Department of Chemistry and Biochemistry CHM Reeves CHM 101 – Chapter Seven Development of the Periodic Table Effective Nuclear Charge Sizes of Atoms and Ions Ionization Energy Electron Affinity Metals, Nonmetals and Metalloids Group trends for Active Metals Group Fends for Selected Nonmetals

Department of Chemistry and Biochemistry CHM Reeves Development of the Periodic Table

Department of Chemistry and Biochemistry CHM Reeves Effective Nuclear Charge In addition to explaining the periodic patterns observed in atomic properties, quantum mechanics also provides insights in the systematic variation in those properties. The concept of effective nuclear charge explains observed variations in atomic size and ionization energy (energy needed to remove an electron from an atom or ion). The effective nuclear charge is the charge that the nucleus exerts on the valence electrons after the screening effects of core electrons are taken into account.

Department of Chemistry and Biochemistry CHM Reeves Effective Nuclear Charge

Department of Chemistry and Biochemistry CHM Reeves Effective Nuclear Charge Consider the variation of atomic sizes for the alkali metals. Going down any group, the size increases as n increases. The valence electrons are shielded from most of the increased nuclear charge by the core electrons.

Department of Chemistry and Biochemistry CHM Reeves Effective Nuclear Charge

Department of Chemistry and Biochemistry CHM Reeves Effective Nuclear Charge Unlike core electrons, valence electrons are not effective screeners of nuclear charge. The sizes of atoms decrease from left to right in any row (shell) because of the increase in their effective charge (Z eff ).

Department of Chemistry and Biochemistry CHM Reeves Sizes of Atoms

Department of Chemistry and Biochemistry CHM Reeves Sizes of Atoms and Ions Metals form cations by loosing their all of their valence s and p electrons. Transition metals can also loose d electrons. Compared to metal atoms, their cations are smaller because they have fewer electrons, with their valence electrons in a lower shell.

Department of Chemistry and Biochemistry CHM Reeves Sizes of Atoms and Ions Nonmetals form anions by gaining enough electrons to complete their valence s and p subshells. Compared to nonmetal atoms, their anions are larger because they have more electrons. Thus, when two species have the same number of protons, the larger species will be the one the more electrons.

Department of Chemistry and Biochemistry CHM Reeves Sizes of Atoms and Ions

Department of Chemistry and Biochemistry CHM Reeves Isoelectronic Species Species with the same total number of electrons are isoelectronic N: N 3- : O:O 2- : Al:Al 3+ :

Department of Chemistry and Biochemistry CHM Reeves Summary of Size Effects Sizes of atoms increase going down any column of the periodic table. Sizes of atoms decrease going across any row of the periodic table. When species have the same number of protons, the larger the number of electrons, the larger the species. When species have the same number of electrons (isoelectronic), the larger the number of protons, the smaller the species.

Department of Chemistry and Biochemistry CHM Reeves Ionization Energy Ionization Energy is the energy required remove an electron from a species.

Department of Chemistry and Biochemistry CHM Reeves Ionization Energy

Department of Chemistry and Biochemistry CHM Reeves Ionization Energy

Department of Chemistry and Biochemistry CHM Reeves Summary of Ionization Energy Going down a column of the periodic table, Ionization Energies decreases as the sizes of the atoms increase. Going across a row of the periodic table, Ionization Energies increase as the sizes of the atoms decrease, except when subshells are filled or half-filled.

Department of Chemistry and Biochemistry CHM Reeves Electron Affinity Electron Affinity is the energy required or released when an electron is added to a neutral species forming an anion Electronic Affinity generally gets more negative (more energy released) from left to right, but is not changed from top to bottom. EA shows the same general trend exceptions as IP.