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Encyclopedia > Covalent radius

edit Atomic radius: Ionic radius Covalent radius Metallic radius van der Waals radius edit Atomic radius, and more generally the size of an atom, is not a precisely defined physical quantity, nor is it constant in all circumstances. ... Ionic radius is a concept for expressing the sizes of ions in ionic crystals. ... Atomic radius: Ionic radius Covalent radius Metallic radius van der Waals radius edit Atomic radius, and more generally the size of an atom, is not a precisely defined physical quantity, nor is it constant in all circumstances. ... The van der Waals radius of an atom is the radius of an imaginary hard sphere which can be used to model the atom for many purposes. ...

The covalent radius, rcov, is a measure of the size of atom which forms part of a covalent bond. It is measured either in picometres (pm) or ångströms (Å), with 1 Å = 100 pm. Properties In chemistry and physics, an atom (Greek ἄτομος or átomos meaning indivisible) is the smallest particle still characterizing a chemical element. ... Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. ... One picometre is defined as 1x10-12 metres, in standard units. ... An angstrom, angström, or Ã¥ngström (symbol Ã…) is a non-SI unit that is internationally recognized. ...


In principle, the sum of the two covalent radii should equal the covalent bond length between two atoms. This relationship does not hold exactly because the size of an atom is not constant but depends on its chemical environment. In particular, polar covalent bonds tend to be shorter than would be expected on the basis of the sum of covalent radii. Tabulated values of covalent radii are either average or idealized values, which nevertheless show a certain transferability between different situations. In molecular geometry, bond length or bond distance is the distance between two bonded atoms in a molecule. ... A polar covalent bond is a form of covalent bonding that happens when atoms of two different elements with different electronegativities bond resulting in an unequal sharing of electrons. ... Transferability, in chemistry, is the assumption that a chemical property that is associated with an atom or a functional group in a molecule will have a similar (but not identical) value in a variety of different circumstances. ...


Covalent radii are measured by X-ray diffraction (more rarely, neutron diffraction on molecular crystals. Rotational spectroscopy can also give extremely accurate values of bond lengths. One method takes the covalent radius to be half the single bond length in the element, e.g. d(H–H, in H2) = 74.14 pm so rcov(H) = 37.07 pm: in practice, it is usual to obtain an average value from a variety of covalent compounds, although the difference is usually small. Sanderson has published a recent set of non-polar covalent radii for the main-group elements,[1] but the availabilty of large collections of bond lengths, which are more transferable, from the Cambridge Crystallographic Database[2] has rendered covalent radii obsolete in many situations. X-ray crystallography is a technique in crystallography in which the pattern produced by the diffraction of x-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. ... Neutron diffraction is a crystallography technique that uses neutrons to determine the atomic structure of a material. ... A molecular crystal is a crystal with a recognizable molecules that are held together by weak bonding such as van der Waals forces or hydrogen bonding as opposed to covalent or metallic bonds. ... Rotational spectroscopy or microwave spectroscopy studies the absorption and emission of electromagnetic radiation (typically in the microwave region of the electromagnetic spectrum) by molecules associated with a corresponding change in the rotational quantum number of the molecule. ... Transferability, in chemistry, is the assumption that a chemical property that is associated with an atom or a functional group in a molecule will have a similar (but not identical) value in a variety of different circumstances. ...


References

  1. ^ Sanderson, R. T. (1983). "Electronegativity and Bond Energy." J. Am. Chem. Soc. 105:2259–61.
  2. ^ Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. (1987). "Table of Bond Lengths Determined by X-Ray and Neutron Diffraction." J. Chem. Soc., Perkin Trans. 2 S1–S19.

The Journal of the American Chemical Society (usually abbreviated as , or JACS), is a peer-reviewed scientific journal, published since 1879 by the American Chemical Society. ...

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