Electron Pairs Push Apart: VSEPR Molecule Shapes

0:00Carbon dioxide and water both have three atoms. Why is one straight and one bent?

0:07Around a central atom, each bond is one electron domain, and so is each lone pair, an unshared pair of electrons.

0:16Electrons are negative, so domains repel and spread as far apart as they can. This is VSEPR: valence shell electron pair repulsion.

0:28Carbon dioxide has two double bonds and no lone pairs on carbon. Each double bond counts as one domain.

0:36They point opposite ways, so the shape is linear. The angle between the bonds, the bond angle, is 180 degrees.

0:45Boron trifluoride has three bonds and no lone pairs on boron. They spread into a flat triangle, 120 degrees apart: trigonal planar.

0:56Methane has four bonds and no lone pairs. In three dimensions, they point to the corners of a tetrahedron, 109.5 degrees apart.

1:07Ammonia also has four domains: three bonds and one lone pair.

1:12A lone pair is held by only one nucleus, so it spreads wider and pushes harder. The bonds squeeze to about 107 degrees.

1:22Shapes are named from the atoms only, so ammonia is trigonal pyramidal, though its lone pair takes up space.

1:31Water has four domains too: two bonds and two lone pairs.

1:36Two lone pairs squeeze the bonds even more, to about 104.5 degrees. So water is bent, not straight: its lone pairs take up space.

1:48Electron pairs push apart to shape a molecule, and lone pairs push hardest.