Double Bonds with two bonding pairs (linear):
oxygen O=O carbon dioxide O=C=O carbon disulfide S=C=STriple Bonds with three bonding pairs (linear):
carbon monoxide C≡O cyanide ion C≡N- nitrogen N≡NIn electron pair repulsion theory, a double or triple bond counts as one region of electron density (one bonding area), because all of its electrons lie between the same two atoms. So it is the number of bonding areas and lone pairs, not the number of electron pairs, that decides the shape.Carbon dioxide has two double bonds and no lone pairs on carbon. The two bonding areas repel as far apart as possible, so O=C=O is linear. Hydrogen cyanide, H-C≡N, also has two bonding areas around carbon and is linear.In ethene, H2C=CH2, each carbon has three bonding areas (two C-H bonds and the C=C bond) and no lone pairs, so the arrangement around each carbon is trigonal planar.Sulfur dioxide, O=S=O, has two bonding areas and one lone pair on sulfur, so the molecule is non-linear (bent).
Worked example: predict the shape of methanal, H2C=O. Carbon uses all four of its outer electrons in bonding (two C-H bonds and one C=O bond), so there are three bonding areas and no lone pairs. Methanal is trigonal planar.
oxygen O=O carbon dioxide O=C=O carbon disulfide S=C=STriple Bonds with three bonding pairs (linear):
carbon monoxide C≡O cyanide ion C≡N- nitrogen N≡NIn electron pair repulsion theory, a double or triple bond counts as one region of electron density (one bonding area), because all of its electrons lie between the same two atoms. So it is the number of bonding areas and lone pairs, not the number of electron pairs, that decides the shape.Carbon dioxide has two double bonds and no lone pairs on carbon. The two bonding areas repel as far apart as possible, so O=C=O is linear. Hydrogen cyanide, H-C≡N, also has two bonding areas around carbon and is linear.In ethene, H2C=CH2, each carbon has three bonding areas (two C-H bonds and the C=C bond) and no lone pairs, so the arrangement around each carbon is trigonal planar.Sulfur dioxide, O=S=O, has two bonding areas and one lone pair on sulfur, so the molecule is non-linear (bent).
Worked example: predict the shape of methanal, H2C=O. Carbon uses all four of its outer electrons in bonding (two C-H bonds and one C=O bond), so there are three bonding areas and no lone pairs. Methanal is trigonal planar.