Enthalpy of atomization
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The enthalpy of atomization (also atomisation in British spelling) is the enthalpy change that accompanies the total separation of all atoms in a chemical substance (either a chemical element or a chemical compound). This is often represented by the symbol ΔatH
o or ΔHat o. All bonds in the compound are broken in atomization and none are formed, so enthalpies of atomization are always positive. The associated standard enthalpy is known as the Standard enthalpy of atomization, ΔatH o/(kJ mol−1), at 298.15 K (or 25 degrees celsius) and 101.3 kPa.
Enthalpy of atomization is the amount of enthalpy change when a compound's bonds are broken and the component atoms are reduced to individual atoms.
When a diatomic element is converted to gaseous atoms, only half a mole of molecules will be needed, as the standard enthalpy change is based purely on the production of one mole of gaseous atoms. When the atoms in the molecule are different isotopes of the same element the calculation becomes non-trivial.
Standard enthalpy of atomization is the enthalpy change when 1 mol of gaseous atoms is formed from its element in its defined physical state under standard conditions (298.15K, 1 atm).
Atomization enthalpy and metallic character of transition metals
Transition elements are typical metals with relatively high atomization enthalpy. These are hard due to strong bonding. More the number of unpaired electrons more will be the number strength of bonds. Cr, Mo, W have maximum number of unpaired electrons so they are very hard and have high enthalpy of atomization. Zn, Cd, Hg have no unpaired electrons and hence very low atomization enthalpy. 4d and 5d series have higher atomization enthalpy than 3d series.