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Key length

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Covalent bond parameters
This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.
Bond length refers to two Nucleus Balance distance between. For example, H two The distance between the two H atoms in the molecule is 74 pm, so the H-H bond length is 74 pm. Key length and Bond energy Both covalent bond The important properties of can be measured by experiments (mainly molecular spectroscopy or thermochemistry) [1] 。
Chinese name
Key length
Foreign name
Bond length [2]
Discipline
Chemistry 、 physics [2] 、 pharmacy [2]
Definition
Distance between bonding nuclei [2]

catalog

  1. one definition
  2. two general rule
  3. three Determination method
  4. four Bond length Bond energy

definition

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Bond length: the average distance between nuclei of two bonding atoms A and B. It is to understand the basic configuration parameters of molecular structure, and also to understand Chemical bond The parameters of strength, strength and property. It can be measured by experiment. Bond length is one of the important parameters of molecular structure, which plays an important role in discussing the properties of chemical bonds, studying the microstructure of substances and clarifying the relationship between microstructure and macro properties.

general rule

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For the chemical bond composed of the same two atoms A and B: the bond length is small, and the bond strength is strong; The number of keys is large, and the value of key length is small.
stay Atomic crystal Medium, Atomic radius The smaller the bond, the shorter the bond length, and the greater the bond energy. The atomic radius of bonding atoms can be deduced from a large number of bond length values; On the contrary, the typical bond length of this chemical bond can be obtained by using the sum of atomic radius. If we consider two more atom If the magnitude of electronegativity difference is properly corrected, it will be in good agreement with the actual measured value.
For the comparison of covalent bond length, we can roughly refer to the following methods: the greater the covalent bond strength, the smaller the bond length; The greater the difference between the electronegativity of the atom that combines with the same atom to form a covalent bond and that of the atom, the smaller the bond length; (For example, the valence bond formed between halogen and carbon atom) At the same time, the bond length is also related to the type and strength of other chemical bonds formed by the atom.
However, there are many factors that affect the bond length and bond energy, such as atomic radius, distance between atomic nuclei, repulsive force between lone pair electrons, feedback bond, etc. In actual molecules, due to the influence of conjugation effect, space blocking effect and electronegativity of adjacent groups, the bond length of the same chemical bond is somewhat different. Therefore, when discussing the problem, it must be analyzed according to the specific situation. [3]

Determination method

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The value of bond length in various molecules has passed through a large number of crystals X-ray diffraction Method; A small number of simple gaseous molecules and X-H bond lengths have been determined by spectroscopy and neutron diffraction Method.
In addition to measuring bond length by physical methods such as spectroscopy and diffraction, bond length can be calculated by ab initio calculation or self consistent field semi empirical method in quantum chemistry [4] 。

