Periodic Trends


The objective of this exercise is to observe periodic trends in bond angles as we move around the period table.


Use the specified calculation method to fill in the bond angles of the following molecules:

Molecule PM3 Bond Angle Ab Initio [HF/6-311G+(d,p)] Experimental Bond Angle
H2O 107.7 106.2 104.5
H2S 93.5 94.2 92.3
H2Se 93.6 94.2 92.3
F2O 101.0 103.6 103.2
Cl2O 109.2 114.1 110.8

In the following section, you will use the WebMO editor to determine the atomic radius of several atoms. Using the WebMO editor, draw a dimer of each of the following atoms. Enter adjust mode and select both atoms; the bond distance should be displayed in the status bar. Dividing this bond length by two yields the atomic radius for that atom.

Atom O S Se H F Cl
Radius .8 1.27 1.40 .36 .74 1.20

Describe and explain the trends observed in bond angle as a function of

Central Atom: As we proceed down the column, the bond angle decreases towards 90°. This pattern can be explained in terms of the Coulomb repulsion between each of the two hydrogen atoms. With a small central atom, the hydrogen atoms lie closer together and the bond angle must increase to maximize the distance between each atom. However, as the central size of the central atom increases, the distance between each hydrogen atom also increases. This results in much less Coulomb repulsion, and thus the bond angle approaches that of an ideal sp directed bond.

Outer Atom: As we proceed down the column, the bond angle varies in an interesting pattern. The bond angles in H2O are the are slightly less than ideal tetrahedral bond angle, due to the presence of a extra lone pair of electrons. However, when we examine F2O, we find that the bond angles are slightly less than those found in a water molecule. Perhaps there is a small bonding interaction resulting from a slightly overlap of the p oribtals of the two fluorine atoms. Interestingly, the bond angles of Cl2O are slightly greater than those found in a molecule of water. This is most likely the result of Pauli repulsion due to the significant overlap of the two chlorine electron clouds.

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