Normally (by which I mean at atmospheric pressure) carbon rich compounds can be pyrolised to give pure carbon in the form of graphite. Here the heat drives off the non-carbon atoms and the carbon atoms rearrange to the form stable at lower pressures.

Once formed neither major allotrope is easy to convert to the other without extreme conditions. Which is why you can wear diamond jewellery without constant anxiety.

In short the stable allotropes of carbon are a function of the way they were made. And there are a variety of ways to join carbon atoms up to give different structures. Once formed they don't easily interconvert. Hence we find a variety of allotropes depending on the variety of conditions that led to their formation.

NB carbon isn't the only element with multiple allotropes. Tin, for example, changes crystal structure and properties depending on the temperature (and, unlike carbon, this interconversion is fast enough to have been a problem in the era when tin cans were actually made of tin and stored in very cold conditions).

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    $\begingroup$ Good explanation, but the tin cans part is misleading. Tin cans are actually steel cans with tin plating. The tin plating still may crumble if cooled, but the effect is to render the steel more prone to corrosion over time, not a catastrophic failure. $\endgroup$ – Oscar Lanzi Mar 19 '18 at 13:12
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    $\begingroup$ Nice answer..Also sulphur has many allotropes $\endgroup$ – santimirandarp Mar 19 '18 at 13:23
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    $\begingroup$ @OscarLanzi That's what some cans are made of today (some are aluminium). But the original cans were made of tin. $\endgroup$ – matt_black Mar 19 '18 at 13:46
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    $\begingroup$ Another relevant issue: "stable" is in most cases relative to time scale and precise definition of the system in question. A diamond sitting in an idealized heat bath at room temperature and under 1 atm of inert atmosphere will, according to thermodynamics, spontaneously convert to graphite eventually, because graphite has slightly less free energy. But the time scale is so staggeringly massive that no one cares. $\endgroup$ – Ian Mar 19 '18 at 22:46
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There are various stable structures for pure carbon. As the 1st answerer said, we now know that Buckyballs and Carbon Nanotubes are other pure carbon structures. Carbon nanotubes come in both single-walled and multi-walled varieties and can be both conductors and semiconductors [https://www.sigmaaldrich.com/technical-documents/articles/materials-science/single-double-multi-walled-carbon-nanotubes.html].

So, graphite forms sheets of graphene, 1 atomic layer thick, with the carbon atoms in a flat hexagonal lattice. There are also interactions between the graphene layers due to stacking energies. Hexagonal lattices were by far the most common form of atomic packing seen in early images from the STM - the scanning tunneling microscope.

And diamond has a tetrahedral covalent packing, which is reasonable for an atom that typically forms 4 covalent bonds and is the essential element for all of organic chemistry.

This piece in Scientific American has a nice description of the structural and other differences between graphite and carbon https://www.scientificamerican.com/article/how-can-graphite-and-diam/

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