Boron atomic radius

The data in Table 7.1 show that, as expected, density, ionic radius, and atomic radius increase with increasing atomic number. However, we should also note the marked differences in m.p. and liquid range of boron compared with the other Group III elements here we have the first indication of the very large difference in properties between boron and the other elements in the group. Boron is in fact a non-metal, whilst the remaining elements are metals with closely related properties. 

The radius of the 24-coordinate metal site in MBs is too large (215-225 pm) to be comfortably occupied by the later (smaller) lanthanide elements Ho, Er, Tm and Lu, and these form MB4 instead, where the metal site has a radius of 185-200 pm. The structure of MB4 (also formed by Ca, Y, Mo and W) consists of a tetragonal lattice formed by chains of Bs octahedra linked along the c-axis and joined laterally by pairs of B2 atoms in the xy plane so as to form a 3D skeleton with tunnels along the c-axis that are filled by metal atoms (Fig. 6.11). The pairs of boron atoms are thus surrounded by trigonal prisms of... 

Boron trichloride, a colorless, reactive gas of BC13 molecules, behaves chemically like BF3. However, the trichloride of aluminum, which is in the same group as boron, forms dimers, linked pairs of molecules. Aluminum chloride is a volatile white solid that vaporizes at 180°C to a gas of Al2Cl6 molecules. These molecules survive in the gas up to about 200°C and only then fall apart into A1C13 molecules. The Al,CI6 molecule exists because a Cl atom in one AlCI, molecule uses one of its lone pairs to form a coordinate covalent bond to the Al atom in a neighboring AICI molecule (33). This arrangement can occur in aluminum chloride hut not boron trichloride because the atomic radius of Al is bigger than that of B. 

Boron forms perhaps the most extraordinary structures of all the elements. It has a high ionization energy and is a metalloid that forms covalent bonds, like its diagonal neighbor silicon. However, because it has only three electrons in its valence shell and has a small atomic radius, it tends to form compounds that have incomplete octets (Section 2.11) or are electron deficient (Section 3.8). These unusual bonding characteristics lead to the remarkable properties that have made boron an essential element of modern technology and, in particular, nan otechn ol ogy. 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

Boron, with a covalent radius of 0.85 A, is too small to coordinate to a porphyrin ligand through all four nitrogen atoms. There are two possible solutions to this problem, either to use a contracted porphyrin-type ligand or to coordinate more than one boron atom to a single porphyrin, and both of these have been realized. 

In the boron trimethyl molecule the boron atom is surrounded by three pairs of valence electrons, which are involved in the formation of single covalent bonds to the three carbon atoms of the methyl groups. An electron-diffraction study10 has shown the molecule to be planar (except for the hydrogen atoms), as would be expected for sp1 hybrid orbitals. The —C distance is 1.56 i 0.02 A, which agrees reasonably well with the value 1.54 A calculated, with the electronegativity correction, by the use of 0.81 A for the boron single-bond radius.11... 

Boron Sulfide. Boron sulfide has essentially covalent bonds. The covalent radius of the boron atom is small so the formation of solid solutions does not appear to be compatible with B2S3. Experimentally we did not observe any solid solution with BeS, ZnS, CdS, and MgS. 

Much of B-N chemistry relates to the fact that a BN unit is isoelectronic with CC and that the electronegativity and atomic radius of carbon are almost exactly intermediate between the values for boron and nitrogen. Consequently, there are many azaborane analogues of hydrocarbons in which B and N replace a pair of carbon atoms, as in 5-II to 5-VIII ... 

If boron is incorporated in the zeolitic framework, its presence leads to a contraction of the imit cell because the atomic radius of the B atom (0.98 A) is smaller than that of the Si atom (1.32 A). The cell parameters and the unit cell... 

The compounds with the antiperovskite LaPdj B-type structure can be considered as a filled-up Cuj Au-type. The boron atoms can be inserted in the Tg octahedra of the binary compound LaPdj (or CePdj in our case), giving rise to a continuous solid solution of composition CePdjB,, with 0 < x < 1 (Parthe and Chabot 1984). Although the atomic radius of B (r = 0.98 Aj is small, other light atoms with similar size, such as Be and Si r = 1.12 A and 1.32 A, respectively) can be also inserted in that... 

Boron has a small atomic radius and a relatively high ionization energy. In consequence its chemistry is largely covalent and it is generaUy classed as a metalloid. It forms a large number of volatile hydrides, some of which have the uncommon bonding characteristic of electron-deficient compounds. It also forms a weakly acidic oxide. In some ways, boron resembles siUcon (see diagonal relationship). 

Boron has a smaller atomic radius than carbon and easily fits into the carbon lattice and can, for example, be found in the diamond lattice. The boron lowers the electrical resistivity and presumably acts as an electron acceptor site and, for valency reasons, some of the joins in... 

Amovilli and March [4] generalized the simple model in March [1] to boron cages using HF calculations. They found that the equilibrium radius of the spheroidal boron cages is proportional to 4n, where n denotes the number of boron atoms in the cluster. The results of Amovilli and March [4] are therefore redrawn in Figure 4.9 to make the above comment quite concrete. 

FIGURE 4.9 Equilibrium radius (in angstroms) of spheroidal boron cages against n, where n is the number of boron atoms. Triangles refer to ab initio computed values. (Redrawn from C. Amovilli and N. H. March, Chem. Phys. Lett. 347,459, 2001.)... 

In the Periodic Table of the Elements, carbon (with an atomic number of six) follows boron (with an atomic number of five) and is just above silicon in the column of Group IVb (see Table 2.1 of Ch. 2). Table 7.1 shows the electronic configuration, the electronegativity, and the atomic radius of these three elements.l lPl... 

As shown in this table, carbon, boron, and silicon have comparable electronic structure. They also have some of the smallest atoms. Silicon and boron are similar elements which can be considered borderline cases between metals and nonmetals. They also have lower electronegativity than carbon and, by convention, their compounds with carbon can be called carbides (see Sec. 2.0 of Ch. 2). The differences between the atomic structure, electronegativity, and atomic radius of these three elements are not as significant as those between carbon and the transition metals (see Ch. 3, Table 3.8). 

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