35

Atoms at the edge of a crystal that have an unsatisfied valence are said to have "dangling bonds." Many elements, in addition to carbon, can have dangling bonds. Dangling bonds is a subject of current interest because of the impact these structures can have on semiconductor properties. These dangling bonds are very similar to free radicals, except since ...


34

Crystals have inspired a great many chemists because they are fascinating for a good reason. Not only are they aesthetically pleasing, but they serve as an excellent subject to tour a variety of theoretical subjects important for understanding high-level chemistry. Crystalline materials are made up of periodic structures. We’re only going to primarily focus ...


26

Is it crystallization? You are correct. The main difference is that sand is crystalline and glass is not—it is amorphous. The main component (> 95%) of common yellow sand is quartz (the mineral whose composition is SiO2). Note that not all sand is quartz. There are white sands containing calcite (CaCO3) and black sand (containing various heavy minerals). ...


25

Diamond is a covalent network solid, like a number of other common materials (quartz, graphite, glass, and a whole bunch of stuff). Because they are not discrete molecules - there is no 'diamond' molecule the same way there are molecules of caffeine, benzoic acid, citric acid, N,N-dimethylaminopyridine, etc. - network solids form one of the two main classes ...


22

It turns out that 2 identical snowflakes have been observed, but... Two Identical Snowflakes Although when we think "snowflake" we usually picture an object with 6-fold radial symmetry, snowflakes actually come in many different shapes (reference 1). Many sites report that in 1988 an NCAR researcher found 2 identical snowflakes of the hollow columnar ...


20

Diamond has dangling bonds on the outer surface of the crystal for pretty much the same reason as graphite. If you understood graphite differently, then you understood it wrong. See, a molecule of oxygen contains 2 atoms, a molecule of sulfur has 8; but how many atoms are there in a "molecule" of diamond or graphite? Try drawing one to the end, so as to ...


19

Boron is a covalent solid with high melting point, like diamond (though not quite), and hence its crystals are hard to make. Unlike diamond crystals, they are not nice and probably wouldn't make a great display. The table on http://periodictable.com/Properties/A/MolarVolume.v.log.html seems to corroborate your findings about boron molar volume being the ...


18

Quartz and diamond are stronger substances because their molecules form network covalent structures. These structures form a lattice-like structure, much the same as ionic compounds. This molecular network is also the reason that diamond and quartz form a crystalline structures, just like you'd see in ionic substances such as NaCl. Some other structures you ...


18

You asked a question, belonging to surface chemistry. It is a relatively new area of research, as it relies heavily on atomic-resolved microscopy and computational methods. Generally, the answer depends on prehistory of the surface and its environment. In case you crack a diamond, making new surface, two processes happens. so-known reconstruction, or ...


16

All quotes will be from Solid State Physics by Ashcroft and Mermin. Bravais Lattice: A fundamental concept in the description of any crystalline solid is that of the Bravais lattice, which specifies the periodic array in which the repeated units of the crystal are arranged. The units themselves may be single atoms, groups of atoms, molecules, ions, etc.,...


15

The thermochromism of $\ce{ZnO}$ results from a minor loss of oxygen upon heating to temperatures around 800 °C, i.e. a non-stoichiometric $\ce{Zn$_{1+x}$O}$ with $x = 7 \times 10^{-5}$ is formed. Under air, this effect is reversible. Heating (and cooling) of the material while hooked up to a vacuum pump might result in a more persistent colour change.


14

It is quite well established that stoichiometric gallium arsenide (CAS 1303-00-0) forms a zincblende analogous structure. It consists of two stacked face-centered cubic lattice systems (Ioffe Physical Technical Institute database). See also Uncle Al's Answer. In other words, the arsenic forms the lattice and the gallium fills half of the tetrahedral ...


13

$\ce{N5P3}$ is more commonly written as $\ce{P3N5}$, and known as triphosphorus pentanitride. It's a crystalline solid at ambient conditions and not a molecular compound. From the first publication that reported the pure compound and its structure [1]: In the solid a three-dimensional cross-linked network structure of corner sharing $\ce{PN4}$ tetrahedra ...


13

The two-dimensional representations you have given are inadequate and should not be taken too seriously; crystal structures are nearly always three-dimensional. Gallium arsenide has the cubic zincblende structure, shown below. Here, blue atoms can are Ga and orange atoms As (it can be the other way round, too, as these atoms are related by symmetry). (Image ...


