Aufbau Principle and Hund’s Rule

Electron Configurations and the Periodic Table

Effective Nuclear Charge Simplest (Zeff = Z – core electrons) Sodium: Zeff = 11 – 10 = 1 [Cl: 7] Use Bohr’s equation, substituting Zeff for Z and using nfinal = ∞ for IE. Sodium: Zeff = 1.84 [Cl: 2.93] Slater’s rule: differing effects of electrons in different orbitals Sodium: Zeff = 2.2 [Cl: 6.1]

Three Trends Atomic radius Ionization Energy Electron Affinity Energy required to remove an electron from a gaseous atom or ion. Electron Affinity Energy change associated with the addition of an electron to a gaseous atom or ion.

IE vs. Atomic Number

IE vs. Atomic Number

IE vs. Atomic Number

IE vs. Atomic Number

Successive IEs

EA vs. Atomic Number

Boxes of different dimensions  

Forming a molecule Simply combining atomic orbitals.

Simplest Model: Valence Bond Theory

But What About… Methane (CH4)? Carbon monoxide (CO)? Why not CH2 (since we have two unpaired 2p electrons)? Why is this a tetrahedral (since the p orbitals are orthogonal to each other) with four equivalent bonds in the Lewis structure? Carbon monoxide (CO)? Why is there a triple bond (we can’t see this from atomic Lewis structures)? Nitrogen monoxide (NO)? Odd number of electrons. Oxygen (O2)? (O2+)? Why is O2 magnetic? Why is the bond strength of O2+ greater than that of O2?

Hybridization (Part of VBT) The 2s and 2p orbitals are similar in energy. Recall that they are identical for hydrogen. Combining the wavefunctions for 2s and 2p gives a solution to the Schrödinger equation. The s orbital is combined with p orbitals – the number of p orbitals used depends on the molecule to be formed. The resulting (new) atomic orbitals are hybridized orbitals. A molecule is not simply atoms stuck together.