Boron atomic properties

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

Table 1 fists many metal borides and their observed melting points. Most metals form mote than one boride phase and borides often form a continuous series of solid solutions with one another at elevated temperatures thus close composition control is necessary to achieve particular properties. The relatively small size of boron atoms facilitates diffusion. 

Properties. Boron carbide has a rhombohedral stmcture consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms ia a linear chain outside the icosahedra (3,4,6,7). Thus a descriptive chemical formula would be [12075-36-4]. 

The eombination of aeid (-OH) and basie (-NH, -N(CH ), -N=N-) groups is also responsible for the properties of generated boron eomplexes. As a rule, sueh eompounds eomprise N B eoordination bond formed due to boron atom striving to eompensate the laek of eleetrons, and to the faet that nitrogen has a lone-eleetron pair on the other. 

The atomic properties of the Group 13 elements (including boron) are compared in Table 7.4. All have odd atomic numbers and correspondingly few stable isotopes. The varying precision of... 

In Chapter 8.34 of CHEC-II(1996) synthesis and properties of the boron-containing heterocycles with ring junction boron atom 1-6 have been considered <1996CHEC-II(8)889>. However, only a few papers have been published during the last decade concerning the systems 1-4 (Figure 1) and so they are discussed in Section 12.13.6. 

Furthermore, we believe that the stabilizing influence of boron in the structure of graphite is connected with enhancement of its acceptor properties, which manifest themselves when Boron atoms substitute carbon atoms in the crystalline structure (hexagon ring) of carbon. Such effects are mentioned in the literature for some types of carbon materials [3] and the influence of boron on TEG can be the similar. 

The Group III elements all have three electrons in their outer shell. Most of them form +3 ions by losing all three electrons. Because the boron atom is so small, its nucleus holds very tightly to all its five electrons. Boron usually combines with other elements which have electrons to share, just as carbon combines. It is called a semi-metal because it has some, but not all, of the metallic properties. 

The dodecahydrododecaborate anion, B H 2-, is termed unique with considerable justification. This ion and its perhalo derivatives, e.g., Bi2C1i22-, are the most symmetrical molecular aggregates known. The boron atoms occupy the vertices of a regular icosahedron and each is bonded terminally to a hydrogen atom all boron atoms are environmentally equivalent.7,8 This anion is the only known example of the 7 symmetry group.8 General spectral, physical, and chemical properties of Bx2Hi22-are detailed in a paper by Muetterties et al.9... 

The amount of boron required for BNCT can be estimated using the neutron capture cross sections, which are atomic properties, and thus pertain to the number, and not the mass, of the atoms present. Conservative estimates for successful therapy result in boron concentrations of around 20 ppm in tumor tissue, to at least match the dose liberated by neutron capture reactions in the other elements of biological tissue. This would correspond to around 109 boron-10 atoms per cell, assuming that one cell corresponds to 10-9 g. 

Scandium - the atomic number is 21 and the chemical symbol is Sc. The name derives from the Latin scandia for Scandinavia , where the mineral were found. It was discovered by the Swedish chemist Lars-Fredrik Nilson in 1879 from an ytterbium sample. In the same year, the Swedish chemist Per Theodore Cleve proved that scandium was Mendeleev s hypothetical element eka-boron , whose properties and position in the Period Table Mendeleev had previously predicted. 

Stable free carbenes derived from four- (13), six-, or seven-membered heterocycles 14 are also known. Carbenes of type 13 can possess a phosphorus [19] or a boron atom [69] within the heterocycle. These carbenes exhibit extreme values regarding their NMR spectroscopic ( carbene-c 285 and 312.6 ppm) and structural properties (angle N-Ccarbene-N 96.72(13) and 94.0(2)°). 

Properties. Boron carbide has a rhombohedral structure consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms in a linear chain outside the icosahedra (3,4,6,7). Thus a descriptive chemical formula would be B12C3 [12075-36 4], Each boron atom is bonded to five others in the icosahedron as well as either to a carbon atom or to a boron atom in an adjacent icosahedron. The structure is similar to that of rhombohedral boron (see Boron, elemental). The theoretical density for B12C3 is 2.52 g/mL. The rigid framework of... 

As noted above, the hardness or softness of an acidic or basic site is not an inherent property of the particular atom at that site, but can be influenced by the substituent atoms The addition of soft, polarizable substituents can soften an otherwise hard center and the presence of electron-withdrawing substituents can reduce the softness of a site. The acidic boron atom is borderline between hard and soft. Addition of three hard, electronegative fluorine atoms hardens the boron and makes it a hard Lewis acid. Conversely, addition of three soft, electropositive hydrogens54 softens the boron and makes it a soft Lewis acid. Examples of the difference in hardness of these two boron acids are... 

---