Terpene

thcscience_admin
edit

The covalent radius, rcov, is a measure of the size of an atom that forms part of one covalent bond. It is measured either in picometres (pm) or ångströms (Å), with 1 Å = 100 pm.

In principle, the sum of the two covalent radii should equal the covalent bond length between two atoms, R(AB) = r(A) + r(B). Moreover, different radii can be introduced for single, double and triple bonds (r1, r2 and r3 below), in a purely operational sense. These relationships are certainly not exact because the size of an atom is not constant but depends on its chemical environment. For heteroatomic A–B bonds, ionic terms may enter. Often the polar covalent bonds are 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, that makes them useful.

The bond lengths R(AB) are measured by X-ray diffraction (more rarely, neutron diffraction on molecular crystals). Rotational spectroscopy can also give extremely accurate values of bond lengths. For homonuclear A–A bonds, Linus Pauling took the covalent radius to be half the single-bond length in the element, e.g. R(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][3] has rendered covalent radii obsolete in many situations.

[edit] Table of covalent radii

The values from ref. 4 in the table below are based on a statistical analysis of more than 228,000 experimental bond lengths from the Cambridge Structural Database.[4] The numbers in parentheses are the estimated standard deviations for the last digit. This fit pre-fixes the radii for C, N and O.

A different approach is to make a self-consistent fit for all elements in a smaller set of molecules. This was done separately for single,[5] double,[6] and triple bonds[7] up to superheavy elements. Both experimental and computational data were used. The single-bond results are often similar to those of Cordero et al.[4] When they are different, the coordination numbers used can be different. This is notably the case for most (d and f) transition metals. Normally one expects that r1 > r2 > r3. Deviations may occur for weak multiple bonds, if the differences of the ligand are larger than the differences of R in the data used.

Z Symbol r (Å)[4] r1(Å)[5] r2(Å)[6] r3(Å) [7]
1 H 0.31(5) 0.32
2 He 0.28 0.46
3 Li 1.28(7) 1.33 1.24
4 Be 0.96(3) 1.02 0.90 0.85
5 B 0.84(3) 0.85 0.78 0.73
6 C (sp3) 0.76(1) 0.75
C (sp2) 0.73(2) 0.67
C (sp) 0.69(1) 0.60
7 N 0.71(1) 0.71 0.60 0.54
8 O 0.66(2) 0.63 0.57 0.53
9 F 0.57(3) 0.64 0.59 0.53
10 Ne 0.58 0.67 0.96
11 Na 1.66(9) 1.55 1.60
12 Mg 1.41(7) 1.39 1.32 1.27
13 Al 1.21(4) 1.26 1.13 1.11
14 Si 1.11(2) 1.16 1.07 1.02
15 P 1.07(3) 1.11 1.02 0.94
16 S 1.05(3) 1.03 0.94 0.95
17 Cl 1.02(4) 0.99 0.95 0.93
18 Ar 1.06(10) 0.96 1.07 0.96
19 K 2.03(12) 1.96 1.93
20 Ca 1.76(10) 1.71 1.47 1.33
21 Sc 1.70(7) 1.48 1.16 1.14
22 Ti 1.60(8) 1.36 1.17 1.08
23 V 1.53(8) 1.34 1.12 1.06
24 Cr 1.39(5) 1.22 1.11 1.03
25 Mn (low spin) 1.39(5)
Mn (high spin) 1.61(8)
Mn 1.19 1.05 1.03
26 Fe (low spin) 1.32(3)
Fe (high spin) 1.52(6)
Fe 1.16 1.09 1.02
27 Co (low spin) 1.26(3)
Co (high spin) 1.50(7)
Co 1.11 1.03 0.96
28 Ni 1.24(4) 1.10 1.01 1.01
29 Cu 1.32(4) 1.12 1.15 1.20
30 Zn 1.22(4) 1.18 1.20
31 Ga 1.22(3) 1.24 1.17 1.21
32 Ge 1.20(4) 1.21 1.11 1.14
33 As 1.19(4) 1.21 1.14 1.06
34 Se 1.20(4) 1.16 1.07 1.07
35 Br 1.20(3) 1.14 1.09 1.10
36 Kr 1.16(4) 1.17 1.21 1.08
37 Rb 2.20(9) 2.1 2.02
38 Sr 1.95(10) 1.85 1.57 1.39
39 Y 1.90(7) 1.63 1.3 1.24
40 Zr 1.75(7) 1.54 1.27 1.21
41 Nb 1.64(6) 1.47 1.25 1.16
42 Mo 1.54(5) 1.38 1.21 1.13
43 Tc 1.47(7) 1.28 1.2 1.1
44 Ru 1.46(7) 1.25 1.14 1.03
45 Rh 1.42(7) 1.25 1.1 1.06
46 Pd 1.39(6) 1.2 1.17 1.12
47 Ag 1.45(5) 1.28 1.39 1.37
48 Cd 1.44(9) 1.36 1.44
49 In 1.42(5) 1.42 1.36 1.46
50 Sn 1.39(4) 1.4 1.3 1.32
51 Sb 1.39(5) 1.4 1.33 1.27
52 Te 1.38(4) 1.36 1.28 1.21
53 I 1.39(3) 1.33 1.29 1.25
54 Xe 1.40(9) 1.31 1.35 1.22
55 Cs 2.44(11) 2.32 2.09
56 Ba 2.15(11) 1.96 1.61 1.49
57 La 2.07(8) 1.8 1.39 1.39
58 Ce 2.04(9) 1.63 1.37 1.31
59 Pr 2.03(7) 1.76 1.38 1.28
60 Nd 2.01(6) 1.74 1.37
61 Pm 1.99 1.73 1.35
62 Sm 1.98(8) 1.72 1.34
63 Eu 1.98(6) 1.68 1.34
64 Gd 1.96(6) 1.69 1.35 1.32
65 Tb 1.94(5) 1.68 1.35
66 Dy 1.92(7) 1.67 1.33
67 Ho 1.92(7) 1.66 1.33
68 Er 1.89(6) 1.65 1.33
69 Tm 1.90(10) 1.64 1.31
70 Yb 1.87(8) 1.7 1.29
71 Lu 1.87(8) 1.62 1.31 1.31
72 Hf 1.75(10) 1.52 1.28 1.22
73 Ta 1.70(8) 1.46 1.26 1.19
74 W 1.62(7) 1.37 1.2 1.15
75 Re 1.51(7) 1.31 1.19 1.1
76 Os 1.44(4) 1.29 1.16 1.09
77 Ir 1.41(6) 1.22 1.15 1.07
78 Pt 1.36(5) 1.23 1.12 1.1
79 Au 1.36(6) 1.24 1.21 1.23
80 Hg 1.32(5) 1.33 1.42
81 Tl 1.45(7) 1.44 1.42 1.5
82 Pb 1.46(5) 1.44 1.35 1.37
83 Bi 1.48(4) 1.51 1.41 1.35
84 Po 1.40(4) 1.45 1.35 1.29
85 At 1.50 1.47 1.38 1.38
86 Rn 1.50 1.42 1.45 1.33
87 Fr 2.60 2.23 2.18
88 Ra 2.21(2) 2.01 1.73 1.59
89 Ac 2.15 1.86 1.53 1.4
90 Th 2.06(6) 1.75 1.43 1.36
91 Pa 2.00 1.69 1.38 1.29
92 U 1.96(7) 1.7 1.34 1.18
93 Np 1.90(1) 1.71 1.36 1.16
94 Pu 1.87(1) 1.72 1.35
95 Am 1.80(6) 1.66 1.35
96 Cm 1.69(3) 1.66 1.36
97 Bk 1.66 1.39
98 Cf 1.68 1.4
99 Es 1.65 1.4
100 Fm 1.67
101 Md 1.73 1.39
102 No 1.76 1.59
103 Lr 1.61 1.41
104 Rf 1.57 1.4 1.31
105 Db 1.49 1.36 1.26
106 Sg 1.43 1.28 1.21
107 Bh 1.41 1.28 1.19
108 Hs 1.34 1.25 1.18
109 Mt 1.29 1.25 1.13
110 Ds 1.28 1.16 1.12
111 Rg 1.21 1.16 1.18
112 Cn 1.22 1.37 1.3
113 Uut 1.36
114 Uuq 1.43
115 Uup 1.62
116 Uuh 1.75
117 Uus 1.65
118 Uuo 1.57

