Need some help
Hi,I am chelsea.I major in urban and rural planning.Now I find a job in a chemical trade platform(lookchem) .I do not know chemistry very well,but I want to make up for it.So if you are kind enough,please tell me how to learn chemistry and where to find the imformation about chemicals.Thank you very much.

The units of resistivity don't come out right.

10^-8 Ohm * m
or
m Ohm * cm

the 'm' should be a 'mu', but unfortunately they both look the same in the latin alphabet.

This stamp commemorates the death of Mendeleev (February 1907), one of the lead figures responsible for the periodic table. Absolutely excellent choice of colours if I might say so! The stamp was sent to me by Prof Gabriel Pinto (Departamento de Ingeniería Química Industrial, ETSI Industriales, UPM, Madrid, Spain) and I quote from his web page:

Scientists at the Lawrence Berkeley National Laboratory in California, USA, have discovered that nanocrystals of germanium embedded in silica glass don't melt until the temperature rises almost 200 degrees Kelvin above the melting temperature of germanium in bulk. What's even more surprising, these melted nanocrystals have to be cooled more than 200 K below the bulk melting point before they resolidify. Such a large and nearly symmetrical "hysteresis" -- the divergence of melting and freezing temperatures above and below the bulk melting point -- has never before been observed for embedded nanoparticles.

Only carbon from the Group 14 elements forms stable double bonds with oxygen under normal conditions. When frozen, carbon dioxide is known as "dry-ice". A non-molecular single-bonded crystalline form of carbon dioxide (phase V) exists at high pressure.

Amorphous forms of silica (a-SiO₂) and germania (a-GeO₂) are known at ambient conditions but only recently has an amorphous, silica-like form of carbon dioxide, a-CO₂. This is labelled a-carbonia and made by compression of CO₂ at room temperature at pressures between 40 and 48 GPa (that's a staggering 400-500 thousand atmospheres).

Group 14 periodicity

This article addresses the periodicity displayed by the Group 14 elements but excluding, largely, ununquadium (element 114) about which virtually nothing is known. One could predict the properties of ununquadium based upno those of the higher elements and this is left as an exercise for the reader.

Nature of the elements

The elements become increasingly metallic down the group. Carbon, at the top, is a typical non-metal while silicon is a semiconductor profoundly important to the electronics industries. Tin and lead are very metallic although one modification of tin known as grey tin has the same diamond structure as does germanium and silicon. The elements lower down the group form complexes while carbon does not. The melting points of the elements decrease down the group as the elements become increasingly metallic.

Multiple bonds

Carbon often forms multiple bonds, both with itself (as in ethene and ethyne) and with other elements such as oxygen (as in carbon dioxide and ketones). In contrast, silicon, germanium, and tin only form analogues of ethene (albeit non-planar) when the elements possess bulky substituents. While the C=C π-bond formed through the overlap of C 2p-orbitals is strong, those lower down the group are much less strong. This also explains why graphite is stable while there are no analogues of graphite lower down the group. Carbon dioxide, CO₂, possesses two carbon-oxygen double bonds (O=C=O) while the corresponding silicon dioxide, SiO₂, possesses an extended lattice structure. This is because the π-bond formed through the overlap of p-orbitals on carbon and oxygen is strong as the overlap is favourable, while lower down the group the π-overlap is less efficient.

Hydrides

The hydrides MX₄ are known for all the elements except ununquadium although the lead compound (plumbane, PbH₄) is poorly characterized. Each is a covalent molecule. The parent hydride for carbon is methane, CH₄, and there is an extensive range of compounds called alkanes of the type C_nH_2n+2 (methane, ethane, propane, butane....). There are relatively few of the corresponding silicon hydrides (silanes) and they are spontaneously flammable. The germane GeH₄ is known while the stannane SnH₄, a colourless gas, decomposes to tin at about 0°C.

Halides

Two types of halide for this group are known: MX₂ and MX₄. The M(IV) halides dominate the top of the group while the M(II) halides dominate at the bottom. All the M(IV) halides MX₄ (M = C, Si, Ge; X = F, Cl, Br, I) are all known for the three elements carbon, silicon, and germanium at the top of the group. However, as the group is descended, the stability of the M(II) state increases relative to the M(IV) state. None of the dihalides MX₂ exist independently for carbon or silicon while most of the divalent halides MX₂ are known for germanium in addition to the germanium tetrahalides. At the bottom of the group the most stable lead halides are PbX₂ and the only known tetrahalide seems to be PbCl₄ (this decomposes exothermically to PbCl₂ and chlorine gas).

Oxides

Ionization Energy

Workers at The University of Wisconsin-Madison in the USA have managed to release thin membranes of semiconductors from a substrate and transfer them to new surfaces. The freed membranes which are just tens of nanometers thick retain all the properties of silicon in wafer form but the nanomembranes are flexible. By varying the thicknesses of the silicon and silicon-germanium layers composing them, membrane shapes are possible ranging from flat to curved to tubular.

Potential applications include flexible electronic devices, faster transistors, nano-size photonic crystals that steer light, and lightweight sensors for detecting toxins in the environment or biological events in cells.

The scientists made a three-layer nanomembrane composed of a thin silicon-germanium layer sandwiched between two silicon layers of similar thinness. The membrane sat upon a silicon dioxide layer in a silicon-on-insulator substrate. The nanomembranes may be etched away from the oxide layer with hydrofluoric acid.

Although the Wisconsin team grew their nanomembranes on silicon-on-insulator substrates, the method should apply to many substances beyond semiconductors, such as ferroelectric and piezoelectric materials. The key requirement is a layer, like an oxide, that can be removed to free the nanomembranes.