• When there are six electron groups around the central atom, and two are lone pairs, the result is called a square planar shape . – The bond angles between equatorial positions are 90°.
Practice Problem on Molecular Geometry and Electron Groups
Using VSEPR to Predict Molecular Geometries The steps: 1. Draw the Lewis structure. 2. Determine the number of electron groups around the central atom. 3. Classify each electron group as a bonding or lone pair, and count each type. – Remember, multiple bonds count as one group. 4. Determine the shape and bond angles.
Practice Problem on Predicting Molecular Geometries Predict the molecular geometry and bond angle of ClNO Predict the molecular geometry of ICl 4 −
Representing Three-Dimensional Shapes on Paper • is difficult. • How to draw a 3-D representation of molecule on paper: – By convention, the central atom is put in the plane of the paper. – Put as many other atoms as possible in the same plane and indicate with a straight line . – For atoms in front of the plane, use a solid wedge . – For atoms behind the plane, use a hashed wedge .
Illustrations of Molecular Geometries of Molecules Using 3-D Notations
Multiple Central Atoms and Their Geometries • Many molecules have larger structures with many interior atoms. – Think of them as having multiple central atoms. • For multiple center molecules: – Each center atom has a designated shape. The shape around the: • N atom is trigonal pyramidal • Left C is tetrahedral • Right C is trigonal planar • O is bent
Practice Problem on Predicting Molecular Geometries for Multi-Centered Molecules Predict the geometry about each interior atom in each: Make a sketch of each of the molecules CH 3 CH 2 CH=CHCH 2 CH 3
Polarity of Molecules • For a molecule to be polar, it must have the following: – Polar bonds • Electronegativity difference • Bond dipole moments—measured – An unsymmetrical shape • Vector addition • Polarity affects the intermolecular forces of attraction. – Example: • Boiling points and solubilities – Like dissolves like • Nonbonding pairs affect molecular polarity, a strong pull in its direction.
Molecule Polarity • The bond between the H atom and Cl atom in HCl molecule is polar. • The bonding electrons are pulled toward the Cl end of the molecule because Cl is more electronegative (EN) than the H atom. – The net result is a polar molecule.
Molecule Polarity • The bond between the C atom and O atoms in CO 2 molecule is polar. • The bonding electrons are pulled toward the O ends of the molecule equally because O is more electronegative (EN) than the C atom. – The net result is a nonpolar molecule.
Molecule Polarity • The bond between the O atom and H atoms in H 2 O molecule is polar. • Both sets bonding electrons are pulled toward the O end of the molecule equally because O is more electronegative (EN) than the H atom.
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