[edit] References

  1. ^ Sanderson, R. T. (1983). "Electronegativity and Bond Energy". Journal of the American Chemical Society 105: 2259–2261. doi:10.1021/ja00346a026. 
  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. doi:10.1039/P298700000S1. 
  3. ^ Orpen, A. Guy; Brammer, Lee; Allen, Frank H.; Kennard, Olga; Watson, David G.; Taylor, Robin (1989). "Supplement. Tables of bond lengths determined by X-ray and neutron diffraction. Part 2. Organometallic compounds and co-ordination complexes of the d- and f-block metals". Journal of the Chemical Society, Dalton Transactions: S1. doi:10.1039/DT98900000S1. 
  4. ^ a b Beatriz Cordero, Verónica Gómez, Ana E. Platero-Prats, Marc Revés, Jorge Echeverría, Eduard Cremades, Flavia Barragán and Santiago Alvarez (2008). "Covalent radii revisited". Dalton Trans.: 2832–2838. doi:10.1039/b801115j. 
  5. ^ P. Pyykkö, M. Atsumi (2009). "Molecular Single-Bond Covalent Radii for Elements 1-118". Chemistry: A European Journal 15: 186–197. doi:10.1002/chem.200800987. 
  6. ^ P. Pyykkö, M. Atsumi (2009). "Molecular Double-Bond Covalent Radii for Elements Li–E112". Chemistry: A European Journal 15: 12770–12779. doi:10.1002/chem.200901472. . Figure 3 of this paper contains all radii of refs. [5-7]. The mean-square deviation of each set is 3 pm.
  7. ^ P. Pyykkö, S. Riedel, M. Patzschke (2005). "Triple-Bond Covalent Radii". Chemistry: A European Journal 11: 3511–3520. doi:10.1002/chem.200401299. PMID 15832398. 
source: https://web.archive.org/web/20110504181639/http://en.wikipedia.org/wiki/Covalent_radius
Personal tools
  • Log in / create account
Namespaces
Variants
Actions

Leave a Reply