Chemistry: Covalent Bonding and Molecular Geometry (VSEPR)
Predicting molecular shapes from Lewis structures and electron domains
Chemistry: Covalent Bonding and Molecular Geometry (VSEPR)
Predicting molecular shapes from Lewis structures and electron domains
Chemistry - Grade 9-12
- 1
Draw the Lewis structure for CO2. Identify the electron-domain geometry, molecular geometry, approximate bond angle, and whether the molecule is polar or nonpolar.
A double bond counts as one electron domain in VSEPR.
CO2 has two double bonds from carbon to oxygen and no lone pairs on the central carbon. It has 2 electron domains, a linear electron-domain geometry, a linear molecular geometry, a bond angle of 180 degrees, and it is nonpolar because the bond dipoles cancel. - 2
For CH4, count the electron domains around the central carbon atom and predict the molecular geometry and approximate H-C-H bond angle.
CH4 has 4 bonding domains and 0 lone pairs around carbon. Its molecular geometry is tetrahedral, and the approximate H-C-H bond angle is 109.5 degrees. - 3
Explain why NH3 has a trigonal pyramidal molecular geometry instead of a trigonal planar geometry.
VSEPR molecular geometry describes the positions of atoms, not lone pairs.
NH3 has three N-H bonds and one lone pair on the central nitrogen atom. The four electron domains form a tetrahedral electron-domain geometry, but the lone pair is not counted as an atom in the molecular shape, so the molecular geometry is trigonal pyramidal. - 4
Water, H2O, has two O-H bonds and two lone pairs on oxygen. Predict its electron-domain geometry, molecular geometry, and approximate bond angle.
Lone pair-lone pair repulsions are stronger than bonding pair-bonding pair repulsions.
H2O has 4 electron domains around oxygen, so its electron-domain geometry is tetrahedral. Its molecular geometry is bent, and its H-O-H bond angle is about 104.5 degrees because the lone pairs compress the bond angle. - 5
BF3 has boron as the central atom with three B-F bonds and no lone pairs on boron. Predict its molecular geometry, approximate bond angle, and polarity.
BF3 has 3 electron domains around boron, so its molecular geometry is trigonal planar. The bond angles are about 120 degrees, and the molecule is nonpolar because the three identical B-F bond dipoles cancel in a symmetrical shape. - 6
Compare NH3 and BF3. Both contain three atoms bonded to the central atom. Explain why they have different molecular geometries.
Look at the number of lone pairs on the central atom.
NH3 has three bonding domains and one lone pair on nitrogen, so its molecular geometry is trigonal pyramidal. BF3 has three bonding domains and no lone pairs on boron, so its molecular geometry is trigonal planar. The lone pair on NH3 changes the molecular shape. - 7
For PCl5, identify the number of electron domains around phosphorus, the molecular geometry, and the common bond angles.
PCl5 has 5 bonding domains and 0 lone pairs around phosphorus. Its molecular geometry is trigonal bipyramidal, with common bond angles of 90 degrees, 120 degrees, and 180 degrees. - 8
For SF6, identify the number of electron domains around sulfur, the molecular geometry, the common bond angles, and whether the molecule is polar or nonpolar.
A fully symmetrical molecule with identical outer atoms is often nonpolar.
SF6 has 6 bonding domains and 0 lone pairs around sulfur. Its molecular geometry is octahedral, the common bond angles are 90 degrees and 180 degrees, and it is nonpolar because the six identical S-F bond dipoles cancel in the symmetrical structure. - 9
SO2 has sulfur as the central atom with two S-O bonding regions and one lone pair on sulfur. Predict the electron-domain geometry, molecular geometry, and whether SO2 is polar.
A lone pair can make a molecule bent even when there are only two bonded atoms attached to the central atom.
SO2 has 3 electron domains around sulfur, so its electron-domain geometry is trigonal planar. Its molecular geometry is bent, and it is polar because the bent shape prevents the S-O bond dipoles from canceling. - 10
The nitrate ion, NO3-, has three equivalent N-O bonds around nitrogen and no lone pairs on nitrogen. Predict its molecular geometry and approximate bond angles.
NO3- has 3 electron domains around nitrogen and no lone pairs on the central atom. Its molecular geometry is trigonal planar, and the O-N-O bond angles are about 120 degrees. - 11
Draw the Lewis structure for NH4+. Then identify the molecular geometry and approximate H-N-H bond angle.
The positive charge means the ion has one fewer total valence electron than neutral NH4 would have.
NH4+ has nitrogen bonded to four hydrogen atoms with no lone pairs on nitrogen. Its molecular geometry is tetrahedral, and the approximate H-N-H bond angle is 109.5 degrees. - 12
XeF4 has four Xe-F bonds and two lone pairs on xenon. Predict its electron-domain geometry, molecular geometry, and polarity.
In AX4E2 molecules, the two lone pairs are placed opposite each other.
XeF4 has 6 electron domains around xenon, so its electron-domain geometry is octahedral. Its molecular geometry is square planar, and it is nonpolar because the four Xe-F bond dipoles cancel in the symmetrical square planar shape. - 13
ClF3 has three Cl-F bonds and two lone pairs on chlorine. Use VSEPR to predict its molecular geometry.
ClF3 has 5 electron domains around chlorine: 3 bonding domains and 2 lone pairs. The electron-domain geometry is trigonal bipyramidal, and the molecular geometry is T-shaped. - 14
SF4 has four S-F bonds and one lone pair on sulfur. Predict its electron-domain geometry and molecular geometry.
A molecule with 5 electron domains, 4 bonded atoms, and 1 lone pair is classified as AX4E.
SF4 has 5 electron domains around sulfur, so its electron-domain geometry is trigonal bipyramidal. With one lone pair and four bonded atoms, its molecular geometry is seesaw. - 15
A student claims that every molecule with polar bonds must be a polar molecule. Use CO2 and H2O as examples to explain why this claim is not always true.
Polarity depends on both bond polarity and molecular geometry.
The claim is not always true because molecular shape determines whether bond dipoles cancel. CO2 has polar C-O bonds, but its linear shape makes the dipoles cancel, so CO2 is nonpolar. H2O has polar O-H bonds and a bent shape, so its dipoles do not cancel and H2O is polar.