Enthalpy change of atomization

The enthalpy change of atomization of an element is the heat change (at constant pressure) when one mole of separate gaseous atoms of the element is formed from the element in its standard state under standard conditions. 

The following thermochemical equations each describe the enthalpy of atomization of an element. In each case the enthalpy change of the reaction is the enthalpy change of atomization. 

Note that the enthalpy change of atomization for a halogen or other diatomic gaseous molecule is equivalent to half the bond enthalpy (Chapter 5). 

All the enthalpy changes of atomization of the noble gases are zero. This is because the elements are already in the form of separate gaseous atoms under standard conditions. 

Enthalpy changes of atomization are always positive, because energy must be absorbed to pull the atoms apart and break the chemical bonds. Enthalpy changes of atomization are usually found indirectly by calculation from other enthalpy changes, using Hess s law. Enthalpy changes of atomization are used in Born-Haber cycle calculations. 

The standard enthalpy change of atomisation of an element, AHtt, is the enthalpy change when one mole of gaseous atoms is produced from the element in its standard state, under standard conditions. 

Atomization of each standard elemental form into gaseous atoms, with associated enthalpy change A//atom... 

The first ionization energy I of an atom is defined as the energy required to remove the outermost electron from a gaseous atom in its ground state. This energy may be expressed as the enthalpy change of the process... 

Decomposition of NH3 (g) into its gaseous atoms breaks 3 N-H bonds and the enthalpy change of the reaction, Ar// can be equated to three (supposedly equal) bond enthalpy terms, thus ... 

Miller and Kusch (3 ) determined the molecular composition of KI vapor by measurement of the velocity distribution of the molecules in the beam produced as the vapor effused through a small slit in a source. The analysis was based on an assumption that the velocity distribution within the oven is Maxwellian and that the vapor effuses through the ideal slit of kinetic theory. The velocity distributions of potassium and thallium atomic beams were found to be in excellent agreement with the theoretical distributions so the determination of the molecular composition of KI beams was tried. Using the derived equilibrium constants, we calculate the enthalpy change of the dissociation reaction by the 2nd and 3rd law methods. The results are presented in the following table. 

Hess s law of heat summation A law stating that the enthalpy change of an overall process is the sum of the enthalpy changes of the individual steps of the process. (192) heteroatom Any atom in an organic compound other than C or H. (459)... 

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