Molecules helium

The Helium Molecule and Molecule-ion.—The simplest example of a molecule containing a three-electron bond is the helium molecule-ion, in which a Is eigenfunction for each of two identical atoms is involved. The two unperturbed states of equal energy are He He+ and He-+ He. The formation of this molecule might be represented by the equation He Is2 >5 + He+ Is 5 —>- He (Is + ls) 2 Three dots in a horizontal line placed between the two atomic symbols may be used to designate a three-electron bond He He+. 

Evidence has been advanced8 that the neutral helium molecule which gives rise to the helium bands is formed from one normal and one excited helium atom. Excitation of one atom leaves an unpaired Is electron which can then interact with the pair of Is electrons of the other atom to form a three-electron bond. The outer electron will not contribute very much to the bond forces, and will occupy any one of a large number of approximately hydrogen-like states, giving rise to a roughly hydrogenlike spectrum. The small influence of the outer electron is shown by the variation of the equilibrium intemuclear distance within only the narrow limits 1.05-1.13 A. for all of the more than 25 known states of the helium molecule. 

I believe that the explanation of these facts is provided by the three-8 W. Weizel, Z. Physik, 59,320 (1929). Weizel and F. Hund [ibid., 63, 719 (1930) ] have discussed the possible electronic states of the helium molecule. Neither one, however, explains why He Is2 + He+ Is form a stable molecule-ion, nor gives the necessary condition for the formation of a three-electron bond. In earlier papers they assumed that both atoms had to be excited in order to form a stable molecule [W. Weizel, ibid., 51,328 (1928) F. Hund, ibid., 51, 759 (1928)]. 

In Sections 42 and 43 we shall describe the accurate and reliable wave-mechanical treatments which have been given the hydrogen molecule-ion and hydrogen molecule. These treatments are necessarily rather complicated. In order to throw further light on the interactions involved in the formation of these molecules, we shall preface the accurate treatments by a discussion of various less exact treatments. The helium molecule-ion, He , will be treated in Section 44, followed in Section 45 by a general discussion of the properties of the one-electron bond, the electron-pair bond, and the three-electron bond. 

The hydrogen molecules are moving, on average, 1.4 times as fast as the helium molecules. 

The Helium Molecule-Ion.—The simplest molecule in which the three-electron bond can occur is the helium molecule-ion, HeJ, consisting of two nuclei, each with one stable Is orbital, and three electrons. The theoretical treatment7 of this system has shown that the bond is strong, with bond energy about 55 kcal/mole and with equilibrium internuclear distance about 1.09 A. The experimental values for these qualities, determined from spectroscopic data for excited states of the helium molecule, are a bout 58 kcal/mole and 1.080 A, respectively, which agree well with the theoretical values. It is seen that the bond energy in He He4 is about the same as that in H H+, and a little more than half as great as that of the electron-pair bpnd in H H. 

We know that Iso is placed higher above the atomic Is level than lsob is placed below. Thus, no energy is released—on the contrary, energy must be expended—in forming the He2 molecule. Therefore, two isolated He atoms have lower energy than a hypothetical helium molecule this is in agreement with the fact that He2 does not exist in nature. 

Difference of the calculated and experimental results for the first excited state of the helium molecule. 

An alternative method of representing the movement of an individual molecule by computational techniques is shown in Figure 2.4 [9], This figure shows the movement of three different permeate molecules over a period of 200 ps in a silicone rubber polymer matrix. The smaller helium molecule moves more frequently and makes larger jumps than the larger methane molecule. Helium, with a molecular diameter of 2.55 A, has many more opportunities to move from one... 

Though no helium molecule He2 exists, an ion He2 does exist. Explain. 

We will now apply the MO model to the helium molecule (He2). Does this model predict that this molecule is stable Since the He atom has a Is2 configuration, Is orbitals are used to construct the MOs. Therefore the molecules will have four electrons. From the diagram shown in Fig. 14.29, it is apparent that two electrons are raised in energy and two are lowered in energy. Thus the bond order is zero ... 

Though the helium molecule He2 does not exist (p. 91 of the textbook explains why), the cation He2 does exist. Why ... 

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