Chemistry Lecture Notes

Chemistry Notes

Dr. Thomas, 8:00-9:50 TR

Tuesday, November 17, 1998

Today's Lecture: Hybridized Orbitals, Molecular Orbitals, Conjugated P-systems

Chapters 16.1-2, 16.8

Hybridized Orbitals (continuing from Thursday)

Definitions:

s-bond - overlap of the orbitals lies along the internuclear axis (directly between the atoms)

p-bond - overlap of the orbitals does not lie along the internuclear axis

Number of Atoms Bonded	Shape	Hybridized Orbital	Example Molecule
6	Octahedral	d²sp³	SF₆
5	Trigonal bipyramid	dsp³	PF₅
4	Tetrahedral	sp³	CH₄
3	Trigonal planar	sp²	NH₃
2	Linear	sp	CO₂

Note: 5 and 6, that is, the hybridized d orbitals, are not going to be on the quiz. They're just to satisfy your raving curiosity.

The hybridized orbitals basically look like stretched-out balloons going to the other atoms (from C to the 2 Os, N to the three Hs, C to the four Hs, etc.). See Diagram 16.11, which shows a BeH₂ atom forming. The yellow orbitals on the right are the H 1s orbitals, while the green ones are the Be sp orbitals. Where they overlap, there is a s-bond.

Molecular Orbitals

formed from combinations of atomic orbitals
if n atomic orbitals are combined, n molecular orbitals come out

Homonuclear diatomics (like H₂, Br₂, Cl₂, etc.)

This drawing shows two atoms, A and B, with one s orbital. Each orbital has a + and a - component (not magnetic charge, but to denote the two electrons with opposing spin). When the s orbital is combined, both the + and the - component come in contact. Where they are the same, there is a bonding orbital, shown as A+B, which can be either + or - in electron spin. However, where they are different, there is a nodal plane (where the + and - cancel, making it 0), and two orbitals of differing spin. This is called an antibonding orbital. The bonding orbital is lower energy than the two atoms apart, and the antibonding orbital is higher energy. The next picture shows H₂, which always occurs when atomic H is brought together.

The table of how homonuclear diatomics bond up to Ne is on pg. 589. You can see that N bonds with four orbitals, two s-bonds with the s orbital and one p orbital, one antibonding s orbital (marked s^*), and two p-bonds with two p orbitals. This makes 4 - 1 bonds, or a bond order of 3. Looking at the Lewis dot diagram, we see that N has a triple bond. These basic rules apply in the same way to heteronuclear bonding, which Thomas will explain Thursday (I think). To understand these a little better, look at pp. 590-592.

Conjugated P-systems

These are best explained by the first three pages of Chapter 16.8. Dr. Thomas will talk more about them on Thursday. Basically, these are long strings of p bonds which stick together into one long orbital. (Diagram 16.34) They are lower energy, and absorb lower energy photons (the chapter is specifically about the ones in the visible spectrum.)