Published on July 21, 2008
Chemical Bonding: Covalent Lecture 2
Covalent compounds outnumbers the ionic. Sharing electrons is the principal way that atoms interacts chemically.
Formation of a bond always results in greater electron density between the nuclei.
There are shared and unshared (lone) electron pairs. H H H-H H F H-F
Shared electron pair is represented by a line (the bond order is 1). H O H H-O-H H H N N H-N-H H
H O H
H N N
Two shared electron pairs are represented by a double line. H C C H H H H 2 C=CH 2
H C C H
H 2 C=CH 2
Two shared electron pairs are represented by a double line (the bond order is 2). O C O CO 2 , O=C=O
O C O
CO 2 , O=C=O
Three shared electron pairs are represented by a triple line (the bond order is 3). N N N 2 , N=N
N 2 , N=N
The bond energy (also bond strength or bond enthalpy) is the energy required to overcome mutual attraction between bonded nuclei and shared electrons.
Bond breakage is an endothermic process, so the bond energy is always positive: A-B (g) A (g) + B (g) ; ∆H bond breaking > 0
Bond formation is an exothermic process, so the bond energy is always negative: A (g) + B (g) A-B (g) ; ∆H bond forming < 0 For A-B, ∆H bond breaking = - ∆H bond forming
The bond length is the distance between the two nuclei or two bonded atoms.
For a given pair of atoms, a higher bond order results in shorter bond length and a higher bond energy.
A sample problem on comparing bond length and bond strength.
Covalent bonds should be strong. So why do covalent compounds melt and boil at low temperatures? Is it an inconsistency or paradox?