Slides10

Lecture #8

CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.

Lecture Outline

The Periodic Table (continued)

Electron configurations (exceptions)

Transition metal ions

Ionization energies

Second and third ionization energies

All the rules we've seen about the electron configuration filling sequence apply to both atoms and ions of the main group elements (s-block and p-block) and to the transition metal atoms. For transition metal ions, the sequence changes!

To explain why the 4s-3d order shifts when ionizing an element, we resort to a familiar example, sodium, and look at its (excited) 4s and 3d excited one-electron orbital energies. Z_eff refers to that for atomic Na in this illustration

The states are graphed at their proper energies, but we've indicated the effective nuclear charges drived from these energies. If an inner electron is removed completely from Na, we can estimate that the Z_eff for the outer valence electron osup by approximately 1.

The other changes are similarly estimated and -- lo and behold -- the 3d is now more stable than the 4s orbital. When these are filled as in the transition metal ions, the 3d is occupied while the 4s is empty.

(Keep in mind that the values shown are just the effective nuclear charges, but the position of the energy level corresponds to E as determined by n and Z_eff.)

For this course, 09-105, at CMU, we will adopt the general rule that notes a different configuration decision about transition metal ions as opposed to atoms and main group ions.

The energy needed to remove the easiest-to-remove electron from a neutral atom is called the first ionization energy.

First ionization energies for light elements. Using this information, we can estimate the effective nuclear charge, Z_eff, for the electron being removed. Helium as an example.

Z_eff for lithium

Z_eff for neon

The detailed trend in ionization energies for the light elements

The n=1 shell filling

The n=2 shell filling after which the n=3 shell starts

Starting the p-subshell causes a break in the smooth trend across the row.

Starting to pair up electrons after half the p-subshell is filled causes a second break, which we referred to as the mid-shell dip, in the smooth trend across the row.

First ionization energies across rows 1 through 3 of the Periodic Table

Overlapping the 2nd and 3rd row element ionization energies to demonstrate the repeating pattern (determined by valence electron configuration)

Second ionization energies

First, second, and third ionization energies for the light elements

First, second, and third ionization energies shifted to show, again, that valence electron configuration is the determining driver

Electron affinity is the energy involved in adding an electron to a neutral atom to form a negative ion. It is numerically equal to minus the ionization energy for that negative ion. As such, we should expect that the electron affinities also depend on electron configuration.