Ionization energies and electron affinities

Key point: Ionization energy increases across a period and decreases down a group. Electron affinities are highest for elements near fluorine, particularly the halogens. We need to be aware of the energies needed to form cations and anions of the elements. Ionization energies are relevant to the formation of cations; electron affinities are relevant to the formation of anions. The ionization energy of an element is the energy required to remove an electron from a gas-phase atom (Section 1.9). Ionization energies correlate strongly with atomic radii, and elements that have small atomic radii generally have high ionization energies. Therefore, as the atomic radius increases down a group, the ionization energy decreases. Likewise, the decrease in radius across a period is accompanied by an increase in ionization energy (Fig. 9.2). As discussed in Section 1.9, there are variations in this trend: in particular, high ionization energies occur when electrons are removed from half-filled or full shells or sub shells. Thus the first ionization energy of nitrogen ([He]2s²2p³) is 1402 kJ mol^-1, which is higher than the value for oxygen ([He]2s²2p⁴, 1314 kJ mol^-1). Similarly, the ionization energy of phosphorus (1011 kJ mol 1) is higher than that of sulfur (1000 kJ mol^-1). The energies of electrons in hydrogenic atoms are proportional to Z²/n². A first approximation to the energies of electrons in many-electron atoms is that they are proportional to Z_eff²/n², where Z_eff is the effective nuclear charge (Section 1.6), although this proportional ity should not be taken too seriously. Plots of first ionization energies against Z_eff²/n² for the outermost electrons for the elements Li to Ne (n = 2) and Hf to Hg (n = 6) are shown

Figure 9.2 The variation of first ionization energy (in kilojoules per mole) through the main groups of the periodic table.

in Figs 9.3 and 9.4, respectively. The graphs confirm that the proportionality is broadly followed, especially at high values of n when the outermost electron experiences an inter action with an almost point-like core. The electron affinity plays a role in assessing the energy required to form an anion. As we saw in Section 1.9c, an element has a high electron affinity if the additional electron can enter a shell where it experiences a strong effective nuclear charge. Therefore, elements close to F (other than the noble gases) have the highest electron affinities as Zeff is then large. The addition of an electron to a singly charged ion (as in the formation of O^-2 from O^-) is invariably negative (that is, the process is endothermic) because it takes energy to push an electron on to a negatively charged species. However, that does not mean it cannot happen, for it is important to assess the overall consequences of ion formation, and it is often the case that the interaction between highly charged ions in the solid overcomes the additional energy needed to form them. When assessing the energetics of compound formation, it is essential to think globally and not to rule out an overall process simply because an individual step is endothermic.

Figure 9.3 Plot of first ionization energies against Z_eff²/n² for the outermost electrons for the elements lithium to neon (n = 2).

Figure 9.4 Plot of first ionization energies against Z_eff²/n² for the outermost electrons for the elements hafnium to mercury (n = 6).

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مواضيع ذات صلة

The atom and its ions

Oxidation states

Occurrence

Enthalpies of atomization

Electronegativity

Computational techniques

Electrochemical techniques

Thermal analysis

X-ray fluorescence elemental analysis

CHN analysis

Atomic absorption spectroscopy

Ionization and detection methods