6.5 Electron Affinity & Metallic Character Trend
Periodic trends in electron affinity
Formation of a negative ion occurs when an electron from some external source enters the atom and become incorporated into the lowest energy orbital that possesses a vacancy. Because the entering electron is attracted to the positive nucleus, the formation of negative ions is usually exothermic. The energy given off is the electron affinity of the atom. For some atoms, the electron affinity appears to be slightly negative, suggesting that electron-electron repulsion is the dominant factor in these instances.
In general, electron affinities tend to be much smaller than ionization energies, suggesting that they are controlled by opposing factors having similar magnitudes. These two factors are, as before, the nuclear charge and electron-electron repulsion. But the latter, only a minor actor in positive ion formation, is now much more significant. One reason for this is that the electrons contained in the inner shells of the atom exert a collective negative charge that partially cancels the charge of the nucleus, thus exerting a so-called shielding effect which diminishes the tendency for negative ions to form.
Figure 6.58(a) Electron Affinity Figure 6.58(b) Anion formation
Because of these opposing effects, the periodic trends in electron affinities are not as clear as are those of ionization energies. This is particularly evident in the first few rows of the periodic table, in which small effects tend to be magnified anyway because an added electron produces a large percentage increase in the number of electrons in the atom.
Figure 6.59 Periodic Trend in Electron Affinity
In general, we can say that electron affinities become more exothermic as we move from left to right across a period (owing to increased nuclear charge and smaller atom size). There are some interesting irregularities, however:
- In the Group 2 elements, the filled 2s orbital apparently shields the nucleus so effectively that the electron affinities are slightly endothermic.
- The Group 15 elements have rather low values, due possibly to the need to place the added electron in a half-filled p orbital; why the electron affinity of nitrogen should be endothermic is not clear. The vertical trend is for electron affinity to become less exothermic in successive periods owing to better shielding of the nucleus by more inner shells and the greater size of the atom, but here also there are some apparent anomalies.
- Electron Affinity Trend
How electron affinity, ionization energy and electronegativity relate to each other
Metallic character
The metallic trend follows the trend of the atomic radius. It increases within a group of the periodic table from the top to the bottom and it decreases within a period of the periodic table from left to right.
What you should be able to do
Make sure you thoroughly understand the following essential concepts that have been presented above.
- You should be able to sketch out the general form of the periodic table and identify the various blocks and identify the groups corresponding to the alkali metals, the transition elements, the halogens, and the noble gases.
- For the first eighteen elements, you should be able to predict the formulas of typical binary compounds they can be expected to form with hydrogen and with oxygen.
- Comment on the concept of the “size” of an atom, and give examples of how radii are defined in at least two classes of of substances.
- Define ionization energy and electron affinity, and explain the periodic general trends.
- State the meaning and significance of electronegativity.
The video below illustrates the concepts of the atomic radius.
Reference: https://www.youtube.com/watch?v=3tFOr1Jh_bs
a. Ionization Energy: It is defined as the amount of energy needed to remove the valence electrons from an atom in the gas phase.The Ionization Energy trend is the opposite of the atomic radii of the atoms within the groups and the periods of the periodic table.Ionization energy increases within the period from the left to the left to the right and decreases within a group from top to bottom.Ionization energies can be measured experimentally.
Example: Aluminum atom ionization energies:
Al(g) …> Al + (g) + 1 e – [First Ionization Energy I1 ]
Al + (g) ….> Al 2+ (g) + 1 e – [Second Ionization Energy I2 ]
Al 2+ (g) …..> Al 3+ (g) + 1 e – [Third Ionization Energy I3 ]
The third ionization is the largest since it will require more energy to remove the last third valence electron because it is very close to the nucleus compared with the other two valence electrons.
The video below illustrates the concepts of the ionization energy.
Reference: https://www.youtube.com/watch?v=LNVEBURpTcQ
b. Electron affinities: The ability to attract valence electrons to an atom to form an ion in the gas phase.
Example: Fluorine ion
Electron affinities are difficult to be measured experimentally compared with the ionization energies’ measurements.
The figure below shows the ionization energies calculated (values are given in black) and the electron affinities estimated (values are given in red)
The videos below illustrate the concepts of the electron affinities in some details:
Reference: https://www.youtube.com/watch?v=KhLijA77WQE
Reference: https://www.youtube.com/watch?v=OcPvtr1EuTc
c. Metallic Trend
The metallic trend follows the trend of the atomic radius. It increases within a group of the periodic table from the top to the bottom and it decreases within a period of the periodic table from left to right.
The summary of variation of the electron properties in periodic table is given in the figure and the videos below:
Figure 6.61Summary of all Periodic Trend
Reference: https://www.youtube.com/watch?v=A3dTn2at0Hw
Reference: https://www.youtube.com/watch?v=K0YokTyEAMg