13

Boron pentachloride is likely not stable except perhaps in extreme conditions, such as under very high pressures. Even then it may be possible that a description such as $\ce{[BCl4^{-}]Cl^+}$ containing a tetrahedral boron anion could turn out to be more accurate than any hypercoordinate structure (a boron atom surrounded by more than four ligand atoms). ...


12

Step 1: Look up Nickel Carbonyl and find out what geometry it has. We need the geometry to know how the $d$ orbitals will split in the ligand field. The geometry can also be predicted: late transition metals with strong field ligands tend to be tetrahedral. Step 2: Find the appropriate crystal field splitting diagram for this geometry. Step 3: ...


12

This is a nice question. Although I do not speak with authority on diamonds, most crystal surfaces have several imperfections which mitigate the hanging valencies that would otherwise appear for the atoms (constituents in general) at the surface. Impurities. This is the primary method which helps satisfy the unfulfilled valencies at the surface. Many ...


12

When salt (NaCl) crystals are grown under normal conditions, the edges of the cubes usually grow faster than the faces (because the edges have more contact with the saturated salt solution than the faces). So the crystals grow with a square-pyramidal indentation on each face. The "X" you observe is your view of the edges of that inverted pyramid. On the ...


12

They are potassium bitartrate crystals (source). The crystals form because the potassium bitartrate is not very soluble. Since solubility is a function of temperature, when wine is chilled the solution can become saturated, causing the precipitate to form in the bottle. Stabilizers are often added to prevent this. In your case, the precipitate is on the ...


12

Ionic crystals are hard because of tight packing lattices, say, the positive and negative ions are strongly attached among themselves. So, if mechanical pressure is applied to an ionic crystal then ions of similar charges may be forced to get closer to each other. Now, by doing so, the electrostatic repulsion can be enough to split or disorient completely ...


12

OK, let's get it straight. A lattice is just a periodic set of points. For the sake of brevity, I'll talk about 2D lattices; trust me, with 3D it is pretty much the same, only more complicated. Initially the lattices were just like that: periodic patterns of one infinitely repeating parallelogram, called the unit cell. Other than that, they were without ...


12

The unit cell for graphene is a two-dimensional rhombus according to the figure shown on page 31 of this paper.$^1$ (also here.) The result is that two atoms are contained per unit cell. The upper right structure actually appearing in graphite, stacked layers of graphene. $^1$Zhou, J; Huang, R. Internal lattice relaxation of single-layer graphene under in-...


12

To calculate the height of a unit cell, consider a tetrahedral void in an hexagonal closed packing arrangement. It can be imagined as a 3 solid spheres touching each other and at the center-point, you have another sphere stacked over them. An interactive version can be viewed on this site. The situation looks like this: If you join the centers of these ...


11

With gas no absolutely. To preserve ice at room temperature using the pressure of a gas you should reach a pressure of one billion Pascals. Too dangerous for Penny safe... In fact there are some imprecisions in the video Leonard cites two different methods to "preserve" snow flakes with an organic polymer. In fact these methods create a snow flakes replicas,...


11

Let's look at structure of $\ce{NaCl}$ ideal cubic crystal As you can see, atoms on the faces have 5 neighbors, atoms on the edges has four neighbors, and atoms on the vertices of the cube have 3 neighbors. This means, that a) edges and vertices pulls new atoms into crystal structure stronger and b) they 'collect' new atoms from larger volume, than faces. ...


11

You've already answered your own question - hydrogen bonding. First, don't think of it as why is the gas state different than solid state. Rather, think about why is the solid state different than the gas state. In the gas state there is very little intermolecular interaction (very, very few species do not follow the ideal gas law to many significant ...


11

In the crystal, carboxylic acids mostly form dimers through pairs of $\ce{OH\bond{~}O=C}$ hydrogen bonds. In some cases, infinite chains of hydrogen bonds are formed instead: here, each carboxylic acid forms hydrogen bonds to two different neighbours. These structures are known as catemers. (Please note that the depiction of the catemer is very much ...


11

The picture you showed does have an unequal number of sodium cations and chloride anions. However, the picture shows only part of a crystal. Every atom that is on a boundary of the shown cube, whether on a face, edge, or vertex of the cube, is shared with other "cubes" in the crystal that aren't shown in the picture. Each of the 8 corner Cl atoms in your ...


11

There are a few reasons, but the most direct answer is that the wavelength of X-ray photons is on the order of the distance between atomic nuclei in solids, e.g. ~ 4 ångströms (bonds are roughly 1.5-2.5 Å). You can think of it like the waves fit nice and snugly between the atoms and "fill" the crystal and thereby give us information about where the "cavities"...


Only top voted, non community-wiki answers of a minimum length are eligible