Bond length Bond energy

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Bond Lengths and Bond Energies of Common Chemical Bonds
Bond length and bond energy of common chemical bonds
Chemical bond
Key length
(Bond length)
/(10 -12 m)
Bond energy
(Bond energy)
/(kJ/mol)
Chemical bond
Key length
(Bond length)
/(10 -12 m)
Bond energy
(Bond energy)
/(kJ/mol)
B—F
-
six hundred and forty-four
N—H
one hundred and one
three hundred and eighty-nine
B—O
-
five hundred and fifteen
N—N
one hundred and forty-five
one hundred and fifty-nine
Br—Br
two hundred and twenty-nine
one hundred and ninety-three
N═N
one hundred and twenty-five
four hundred and fifty-six
C—B
one hundred and fifty-six
three hundred and ninety-three
N≡N
one hundred and ten
nine hundred and forty-six
C—Br
one hundred and ninety-four
two hundred and seventy-six
N—O
one hundred and forty-six
two hundred and thirty
C—C
one hundred and fifty-four
three hundred and thirty-two
N═O
one hundred and fourteen
six hundred and seven
C═C
one hundred and thirty-four
six hundred and eleven
Na—Br
two hundred and fifty
three hundred and sixty-seven
C≡C
one hundred and twenty
eight hundred and thirty-seven
Na—Cl
two hundred and thirty-six
four hundred and twelve
C—Cl
one hundred and seventy-seven
three hundred and twenty-eight
Na—F
one hundred and ninety-three
five hundred and nineteen
C—F
one hundred and thirty-eight
four hundred and eighty-five
Na—H
one hundred and eighty-nine
one hundred and eighty-six
C—H
one hundred and nine
four hundred and fourteen
Na—I
two hundred and seventy-one
three hundred and four
C—I
two hundred and fourteen
two hundred and forty
O—H
ninety-eight
four hundred and sixty-four
C—N
one hundred and forty-eight
three hundred and five
O—O
one hundred and forty-eight
one hundred and forty-six
C═N
one hundred and thirty-five
six hundred and fifteen
O═O
one hundred and twenty
four hundred and ninety-eight
C≡N
one hundred and sixteen
eight hundred and ninety-one
P—Br
two hundred and twenty
two hundred and seventy-two
C—O
one hundred and forty-three
three hundred and twenty-six
P—Cl
two hundred and three
three hundred and thirty-one
C═O
one hundred and twenty
seven hundred and twenty-eight
P—H
one hundred and forty-two
three hundred and twenty-two
C═O(CO two )
-
eight hundred and three
P—O
one hundred and sixty-three
four hundred and ten
C—P
one hundred and eighty-seven
three hundred and five
P═O
one hundred and thirty-eight
five hundred and eighty-five
C—S
one hundred and eighty-two
two hundred and seventy-two
P—P
-
two hundred and thirteen
C═S
one hundred and fifty-six
five hundred and thirty-six
Pb—O
one hundred and ninety-two
three hundred and eighty-two
C═S(CS two )
-
five hundred and seventy-seven
Pb—S
two hundred and thirty-nine
three hundred and forty-six
C—Si
one hundred and eighty-six
three hundred and forty-seven
Rb—Br
two hundred and ninety-four
three hundred and eighty-one
Cl—Cl
one hundred and ninety-nine
two hundred and forty-three
Rb—Cl
two hundred and seventy-nine
four hundred and twenty-eight
Cs—I
three hundred and thirty-seven
three hundred and thirty-seven
Rb—F
two hundred and twenty-seven
four hundred and ninety-four
F—F
one hundred and forty
one hundred and fifty-three
Rb—I
three hundred and eighteen
three hundred and nineteen
H—H
seventy-five
four hundred and thirty-six
S—H
one hundred and thirty-five
three hundred and thirty-nine
H—Br
one hundred and forty-two
three hundred and sixty-six
S—O
-
three hundred and sixty-four
H—Cl
one hundred and twenty-seven
four hundred and thirty-one
S═O
one hundred and forty-three
-
H—F
ninety-two
five hundred and sixty-five
S—S
two hundred and seven
two hundred and sixty-eight
H—I
one hundred and sixty-one
two hundred and ninety-eight
S═S
one hundred and eighty-nine
-
I—I
two hundred and sixty-six
one hundred and fifty-one
Se—H
one hundred and forty-seven
three hundred and fourteen
K—Br
two hundred and eighty-two
three hundred and eighty
Se—Se
two hundred and thirty-two
-
K—Cl
two hundred and sixty-seven
four hundred and thirty-three
Se═Se
two hundred and fifteen
-
K—F
two hundred and seventeen
four hundred and ninety-eight
Si—Cl
-
three hundred and sixty
K—I
three hundred and five
three hundred and twenty-five
Si—F
-
five hundred and fifty-two
Li—Cl
two hundred and two
four hundred and sixty-nine
Si—H
-
three hundred and seventy-seven
Li—H
two hundred and thirty-nine
two hundred and thirty-eight
Si—O
-
four hundred and sixty
Li—I
two hundred and thirty-eight
three hundred and forty-five
Si—Si
-
one hundred and seventy-six
By comparison, it can be found that the bond length of H-F, H-Cl, H-Br, H-I increases in turn, while the bond energy decreases in turn; The bond length of single bond, two bond and three bond is shortened in turn, but the bond energy is gradually increased, but the bond length of two bond and three bond is not two or three times that of single bond. Therefore, when reusing the general laws of bond length and bond energy, we should make specific analysis based on the empirical laws and combined with the actual chemical environment of different atoms. [5]
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