Quantum Mechanical Model of the Atom

For the last four years, we learned a very simplified model of the atom. It worked very well to explain the chemical properties of the first 20 elements, alkali metals, earth metals, carbon, the halogens and the Noble gases. However, we are about to venture in to the world of the Transition Metals and other atoms that do not obey the Octet Rule. For this, we need a better model...

What has stayed the same?

1. The Nucleus. As chemists, we really only focus on the protons, as they attract electrons.
2. Electron Shells. While it would be more correct to think of these as "Energy Levels", we can still imagine electrons existing in "shells" at different distances from the nucleus. We still (generally) fill these from the inside-out. The first energy level (shell) still takes 2 electrons. The second still takes 8 electrons. After we put 8 in the third energy level, we start using the fourth energy level. However, things get interesting at #21 (Scandium). 

What has changed? What is new?

1. Sub-levels (sub-shells). The electrons are arranged into sub-levels or sub-shells. In the first Energy Level, there is only one sub-level (called "1s"). In the second Energy Level, there are two (called "2s" and "2p"). The third and fourth Energy levels each have three ("3s", "3p", "3d", "4s", "4p" and "4d").
2. Electron configuration. We use the sub-levels to write electron configurations now. For example, magnesium used to be 2,8,2. Now it is 1s² 2s² 2p⁶ 3s²

We are going to use our understanding of atomic structure to:

• give electron configurations of ions
• explain properties of transition metals (and their ions)
• explain periodic trends (such as electronegativity and atomic radius)