Opposites attract

attract

repel

In a many-electron atom, each electron feels both the attraction to the protons in the nucleus and the repulsion from other electrons Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill. p. 249 “In many-electron atoms, each electron “feels” not only the attraction to the nucleus but also the repulsion from other electrons. This repulsion counteracts the nuclear attraction somewhat, making each electron easier to remove by, in effect, helping to push it away. Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

Effective nuclear charge

In a many-electron atom, each electron feels both the attraction to the protons in the nucleus and the repulsion from other electrons Force of attraction increases as nuclear charge (# of protons) increases Force of attraction decreases as the electron goes farther from the nucleus Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

In effect, the charge felt by an electron is less than the full nuclear charge Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

An electron is shielded from the full charge of the proton In effect, the charge felt by an electron is less than the full nuclear charge The effective nuclear charge (Zeff) is the nuclear charge that is actually felt by an electron An electron is shielded from the full charge of the proton greatly by the inner electrons only slightly by the other electrons in the same principal quantum number (n) not at all by outer electrons Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill. p. 249 “We speak of each electron “shielding” the other electrons somewhat from the nucleus. Shielding (also called screening) reduces the full nuclear charge to an effective nuclear charge (Zeff), the nuclear charge an electron actually experiences. This lower nuclear charge makes the electron easier to remove.” “Much greater shielding is provided by inner electrons. Because they spend nearly all their time between the outer electrons and the nucleus, inner electrons shield outer electrons very effectively, in fact, much more effectively that do electrons in the same subshell. Shielding by inner electrons greatly lowers the Zeff felt by outer electrons.” Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. p. 297 “In a neutral lithium atom, whose electron configuration is 1s22s1, the 2s electrorn is shielded by the two 1s electrons, but the 2s electron does not have a shielding effect on the 1s electrons.” Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

The effective nuclear charge and electron configuration are key in understanding the periodic trends Atomic radius Ionic radius neutral vs. charged isoelectronic series Ionization energy one atom vs. another same atom Electron affinity Physical properties Chemical properties Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. p. 303 “Not only is there a correlation between electron configuration and physical properties, but a close correlation also exists between electron configuration and chemical behavior.”

Atomic radius

Atomic radius is ½ the distance between the two nuclei in two adjacent atoms Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. pp. 297-298 “The electron density in an atom extends far beyond the nucleus, but we normally think of atomic size as the volume containing about 90 percent of the total electron density around the nucleus. When we must be even more specific, we define the size of an atom in terms of its atomic radius, which is one-half the distance between the two nuclei in two adjacent metal atoms.” Figure a. In metals such as beryllium b. for elements that exist as diatomic molecules Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

*Radii in pm Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. 1012 pm = 1 m 10-12 m = 1 pm *Radii in pm Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Left to right: decreasing atomic radius Zeff dominates number of protons increases electrons are added to the same n, so shielding by inner electrons does not change while shielding by electrons belonging to the same n is poor Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

Top to bottom: increasing atomic radius n dominates going down the group, each member has one more level of inner electrons that shield the outer electrons very effectively Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

Ionic radius: Neutral vs. charged Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. p. 299 “when a neutral atom is converted to an ion, we expect a change in size.”

If the atom forms a cation, its radius decreases same number of protons less electrons electron-electron repulsion is reduced the electron cloud becomes smaller Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

If the atom forms an anion, its radius increases same number of protons more electrons electron-electron repulsion is enhanced the electron cloud becomes bigger Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Ionic radius: Isoelectronic series

Ions having the same electron configuration and same number of electrons are said to be isoelectronic Ion Electron Configuration Number of Electrons F- [Ne] 10 O2- N3- Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Left to right: decreasing ionic radius same number of electrons increasing number of protons stronger attraction between the protons and the electrons

Ionization energy: One atom vs. another Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

energy + X(g)  X+(g) + e- Ionization energy (IE) is the minimum energy required to remove an electron from a gaseous atom energy + X(g)  X+(g) + e- The higher the IE, the more difficult it is to remove the electron Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. 8th ed. Phils: Pearson Education Asia Pte. Ltd.

Left to right: increasing ionization energy Zeff dominates number of protons increases same n stronger attraction between the protons and the electrons harder to remove an electron Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. p. 306 “Metals have relatively low ionization energies compared to nonmetals…The difference in ionization energies suggests why metals always form cations and nonmetals form anions in ionic compounds.” Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. Phils: Pearson Education Asia Pte. Ltd. Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Top to bottom: decreasing ionization energy n dominates there are more electrons in between the protons and the outer electrons weaker attraction easier to remove an electron Decreasing ionization energy Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. Phils: Pearson Education Asia Pte. Ltd. Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Ionization energy: Same atom

If more than one electron could be removed from the same atom, there will be different IE values X(g)  X+(g) + e- IE1 X+(g)  X2+(g) + e- IE2 X2+(g)  X3+(g) + e- IE3 . . Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Silberberg, M. 2010. Principles of General Chemistry. 2nd ed Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

For the same atom, IE1 < IE2 <IE3 same number of protons, less electrons same nuclear charge, less electron-electron repulsion greater attraction between the proton and the remaining electrons harder to remove another electron Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

the noble gas configuration is stable There is a dramatic increase in IE when an electron is removed from an atom/ion with a noble gas configuration the noble gas configuration is stable removing another electron from it will result in instability Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.

Electron affinity

X(g) + e-  X-(g) energy gained (+) or released (-) Electron affinity (EA) is the energy change accompanying the addition of electrons to atoms or ions X(g) + e-  X-(g) energy gained (+) or released (-) *usually released The more negative the electron affinity, the greater the tendency to accept an electron Note: different books may use different sign conventions Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill. p. 265 “In most cases, energy is released when the first electron is added because it is attracted to the atom’s nuclear charge. Thus, EA1 is usually negative. The second electron affinity, however, is always positive because energy must be absorbed to overcome electrostatic repulsions and add another electron to a negative ion.” Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. p. 308 Different convention: “The more positive the electron affinity, the greater the tendency too accept an electron” Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

Silberberg, M. 2010. Principles of General Chemistry. 2nd ed Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill. p. 265-266 “Negative values indicate that energy is released when the anion forms. Positive values, which occur in Group 8A, indicate that energy is absorbed to form the anion; in fact, these anions are unstable and the values are estimated.” “Electron affinities are difficult to measure, so values are frequently updated with more accurate data.” *Energies in kJ/mol Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

Left to right: increasing(?) electron affinity Zeff dominates size decreases stronger attraction between the protons and the added electron Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

Top to bottom: no general trend except for Group 1A the nucleus is farther away from an electron being added weaker attraction between the protons and the added electron Decreasing electron affinity Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.

Factors other than Zeff and atomic size affect electron affinities, so trends are not as regular as those for the other properties Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill. p. 266 “These exceptions (in the trend) arise from changes in sublevel energy and in electron-electron repulsion.”