Mohs elements

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

This rule conforms with the principle of equipartition of energy, first enunciated by Maxwell, that the heat capacity of an elemental solid, which reflected the vibrational energy of a tliree-dimensional solid, should be equal to 3f JK moH The anomaly that the free electron dreory of metals described a metal as having a tliree-dimensional sUmcture of ion-cores with a three-dimensional gas of free electrons required that the electron gas should add anodier (3/2)7 to the heat capacity if the electrons behaved like a normal gas as described in Maxwell s kinetic theory, whereas die quanmtii theory of free electrons shows that diese quantum particles do not contribute to the heat capacity to the classical extent, and only add a very small component to the heat capacity. 

The most important nitride of Si is Si3N4 this is formed by direct reaction of the elements above 1300 or more economically by heating Si02 and coke in a stream of N2/H2 at 1500". The compound is of considerable interest as an engineering material since it is almost completely inert chemically, and retains its strength, shape and resistance to corrosion and wear even above 1000°. ° Its great hardness (Mohs 9), high... 

MSH + MOH). Accordingly, solubilities depend sensitively not only on temperature but also on pH and partial pressure of H2S. Thus, by varying the acidity. As can be separated from Pb, Pb from Zn, Zn from Ni, and Mn from Mg. In pure water the solubility of Na2S is said to be 18.06g per 100 g H2O and for Ba2S it is 7.28 g. In the case of some less-basic elements (e.g. AI2S3, Cr2S3) hydrolysis is complete and action of H2S on solutions of the metal cation results in the precipitation of the hydroxide likewise these sulfides (and SiS2, etc.) react rapidly with water with evolution of H2S. 

Silvery-white, brittle metallic element crystal system-hexagonal, rhombo-hedral also, exists in two unstable allotropic forms— a yellow modification and a dark-grey lustrous amorphous powder—both of which revert to crystalline form hardness 3.0 to 3.5 Mohs density 6.697g/cm3 melting point 630.5°C boiling point 1635°C electrical resistivity 39.1 microhm-cm at 0°C magnetic susceptibifity —0.87 x 10 emu/g. 

CdAs2, obtained by fusing the elements in the requisite proportions under fused alkali chloride,10 is bluish-grey in colour, of density 5-86 at 20° C., and of hardness 3-5 to 4-0 on Mohs scale. It melts at 621° C. 

Element Hardness in units of 10-degree Mohs scale measured at 293 K Theoretical j hardness j calculated j Additional from formulal characterization of sample W1 ) = a- r/ y Measure- ment tempera- ture K Measured hardness ... 

CHROMIUM. [CAS 7440-47-3[. Chemical element, symbol Cr. at. nil. 24. at. wi. 51.996, periodic table group 6. mp 1837- 1877°C. bp 2672 C, density 7.2 g/cm. Elemental chromium has a body-centered cubic crystal structure The metal is silver-white with a slight gray-blue tinge, very hard (9.0 on the Mohs scale), capable of taking a brilliant polish, not appreciably ductile or malleable. The element is not aflected by air or HyO al ordinary temperatures, but when heated above 200°C, chromic oxide CryOt is formed. There tire four stable isotopes f0Cr. and, Cr through wCr. Fuur radioactive isotopes have been identihed, all with comparatively short half-lives 4 Cr. Jl,Cr. 1 Cr. and 5Cr, The element was first identified by Vauquelin in 1797. 

Reaction with Water The most well-known and dramatic reaction of the alkali metals is with water to yield hydrogen gas and an alkali metal hydroxide, MOH. In fact, it s this reaction that gives the elements their group name The solution of metal hydroxide that results from adding an alkali metal to water is alkaline, or basic. 

In carbides, carbon is bound to elements with lower or similar EN-values. We distinguish three types of carbides. The salt-like carbides with elements from groups 1, 2 and 3 are decomposed by water A14C3 +12 H20 — 4 Al(OH)3 + 3 CH4. In addition, there are the covalent carbides like SiC and B4C and a intermediate group with most transition metals. In the intermediate group C atoms are located in the octahedral cavities of metal close packings. The melting points vary from 3000 to some extreme values of about 4800 °C and their hardness lies between 7 and 10 on the Mohs scale. Furthermore, the... 

Monoxide (MO+) and hydroxide (MOH+) ions, where M can be any one of many elements, are observed in ICP-MS [140], Typically the molecular oxide or molecular hydroxide signals are small (<3%) relative to the elemental ion signal. However, if one is trying to measure a small concentration of one element in the presence of a high concentration of a second element that forms a molecular oxide or hydroxide ion at the same mass as an analyte, the problem can be severe. Furthermore, the molecular ions may overlap with an elemental ion isotope that is... 

See Eydberg, Zeitsch. physikal. Chem., 1900,33,353. The system, which is based on Mohs scale, gives a numerical value for the hardness of the elements. 

Figure 6.4. Hydrolysis of metal ions, (a) Predominant pH range for the occurrence of aquo, hydroxo, hydroxo-oxo, and 0x0 complexes for various oxidation states. The scheme attempts to show a useful generalization, but many elements cannot be properly placed in this simplified diagram because other factors, such as radius and those related to electron distribution, have to be considered in inteipreting the acidity of metal ions, (b) The linear dependence of the log,o of the first hydrolysis constant Ki = MOH H / M on the ratio of the charge to the M-O distance (z/d) for four groups of cations (25°C). (Note change of abcissa zero for different groups.) (From Baes and Mesmer, 1976.) (c) Hydrolysis constants of some important metal ions.

Table 6-4 gives, in the first column, standard values of Gibbs energies of formation from the elements AGj° for a variety of pure solids, gases, and liquids as well as values for substances in solution at the hypothetical 1M activity As an example, consider the value of AGj ° for pure liquid acetic acid, -389.1 kj moH. The equation for its formation from the elements is ... 

The densities and atomic volumes are normal for the places occupied in the Periodic Table. Boron s extremely high m.p. indicates very strong binding forces the structure of several crystalline forms of pure boron have been clearly established. Crystals of the purest material are very hard, 9-10 on Mohs scale. The specific conductance increases about 100 times between 20 and 600 . Aluminium has a low m.p. compared with neighbouring elements its face-centred cubic lattice is characteristic of a true metal it is soft, and its conductance is high. 

Element Flame test Hardness (Mohs scale) Melting Point (°C) Boiling Point ( C) Density (g/cm ) Atomic radius (pm)... 

Many different complexes of elements in a given oxidation state may exist in water. The amphoteric nature of Al(III) and Fe(III) results from the formation of a series of dissolved species, MOH, M(OH)J, M(0H)3, and other forms, in addition to the more common M ". The speciation of soluble A1 and Fe is thus a sensitive function of pH. 

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