The atom and its ions
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
275
2025-09-03
390
The atom and its ions
Key points: The proton, H+, is always found in combination with a Lewis base and is highly polarizing; the hydride ion, H-, is highly polarizable. There are three isotopes of hydrogen: hydrogen itself (1H), deuterium (D, 2H), and tritium (T, 3H); tritium is radioactive. The lightest isotope, 1H (very occasionally called protium), is by far the most abundant. Deuterium has variable natural abundance with an average value of about 16 atoms in 100 000. Tritium occurs to the extent of only 1 in 1021. The different names and symbols for the three isotopes reflect the significant differences in their masses and the chemical properties that stem from mass, such as the rates of diffusion and bond-cleavage reactions. The nuclear spin of 1H (I =1/2) is exploited in NMR spectroscopy (Section 8.5) for identifying hydrogen-containing molecules and determining their structures. The free hydrogen cation (H+, the proton) has a very high charge/radius ratio and it is not surprising to find that it is a very strong Lewis acid. In the gas phase it readily attaches to other molecules and atoms; it even attaches to He to form HeH. In the condensed phase, H is always found in combination with a Lewis base, and its ability to transfer between Lewis bases gives it the special role in. The molecular cations H+2 and H+3 have only a transitory existence in the gas phase and are unknown in solution. In contrast to H+, which is highly polarizing; the hydride ion, H-, is highly polarizable because two electrons are bound by just one proton. The radius of H- varies considerably depending on the atom to which it is attached. The lack of core electrons to scatter X-rays means that bond distances and angles involving a H atom in a compound are difficult to measure by X-ray diffraction: for this reason, neutron diffraction is used when it is crucial to determine the precise positions of H atoms.
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