Atoms, existence

The nitroparaffiiis in which the nitro group is attached to a primary or secondary carbon atom exist in tautomeric forms, for example ... 

The degree of ionization increases with temperature, and at 6000-8000 K, where ionization efficiencies are 90 to 95%, nearly all atoms exist as ions in the plasma. Operation of the plasma torch under conditions that produce a cooler flame (cold plasma) has important advantages for some applications, which are discussed after the next section. 

A) Cold vapour technique. This procedure is strictly confined to the determination of mercury,45 which in the elemental state has an appreciable vapour pressure at room temperature so that gaseous atoms exist without the need for any special treatment. As a method for determining mercury compounds the procedure consists in the reduction of a mercury(II) compound with either... 

Since at the absolute zero the kinetic energy is zero, the maximum work is the sum of the differences between the potential energies of all the atoms before and after the reaction. By the motion of the atoms which corresponds to a definite elevation of the temperature above absolute zero these potential energies are evidently changed. The above equation requires that this change shall be either infinitely small, or independent of the state in which the atom exists. ... 

Today, modern instrumentation provides much more direct evidence of atoms (Fig. B.3). There is no longer any doubt that atoms exist and that they are the units that make up the elements. In fact, chemists use the existence of atoms as the definition of an element an element is a substance composed of only one kind of atom. By 2006, 111 elements had been discovered or created but in some cases in only very small amounts. For instance, when element 110 was made, only two atoms were produced, and even they lasted for only a tiny fraction of a second before disintegrating. 

Structural evidence Only one dichlorobenzene in which the two chlorine atoms are attached to adjacent carbon atoms exists. 

It is found that chromium atoms, manganese atoms, and tin atoms exist in metals in two forms, a small, high-valent form and a larger, low-valent form. The two kinds of manganese atoms coexist in a-manganese and in /3-manganese. 

Cyclic monomers with five- and six-membered ring atoms exist in strainless puckered conformations their heats of polymerization are either negative or have small positive values due to the repulsion of eclipsed hydrogen atoms. Because the nthalpy and entropy contributions are comparable, the free energies of polymerization are either positive or may become positive at high temperatures. 

With the exception of ionic sulfides formed from highly electropositive elements (i.e., Na, K, Ca, Mg), sulfur bonding in natural environments is covalent. When fully oxidized, however, the covalently bonded sulfur atom exists... 

A cluster with five Au atoms exists in Au5(dppm)3[(Ph2P)2CH]KN03)2. This cluster has a AU4 core, with an additional Au incorporated in the phosphine ligand, linearly coordinated between a phosphorus and a carbon from the methanido bridge (IV) ... 

Very few reports concerning transformations of ligands with other donor atoms exist (Table V). P—H activation at secondary phosphines is the most common motif, with the metal-metal bonds at the heterometallic faces stabilizing the resulting fragments in each case (Figs. 31. 32, 33).In the formation of both... 

Cupric sulfide, CuS, occurring as the mineral covellite (also known as covelline), exhibits a very unusual structure, in which the Cu is again partly 3-coordinate and partly 4-coordinate, with two-thirds of the sulfur atoms existing as S2 groups like those in pyrites. The low-temperature form of CuSe has also a covellite structure, the high-temperature modification (P-CuSe) being orthorhombic. All CuX2 compounds assume pyrite-type structures. 

Figure 13. Normalized catalytic activity (in mmol H2 per mmol surface Pd per s) as a function of metal composition of PVP-stabilized Pd/Pt bimetallic nanoparticles. The normalization was determined by the number of Pd atoms on the surface of the nanoparticle, assuming that Pd atoms exist selectively on the surface. (Reprinted from Ref. [48], 1993, with permission from Royal Society of Chemistry.)...

In 1804, John Dalton proposed the existence of atoms. He not only postulated that atoms exist, as had ancient Greek philosophers, but he also attributed to the atoms certain properties. His postulates were as follows ... 

In general, the Henry reaction gives a mixture of diastereomers and enantiomers. The lack of selectivity is due to the reversibility of the reaction and the easy epimerization at the nitro-substituted carbon atom. Existing reviews have hardly mentioned the stereochemistry of the Henry reaction. Recently, Shibasaki has found that the modification of the Henry reaction can control the stereochemistry to give (3-nitro alcohols with high diastereo- and enantio-selectivity.6 In Section 3.3, the progress of the stereoselective Henry reaction and its application to biologically active compounds are discussed. 

Enantiomer A molecule with a chiral centre (asymmetric carbon atom) existing as two isomers designated D and L that may have different physiological or pharmacological activities. 

An extensive coverage of the reactivity of substituents attached to the 1,2,5-thiadiazole ring carbon atoms exists in both CFIEC(1984) and CFIEC-II(1996). Recent developments are described in this section. 

The question whether atoms exist or not.. . belongs rather to metaphysics. In chemistry we have only to decide whether the assumption of atoms is an hypothesis adapted to the explanation of chemical phenomena. . . [and] whether a further development of the atomic hypothesis promises to advance our knowledge of the mechanisms of chemical phenomena.. .. I rather expect that we shall some day find, for what we now call atoms, a mathematico-mechanical explanation, which will render an account of atomic weight, of atomicity, and of numerous other properties of the so-called atoms. 50... 

Thinking Critically How can the existence of spectra help to prove that energy levels in atoms exist ... 

---