what is the difference betwen atomisation enthalpy and dissosiation enthalpy?

Dear Vishnu,

If i get it right, you are solving or trying to learn something related to Hess Law or energy of reactions.

If so, for Hess Law, you have equations that corresponds to /\H Electron Affinity, /\H Ionization etc...which occur all in gas phase. If you are to combine all the equations you would need to have an equation that converts your element or compound to gas.

Now the enthalpy of atomization is just the enthalpy involved in converting atoms to the gas phase. It can be of different forms like...

enthalpy of sublimation - enthalpy involved in converting a solid material to gas
ie Al(s) ----> Al(g) /\H sublimation

enthalpy of vaporization 
ie Hg (l) ----> Hg(g) /\H vaporization

enthalpy of bond dissociation
ie. H-OH (g) ----> H(g) + OH(g) /\H bond dissociation

You can think of enthalpy of sublimation as a 'form' of enthalpy of atomization.

The enthalpy of atomisation of hydrogen is equal and (in principle) identical to the bond dissociation enthalpy of the H-H bond. However, IF the first is measured by calorimetry and the second by spectrometry there might be a systematic difference.

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