CLASS :- 11
Topic :- Bond length Bond angle Bond enthalpy and Bond . order
Bond length
In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule
the size of the atom very small 1.2 X 10 - 10 m
generally isolated atom not find out in our nature It exist in the form of
homo atomic or hetero atomic molecules form Then the size of atom also changed from one
state to another
As H-H , H-Cl , H-O-H
in covalent molecules Internuclear distance two time (twice) from the radius of an atom that
is now as covalent radius
internuclear distance between two hydrogen atom in hydrogen molecules 74 pm
internuclear distance two chlorine atom in chlorine molecules 198 pm
In the heteronuclear molecules the distance between the centre of nucleus of the Two bonded an atom
H-Cl heteroatomic molecules 127 pm
in ionic compound
internuclear distance = radius of cations + radius of anion
example the inter nucleoid distance of sodium chloride 276 pm which corresponding to the sum of
the ionic radii of sodium and chloride ions
factor affecting of Bond length
1:- multiple Bond in covalent molecules:-The higher the bond order, the stronger the pull of electrons between the two atoms of nuclei and the shorter the bond length. Generally, the length of the bond between two atoms is approximately the sum of the covalent radii of the two atoms.
C- C 154 pm
C=C 134 pm
C≡C 120 pm
2:- size of the atom
As the size of the atom changes the bond length also changes. As the size of the atom increases the bond length also increases because the bond is joined from the center of the atom. And as the size of the atom decreases the bond length also decreases.
C - C くSi - Si く Ge - Ge
Bond angle
The bond angle is defined as the average angle between the orbitals containing bonding electron pairs around the central atom within a molecule.
It is measured in degree or minute or second. ... The bond angle depends upon the number of lone pairs of electrons , size and electronegativity of the central atom and also size of surrounding atoms.
For Example
in Methane molecules Bond angle between H-C-H 109.5 degree
The Bond angle in water molecules 104.5 degree
the word angle in Ammonia molecules 107 degree
Bond enthalpy
Bond enthalpy is defined as the amount of energy required to break one mole of a bond of one type, present between two atoms in a gaseous state.
bond energy (BE), also called the · mean bond enthalpy or · average bond enthalpy is the measure of · bond strength in a chemical bond
The bond dissociation enthalpy Express in the term of closure permit kilo joule per mole
bond dissociation enthalpy
hydrogen molecules :-
the bond dissociation enthalpy of hydrogen hydrogen bond in hydrogen molecule is 435.8 closure per Mall
H2 → H+ H ΔH = 435.8 KJ/mol
chlorine molecules
chlorine molecules is 242.5kj per mole
Cl2 → Cl + Cl ΔH = 242.5
hydrogen iodide bond dissociation enthalpy 298.3 kj per mole
HI → H + I ΔH = 298.3 kj/mol
the bond dissociation enthalpy of double bond in oxygen molecules 498.0 kilo joule per mole
O=O → O + O ΔH = 498.0 kj/mol
the bond dissociation enthalpy of triple Bond in nitrogen molecules 946.0 kj /mol
N ≡ N → N + N ΔH = 946.0 kj/mol
on the basis of the above what dissociation enthalpy , the bond dissociation enthalpy larger if the bond between the atom is stronger
factor affecting of bond dissociation enthalpy
1:- size of the bonded atom
Greater the size of the atom,larger is the bond length then low bond dissociation energy
For example
hydrogen molecules :-
the bond dissociation enthalpy of hydrogen hydrogen bond in hydrogen molecule is 435.8 closure per Mall
H2 → H+ H ΔH = 435.8 KJ/mol
chlorine molecules
chlorine molecules is 242.5kj per mole
Cl2 → Cl + Cl ΔH = 242.5
2 :- Bond length
Greater is the multiplicity of the bond and shorte bond length , greater is the bond dissociation enthalpy.
the bond dissociation enthalpy of double bond in oxygen molecules 498.0 kilo joule per mole
O=O → O + O ΔH = 498.0 kj/mol
the bond dissociation enthalpy of triple Bond in nitrogen molecules 946.0 kj /mol
N ≡ N → N + N ΔH = 946.0 kj/mol
average Bond enthalpy( bond dissociation enthalpy of polyatomic molecules)
The bond dissociation enthalpy is applies to diatomic molecules where as mean or average bond energy is applies to poly atomic molecules.
The average of the bond dissociation enthalpy is called average Bond enthalpy
In water molecules the bond dissociation enthalpy total
502 (H-O-H → H + OH) and 427 (OH → O + H) kilo joule per mole (TOTAL 502 + 427 =929 )the average bond dissociation enthalpy is 464. 5 kj /mol
It is obtained by dividing total bond dissociation enthalpy by the number of bonds broken.
Bond order
Bond order is the number of bonding pairs of electrons between two atoms. In a covalent bond between two atoms,
Bond order is the number of chemical bonds between a pair of atoms; in diatomic nitrogen (N≡N) for example, the bond order is 3
H - H Single bonds have a bond order of 1.
O = O Double bonds have a bond order of 2.
N ≡ N Triple bonds have a bond order of 3.