How to Determine the Molecular Geometry for a …



How to Determine the Molecular Geometry for a Compound

1. Draw the Lewis Dot Structure for the compound.

2. Establish the AXE designation for the Dot Structure as follows:

A = central atom

# of X’s = # of BONDED atoms to the central atom

# of E’s = # of LONE PAIRS of electrons on the central atom

The numbers of X’s and E’s are then written as subscripts.

DO NOT count Lone Pair electrons on the atoms which are connected to the central atom. ONLY the lone pairs of electrons on the central atom count as E’s.

3. The total number of X’s and E’s determine the ELECTRON PAIR geometry. (There are only 5 basic electron pair geometries)

NOTES:

1. If you only have 2 atoms bonded together, there is NO central atom and NO AXE designation. Since two points make a straight line, this geometry is always LINEAR.

2. If you have more than one central atom, determine the geometry on each atom separately. If the molecule is symmetrical, the geometry on each atom will be the same. If not, assign the geometry individually to each central atom.

3. If an atom has multiple bonds to a central atom, it is only counted ONCE. You count # of bonded atoms in this model, NOT # of bonds.

How to Determine the Hybridization for a Compound

1. Draw the Lewis diagram for the compound.

2. The hybridization must follow the format spxdy (where x cannot be higher than 3 and y cannot be higher than 2 in this class)

• The total number of bonds to the central atom (X + E) must add up to the same number of letters in the hybridization.

Ex.) AX3 = 3 bonds to the central atom so hybridization = sp2 (3 total letters)

AX4E = 5 total bonds to central atom so hybridization = sp3d (5 total letters)

Total Bonds

(X + E) |# bonded atoms (X) |# lone pairs (E) |AXE Notation |Molecular

Geometry |Bond

Angles |Example |Polarity | |

2

(Linear) |2 |0 |AX2 |Linear |180° |BeCl2 |POLAR, for AX2 ONLY if X groups not identical. | |

3

(Trigonal Planar) |3 |0 |AX3 |Trigonal Planar |120° |BCl3 |POLAR, ONLY if X groups not identical. | | |2 |1 |AX2E |Bent |118° |NO2- |YES, always polar. | |

4

(Tetra-hedral) |4 |0 |AX4 |Tetrahedral |109.5° |CCl4 |POLAR, ONLY if X groups not identical. | | |3 |1 |AX3E |Trigonal Pyramid |< 109.5° |NH3 |YES, always polar. | | |2 |2 |AX2E2 |Bent |< 109.5° |H2O |YES, always polar. | |

5

(Trigonal bipyramid) |5 |0 |AX5 |Trigonal Bipyramid |90°, 180°,120° |AsCl5 |POLAR, ONLY if X groups not identical. | | |4 |1 |AX4E

|See-saw |< 90°, 180°,120° |SeCl4 |YES, always polar. | | |3 |2 |AX3E2 |T-Shaped | ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download