Electron affinity trends

Electron Affinity movie Periodic Trends Electron Affinity movie... 

Figure 6.22 I The graph shows the electron affinity (in kj/moi) vs. atomic number for the first 20 elements of the periodic table. The inset at the upper right emphasizes the general periodic trend Electron affinity increases —meaning that the value becomes more negative—from left to right and bottom to top in the periodic table.

Electron affinity and metallic character also exhibit periodic trends. Electron affinity is a measure of how easily an atom will accept an additional electron and is crucial to chemical bonding because bonding involves the transfer or sharing of electrons. Metallic character is important because of the high proportion of metals in the periodic table and the large role they play in our lives. Of the roughly 110 elements, 87 are metals. We examine each of these periodic properties individually in this section. 

EXAMPLE 1.12 Sample exercise Predicting trends in electron affinity... 

Account for periodic trends in atomic radii, ionization energies, and electron affinities (Examples 1.11 and 1.12). 

All the elements in a main group have in common a characteristic valence electron configuration. The electron configuration controls the valence of the element (the number of bonds that it can form) and affects its chemical and physical properties. Five atomic properties are principally responsible for the characteristic properties of each element atomic radius, ionization energy, electron affinity, electronegativity, and polarizability. All five properties are related to trends in the effective nuclear charge experienced by the valence electrons and their distance from the nucleus. 

The aufbau principle must be obeyed when an electron is added to a neutral atom, so the electron goes into the most stable orbital available. Hence, we expect trends in electron affinity to parallel trends in orbital stability. However, electron-electron repulsion and screening are more important for negative ions than for neutral atoms, so there is no clear trend in electron affinities as ft increases. Thus, there is only one general pattern ... 

The electron affinity of atoms varies with atomic number. In moving across any main group blue lines), the electron affinity becomes more negative, but this is the only clear trend. 

Although electron affinity values show only one clear trend, there is a recognizable pattern in the values that are positive. When the electron that is added must occupy a new orbital, the resulting anion is unstable. Thus, all the elements of Group 2 have positive electron affinities, because their valence S orbitals are filled. Similarly, all the noble gases have positive electron affinities, because their valence a p orbitals are filled. Elements with half-filled orbitals also have lower electron affinities than their neighbors. As examples, N (half-filled 2 p orbital set) has a positive electron affinity, and so does Mn (half-filled 3 d orbital set). 

A limited number of elements form ionic compounds. As we describe in the next two chapters, most substances contain neutral molecules rather than charged ions. The trends in ionization energies and electron affinities indicate which elements tend to form ions. Ionic compounds form when the stabilization gained through ionic attraction... 

Attempts were made at explaining the trends in reactivity through the use of both an electron-transfer model85 and a resonance interaction model,86,87 but without success. It seems that the trends in reactivity on a fine scale cannot be easily explained by such simple models, but instead depend on a multitude of factors, which may include the ionization potential of the metal, the electron affinity of the oxidant molecule, the energy gap between dns2 and dn+1s1 states, the M-O bond strength, and the thermodynamics of the reaction.57-81... 

Although the same limitations apply to the use of F as those described above for the anodic counterpart, the global trend in Fig. 7 shows that gas-phase electron affinities also generally reflect the trend in the reduction potentials measured in solution for the large variety of (uncharged) acceptor structures included in Table 4.71... 

However, for other reactions an opposite trend is observed. There are undoubtedly several factors involved, which include F forming the strongest bridge but I being the best "conductor" for the electron being transferred because it is much easier to distort the electron cloud of I- (it is much more polarizable and has a lower electron affinity). Therefore, in different reactions these effects may take on different weights, leading to variations in the rates of electron transfer that do not follow a particular order with respect the identity of the anion. 

Note there is much disagreement in the literature about the exact values of electron affinity. These values are taken from the Chemistry Data Book by Stark and Wallace. If we use a different data source, we may find slightly different numbers. The trends will be the same, whichever source we consult. 

Ionization energy and electron affinity Periodic trends... 

Nonmetals follow the general trends of atomic radii, ionization energy, and electron affinity. Radii increase to the left in any row and down any column on the periodic table. Ionization energies and electron affinities increase up any column and towards the right in any row on the periodic table. The noble gases do not have electron affinity values. Ionization energies are not very important for the nonmetals because they normally form anions. Variations appear whenever the nonmetal has a half-filled or filled subshell of electrons. The electronegativity... 

Trends for electron affinity are more irregular than those for atomic radius and ionization energy, because factors other than atomic size and Zeff are involved. In future chemistry courses, you will learn about these factors and how they explain the irregularities. However, the property of electron affinity is still significant when you consider it in combination with ionization energy. The trends that result from this combination are important for chemical bonding. 

Sketch an outline of the periodic table and use it to compare the trends in atomic size, first ionization energy, and electron affinity. 

As can be seen, generally all electron affinities predicted by ASCF are negative, indicating a more stable neutral system with respect to the anion. The inclusion of correlation via CCSD(T) and NOF approximates them to the available adiabatic experimental EAs, accordingly with the expected trend. The EAs tend to increase in moving from ACCSD(T) to ANOF and then from ANOF to the experiment. It should be noted that the NH anion is predicted to be unbound by CCSD(T), whereas the positive vertical EA value via NOE corresponds to the bound anionic state. 

Figure 1.2 Some important trends in the periodic table for (a) ionization energy, (b) electron affinity, (c) atomic and ionic radii, and (d) electronegativity. Increasing values are in the direction of the arrow.

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