The energy of a diatomic molecule
When two atoms combine to form a stable covalent molecule, for example HCl, they may be said to do because of some internal electronic rearrangement. There is a repulsion between the positively charged nuclei of both atoms, and between their negative electron ‘clouds’ ; on the other hand there is an attraction between the nucleus of one atom and the electrons of the other and vice versa. The two atoms settle at a mean internuclear distance such that these forces are just balanced and the total energy of the whole system is a minimum. Squeezing the atoms more closely together will cause the repulsive force to rise rapidly, while pulling them apart is resisted by the attractive force. Any attempt to distort the bond length requires an input of energy and we may plot energy against internuclear distance as in Fig 1 where we have ‘anchored’ the chlorine atom on one axis and imagine pushing and pulling the hydrogen atom closer to or further from the chlorine – a bigger push or pull results in raising the energy more. At the energy minimum the internuclear distance is reffered to as the equilibrium distance r eq , or more simply as the bond length. The compression and extension of a bond may be likened to the behaviour of a spring, and we may extend the analogy by assuming that the bond, like a spring, obeys Hooke’s law.
#LearningScience
#Vibrationalspectroscopy
#spectroscopy

свернуть