- Ionic radius
- Covalent radius
- Metallic radius
- Van der Waals radius
The covalent radius, rcov, is a measure of the size of atom which forms part of a covalent bond. 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 ionic radius, r ion is a measure of the size of an Ion in a Crystal lattice. Atomic radius, and more generally the size of an atom, is not a precisely defined Physical quantity, nor is it constant in all circumstances Van der Waals Volume The van der Waals volume, V, also called the atomic volume or molecular volume, is the atomic property most directly History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny It is measured either in picometres (pm) or ångströms (Å), with 1 Å = 100 pm. A picometre ( American spelling: picometer, symbol pm) is a unit of Length in the Metric system, equal to one trillionth An ångström or angstrom (symbol Å) (ˈɔːŋstrəm Swedish: ˈɔ̀ŋstrœm is an internationally recognized non- SI unit of length equal
In principle, the sum of the two covalent radii should equal the covalent bond length between two atoms. In Molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded Atoms in a Molecule. 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. "Polar molecule" and "Non-polar" redirect here Tabulated values of covalent radii are either average or idealized values, which nevertheless show a certain transferability between different situations. This page is about transferability in chemistry Transferability in economics also exists
Covalent radii are measured by X-ray diffraction (more rarely, neutron diffraction on molecular crystals). X-ray scattering techniques are a family of non-destructive analytical techniques which reveal information about the crystallographic structure chemical composition Neutron diffraction is a crystallographic method for the determination of the atomic and/or magnetic structure of a material A molecular crystal is a Crystal whose molecules are held together by weak physical bonding such as Van der Waals forces or Hydrogen bonding as Rotational spectroscopy can also give extremely accurate values of bond lengths. Rotational spectroscopy or microwave spectroscopy studies the absorption and emission Electromagnetic radiation (typically in the Microwave 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 availability of large collections of bond lengths, which are more transferable, from the Cambridge Crystallographic Database[2] has rendered covalent radii obsolete in many situations. This page is about transferability in chemistry Transferability in economics also exists The Cambridge Structural Database (CSD is a repository for small Molecule Crystal structures.
Table of covalent radii
The values in this table are based on a statistical analysis of more than 228,000 experimental bond lengths from the Cambridge Structural Database. [3] The numbers in parentheses are the estimated standard deviations for the last digit.
| Z | Symbol | r (Å) |
|---|---|---|
| 1 | H | 0. 31(5) |
| 2 | He | 0. 28 |
| 3 | Li | 1. 28(7) |
| 4 | Be | 0. 96(3) |
| 5 | B | 0. 84(3) |
| 6 | C (sp3) | 0. 76(1) |
| C (sp2) | 0. 73(2) | |
| C (sp) | 0. 69(1) | |
| 7 | N | 0. 71(1) |
| 8 | O | 0. 66(2) |
| 9 | F | 0. 57(3) |
| 10 | Ne | 0. 58 |
| 11 | Na | 1. 66(9) |
| 12 | Mg | 1. 41(7) |
| 13 | Al | 1. 21(4) |
| 14 | Si | 1. 11(2) |
| 15 | P | 1. 07(3) |
| 16 | S | 1. 05(3) |
| 17 | Cl | 1. 02(4) |
| 18 | Ar | 1. 06(10) |
| 19 | K | 2. 03(12) |
| 20 | Ca | 1. 76(10) |
| 21 | Sc | 1. 70(7) |
| 22 | Ti | 1. 60(8) |
| 23 | V | 1. 53(8) |
| 24 | Cr | 1. 39(5) |
| 25 | Mn (low spin) | 1. 39(5) |
| Mn (high spin) | 1. 61(8) | |
| 26 | Fe (low spin) | 1. 32(3) |
| Fe (high spin) | 1. 52(6) | |
| 27 | Co (low spin) | 1. 26(3) |
| Co (high spin) | 1. 50(7) | |
| 28 | Ni | 1. 24(4) |
| 29 | Cu | 1. 32(4) |
| 30 | Zn | 1. 22(4) |
| 31 | Ga | 1. 22(3) |
| 32 | Ge | 1. 20(4) |
| 33 | As | 1. 19(4) |
| 34 | Se | 1. 20(4) |
| 35 | Br | 1. 20(3) |
| 36 | Kr | 1. 16(4) |
| 37 | Rb | 2. 20(9) |
| 38 | Sr | 1. 95(10) |
| 39 | Y | 1. 90(7) |
| 40 | Zr | 1. 75(7) |
| 41 | Nb | 1. 64(6) |
| 42 | Mo | 1. 54(5) |
| 43 | Tc | 1. 47(7) |
| 44 | Ru | 1. 46(7) |
| 45 | Rh | 1. 42(7) |
| 46 | Pd | 1. 39(6) |
| 47 | Ag | 1. 45(5) |
| 48 | Cd | 1. 44(9) |
| 49 | In | 1. 42(5) |
| 50 | Sn | 1. 39(4) |
| 51 | Sb | 1. 39(5) |
| 52 | Te | 1. 38(4) |
| 53 | I | 1. 39(3) |
| 54 | Xe | 1. 40(9) |
| 55 | Cs | 2. 44(11) |
| 56 | Ba | 2. 15(11) |
| 57 | La | 2. 07(8) |
| 58 | Ce | 2. 04(9) |
| 59 | Pr | 2. 03(7) |
| 60 | Nd | 2. 01(6) |
| 61 | Pm | 1. 99 |
| 62 | Sm | 1. 98(8) |
| 63 | Eu | 1. 98(6) |
| 64 | Gd | 1. 96(6) |
| 65 | Tb | 1. 94(5) |
| 66 | Dy | 1. 92(7) |
| 67 | Ho | 1. 92(7) |
| 68 | Er | 1. 89(6) |
| 69 | Tm | 1. 90(10) |
| 70 | Yb | 1. 87(8) |
| 71 | Lu | 1. 87(8) |
| 72 | Hf | 1. 75(10) |
| 73 | Ta | 1. 70(8) |
| 74 | W | 1. 62(7) |
| 75 | Re | 1. 51(7) |
| 76 | Os | 1. 44(4) |
| 77 | Ir | 1. 41(6) |
| 78 | Pt | 1. 36(5) |
| 79 | Au | 1. 36(6) |
| 80 | Hg | 1. 32(5) |
| 81 | Tl | 1. 45(7) |
| 82 | Pb | 1. 46(5) |
| 83 | Bi | 1. 48(4) |
| 84 | Po | 1. 40(4) |
| 85 | At | 1. 50 |
| 86 | Rn | 1. 50 |
| 87 | Fr | 2. 60 |
| 88 | Ra | 2. 21(2) |
| 89 | Ac | 2. 15 |
| 90 | Th | 2. 06(6) |
| 91 | Pa | 2. 00 |
| 92 | U | 1. 96(7) |
| 93 | Np | 1. 90(1) |
| 94 | Pu | 1. 87(1) |
| 95 | Am | 1. 80(6) |
| 96 | Cm | 1. 69(3) |
References
- ^ Sanderson, R. T. (1983). "Electronegativity and Bond Energy. " J. Am. Chem. Soc. 105:2259–61. The Journal of the American Chemical Society (usually abbreviated as J
- ^ 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.
- ^ Beatriz Cordero, Verónica Gómez, Ana E. Platero-Prats, Marc Revés, Jorge Echeverría, Eduard Cremades, Flavia Barragán and Santiago Alvarez. Covalent radii revisited. Dalton Trans. , 2008, 2832-2838, doi:10.1039/b801115j
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