Lewis Structures, Formal Charge, VSEPR & Polarity

How total valence electrons are adjusted for polyatomic ions

Add one electron for every negative charge; subtract one electron for every positive charge.

Formula for Formal Charge (FC)

FC = (Valence e-) - (Unshared lone pair electrons) - 1/2(Bonding electrons).

Formal charge criteria for selecting the most optimal Lewis structure

Formal charges are minimized (closest to 0), and any negative formal charge is placed on the most electronegative atom.

Calculated formal charge on the Carbon atom in Carbon Dioxide (CO2, O=C=O)

0 (4 valence - 0 lone dots - 4 bonds = 0).

Calculated formal charge on Nitrogen in the Ammonium ion (NH4+)

+1 (5 valence - 0 lone dots - 4 bonds = +1).

Elements capable of forming hypervalent expanded octets

Nonmetals in Period 3 or below (n >= 3, e.g., P, S, Cl, Xe) due to accessible d-orbitals.

Stable valence electron count for Boron in neutral covalent compounds like BF3

6 electrons (sub-octet exception).

Definition of Electron Geometry vs. Molecular Shape

Electron geometry describes the 3D arrangement of all electron regions; molecular shape describes the positions of the bonded atoms only.

Number of electron regions around central atom in a double or triple bond

Exactly 1 electron region (bond multiplicity does not increase steric number).

Why lone pairs distort ideal bond angles downward

Lone pairs are held by only one nucleus and occupy more space, exerting greater repulsion on neighboring bonding pairs.

Electron geometry, molecular shape, and bond angle of CO2

Linear electron geometry, Linear shape, 180°.

Electron geometry, molecular shape, and bond angle of SO2

Trigonal Planar electron geometry, Bent shape, <120° (due to 1 lone pair on S).

Electron geometry, molecular shape, and bond angle of Ammonia (NH3)

Tetrahedral electron geometry, Trigonal Pyramidal shape, <109.5° (~107°).

Electron geometry, molecular shape, and bond angle of Water (H2O)

Tetrahedral electron geometry, Bent shape, <109.5° (~104.5°).

Electron geometry and molecular shape of Sulfur Tetrafluoride (SF4)

Trigonal Bipyramidal electron geometry, Seesaw shape (5 regions: 4 bonds, 1 equatorial lone pair).

Electron geometry and molecular shape of Chlorine Trifluoride (ClF3)

Trigonal Bipyramidal electron geometry, T-shaped (5 regions: 3 bonds, 2 equatorial lone pairs).

Electron geometry and molecular shape of Xenon Difluoride (XeF2)

Trigonal Bipyramidal electron geometry, Linear shape (5 regions: 2 axial bonds, 3 equatorial lone pairs).

Electron geometry, molecular shape, and bond angle of Sulfur Hexafluoride (SF6)

Octahedral electron geometry, Octahedral shape, 90°.

Electron geometry and molecular shape of Bromine Pentafluoride (BrF5)

Octahedral electron geometry, Square Pyramidal shape (6 regions: 5 bonds, 1 lone pair).

Electron geometry, molecular shape, and polarity of Xenon Tetrafluoride (XeF4)

Octahedral electron geometry, Square Planar shape, Nonpolar (lone pairs opposite at 180°, 4 Xe-F dipoles cancel).

Two conditions necessary for a molecule to possess an overall net dipole moment (polar)

1) It must contain polar covalent bonds; 2) Its 3D geometry must be asymmetric so bond dipoles do not cancel.

Polarity comparison: Carbon tetrachloride (CCl4) vs. Dichloromethane (CH2Cl2)

CCl4 is nonpolar (tetrahedral symmetry cancels dipoles); CH2Cl2 is polar (asymmetric dipoles do not cancel).

Molecular shape and overall polarity of Ozone (O3)

Bent molecular shape; Polar (lone pair on central oxygen causes dipole asymmetry).

Number of lone pairs on the central atom of the Triiodide ion (I3-)

3 lone pairs (gives 5 electron regions, resulting in a Linear molecular shape).