Diatomic molecules, atomic pressures

If a homonuclear diatomic molecule (e.g., H2, 02, N2, etc.) is adsorbed with dissociation into atoms, the adsorption isotherms (121) and (123) give 8 as the functions of partial pressure of atoms in gas phase, pat, that would correspond to dissociation equilibrium at the partial pressure of diatomic molecules, p. These values are linked by... 

Step 1 is the unimolecular decomposition of a diatomic molecule, and step —1 is a recombination of two atoms. This recombination is a highly exothermic reaction, and this energy will reside in the newly formed bond, which is thus highly excited. If this energy is not removed in the time of one vibration, the molecule will split up on vibration and recombination will not occur. If a collision occurs before the first vibration, then energy transfer takes place and the bond will be stabilized. The third body fulfils this function. This is more effective at high pressures and low temperatures. 

The preceding treatment is, undoubtedly, an oversimplification. For example, many diatomic molecules dissociate upon adsorption (e.g., H2, SiH, GeH). Each atom from the dissociated molecule then occupies its own distinct surface site and this naturally changes the rate law expression. When these types of details are accounted for, the Langmuir-Hinshelwood mechanism has been very successful at explaining the growth rates of a number of thin-film chemical vapor deposition (CVD) processes. However, more important, our treatment served to illustrate how crystal growth from the vapor phase can be related to macroscopic observables namely, the partial pressures of the reacting species. 

Theory shows that the vapour pressures of ortho- and para- states of diatomic molecules ( 23.IV) should be diflferent, and this has been confirmed experimentally for hydrogen.3 According to Herzfeld and Teller, the heavier isotope may have a lower or higher vapour pressure than the lighter one. For neon isotopes, the vapour pressure is a linear function of the atomic weight. ... 

The molecular orbital description of He 2 predicts two electrons in a bonding orbital and two electrons in an antibonding orbital, with a bond order of zero—in other words, no bond. This is what is observed experimentally. The noble gas He has no significant tendency to form diatomic molecules and, like the other noble gases, exists in the form of free atoms. He2 has been detected only in very low pressure and low temperature molecular beams. It has a very low binding energy, approximately 0.01 J/mol for comparison, H2 has a bond energy of 436 kJ/mol. 

The most abundant species in the equilibrium vapor over condensed metals are generally atoms as shown by Knudsen effusion mass spectrometry. The relative abundance of the homonuclear diatomic molecules was determined to be between 10 and 10 percent if they are detectable. Exceptions are bismuth and antimony. The dimer partial pressure over liquid bismuth at temperatures below 1000 K exceeds that of the monomer [83, 164, 165]. The tetramer is the most abundant species over liquid antimony [85]. Other polyatomic homonuclear species have so far been observed under equilibrium conditions for some of the alkali metals, as well as the group Ib, IVb, and Vb metals. Particularly large polymers up to 667 and Sn7 were detected for germanium [84,166,167] and tin [168],... 

Figure J Bar graph of the atomic pressures in diatomic hydrides. The largest atomic pressure is for the negatively charged hydrogen atom in the polar molecule BH.

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