Valence shell -electron ionization energies

The minimum energy required to remove a 2s electron from B, lE(ni ), is equal to the first ionization energy plus the energy A required to promote the B+ cation from the [He]25 ground state to the lowest of several states with the electron configuration l 2s 2p  

---

Because of the general validity of Koopmans theorem for closed-shell molecules ionization energies and, as we shall see, the associated vibrational sttucture represent a vivid illustration of the validity of quite simple-minded MO theory of valence electrons. 

Based on their valence-shell electron configurations which of the following species would you expect to have the lowest ionization energy (a) C2+ (b) C2 (c) C2. ... 

Other treatments " have led to scales that are based on different principles, for example, the average of the ionization potential and the electron affinity, " the average one-electron energy of valence shell electrons in ground-state free atoms, or the compactness of an atom s electron cloud.In some of these treatments electronegativities can be calculated for different valence states, for different hybridizations (e.g., sp carbon atoms are more electronegative than sp, which are still more electronegative than and even differently for primary, secondary,... 

The initial ionization is vertical , i.e., without change in position and kinetic energy of the nuclei while it takes place. With the usual electron energy any valence shell electron may be removed. 

We need to pay particular attention to the valence-shell electrons because they affect the chemical properties most directly. Each element in one of the main groups has a characteristic electron configuration shared by its congeners. As well as electron configurations, which control the valence of the element (the number of bonds it can form), there are five main atomic properties to keep in mind atomic radius, ionization energy, electron affinity, electronegativity, and polarizability. 

Look at their positions in the periodic table. The group 4A element germanium has four valence-shell electrons and thus has four relatively low ionization energies, whereas the group 5A element arsenic has five valence-shell electrons and has five low ionization energies. 

The lack of reactivity of the noble gases is a consequence of their unusually large ionization energies (Figure 6.3) and their unusually small electron affinities (Figure 6.6), which result from their valence-shell electron configurations. 

Why does the octet rule work What factors determine whether an atom is likely to gain or to lose electrons Clearly, electrons are most likely to be lost if they are held loosely in the first place—that is, if they feel a relatively low effective nuclear charge, Zeff, and therefore have small ionization energies. Valence-shell electrons in the group 1A, 2A, and 3A metals, for example, are shielded from the nucleus by core electrons. They feel a low Zeff, and they are therefore lost relatively easily. Once the next lower noble gas configuration is reached, though, loss of an additional electron is much more difficult because it must come from an inner shell where it feels a high Zeff. 

Ionization is not limited to the removal of a single electron from an atom. Two, three, or even more electrons can be removed sequentially from an atom, although larger amounts of energy are required for each successive ionization step. In general, valence-shell electrons are much more easily removed than core electrons. 

Table 2.1 Valence shell electron configurations, first and second ionization energies E, atomic radii and some ionic radii of the d-block metals...

A technique called photoelectron spectroscopy is used to measure the ionization energy of atoms. A sample is irradiated with UV light, and electrons are ejected from the valence shell. The kinetic energies of the ejected electrons are measured. Since the energy of the UV photon and the kinetic energy of the ejected electron are known, we can write... 

Electronic transitions within the valence shell of atoms and molecules appear in the energy-loss spectrum from a few electron volts up to, and somewhat beyond, the first ionization energy. Valence-shell electron spectroscopy employs incident electron energies from the threshold required for excitation up to many kiloelectron volts. The energy resolution is usually sufficient to observe vibrational structure within the Franck-Condon envelope of an electronic transition. The sample in valence-shell electron energy-loss spectroscopy is most often in the gas phase at a sufficiently low pressure to avoid multiple scattering of the... 

The energy difference between valence shell. y and p electrons in an atom is of secondary importance for our description of a molecule if the atom uses both s and p electrons for bond formation, or if we describe the molecule using molecular orbitals formed from sp hybridized atomic orbitals. The mean valence electron ionization energies may therefore be used to define a set of electronegativity coefficients. The magnitudes of these electronegativity coefficients are found to increase across a period, and - with a few exceptions - to decrease down a group. 

Table 3.1. First ionization energies lEl, valence shell s electron ionization energies lE(nr), and mean valence shell ionization energies MIE (all in kJ mol ) of the first 20 elements.

The energy required to remove an electron from an isolated atom to infinity is called the ionization potential I, that of eliminating the -th electron is called the -th ionization potential (/ ). Ionization of atoms (or molecules) can be caused by a collision with an electron or another ion or molecule, by strong electric fields, or by thermal emission of electrons. Spectroscopic methods can determine the first ionization potentials (/i) of atoms or molecules with the accuracy of 0.01-0.001 eV, and occasionally as high as 0.0005 eV. For successive ionization potentials the errors increase to tenths or even units of eV [1 ]. The values of / for valence-shell electrons are listed in Table 1.1. 

---