Atomic and physical properties of boron

Boron has 2 stable naturally occurring isotopes and the variability of their concentration (particularly the difference between borates from California (low in °B) and Turkey (high in °B) prevents the atomic weight of boron being quoted more precisely than 10.811(7) (p. 17). Each isotope has a nuclear spin (Table 6.1) and this has proved particularly valuable in nmr spectroscopy, especially for The great 

Boron is the fifth element in the periodic table and its ground-state electronic configuration is [He]2s 2p. The first 3 ionization energies are 800.6, 2427.1 and 3659.7 kJmor , all substantially larger than for the other elements in Group 13. (The values for this and other properties of B are compared with those for Al, Ga, In and T1 on p. 222). The electronegativity (p. 25) of B is 2.0, which is close to the values for H (2.1) Si (1.8) and Ge (1.8) but somewhat less than the value for C (2.5). The implied reversal of the polarity of B-H and C-H bonds is an important factor in discussing hydroboration (p. 166) and other reactions. 

The determination of precise physical properties for elemental boron is bedevilled by the twin difficulties of complex polymorphism and contamination by irremovable impurities. Boron is an extremely hard refractory solid of high mp, low density and very low electrical conductivity. Crystalline forms are dark red in transmitted light and powdered forms are black. The most stable ()3-rhombohedral) modification has mp 2092°C (exceeded only by C among the non-metals), bp 4000°C, d 2.35 gcm (a-rhombohedral form 2.45gcm ), A77sublimation 570kJ per mol of B, electrical conductivity at room temperature 1.5 x 10 ohm cm- . 

Boron has 2 stable naturally occurring isotopes and the variability of their concentration (particularly the difference between borates from California (low in °B) and Turkey (high in B) prevents the atomic weight of boron being quoted more precisely than 10.811(7) (p. 17). Each isotope has a nuclear spin (Table 6.1) and this has proved particularly valuable in nmr spectroscopy, especially for B. The great difference in neutron absorption cross-section of the 2 isotopes is also notable, and this has led to the development of viable separation processes on an industrial scale. The commercial availability of the separated isotopes has greatly assisted the solution of structural and mechanistic problems in boron chemistry and has led to the development of boron-10 neutron capture therapy for the treatment of certain types of brain tumour (see p. 179). 

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Boron remarks on its crystal structure. The atomic and physical properties of boron are reported in 5.13.2 with a few data about its crystal structures in the elemental state. A few more comments will be added here in order to insert also boron in this short summary of the crystal properties of the various elements. In this general although partial picture, the peculiar characteristics of boron have indeed to be underlined. 

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