Is there a simple way to remember the shapes of molecules & their bond angles?

1 Answer

User Comments

llltkl's profile pic

llltkl | College Teacher | (Level 3) Valedictorian

Posted on

There is no simple shortcut to accommodate all the molecules, though there are a few rules of thumb. The best way to remember the structure and bond ngles of a molecule is by looking through its central atom and applying VSEPR rules. The steps are:

1) Write the molecule in Lewis form.

2) Count the total number pairs of valence electrons around the central atom, by the formula (V+A±-charge)/2, where v=no. of own valence electrons of the central atom, A = no. of surrounding bonds from it.

3) Subtract the number of pi-electron pairs (if any) to get X,

4) If X is,  2 = linear, 180 degrees, 3 = trigonal planar, 120 degrees, 4 = tetrahedral, 109.5 degrees, 5 = trigonal bipyramidal, 120, 180 & 90 degrees, 6 = Octahedral (Square bipyramidal) 90 & 180 degrees.

5) Look for the presence of lone electron pairs from the difference in count obtained in 2) and the total number of bonds. If present, modify the structure and bond angles, keeping in mind that the lone pairs repel other lone pairs or bond pairs most dominantly. For molecules where lone pair is absent, pure bond angles and geometries stated above shall prevail.