VSEPR THEORY

VSEPR Theory: (Valence Shell Electron Pair Repulsion) 
valence electrons in an atom will orient themselves to be as far apart as possible

* CLASSES OF MOLECULES

A = central atom
B = surrounding atoms
E = unshared electron pairs on central atom
*** subscript tells how many of each you have ***

EXAMPLE:    CH4            H
                                   ..
                              H : C : H
                                   ..
                                   H

1 central atom, 4 surrounding atoms, no unshared e- prs. on central atom
CLASS = AB4

EXAMPLE 2:    NH3            ..
                                 H : N : H
                                      ..
                                      H

1 central atom, 3 surrounding atoms, 1 unshared e- pr. on central atom
CLASS = AB3E


* SHAPES OF MOLECULES

CLASS                    SHAPE  
AB2                        linear
AB2E                      bent
AB2E2                     bent
AB3                        trigonal planar
AB3E                      trigonal pyramid
AB4                        tetrahedral


* POLARITY OF MOLECULES (symmetry of molecules) – different from bond polarity

A MOLECULE is nonpolar if...
    - there are no unshared e- pairs around the central atom
        AND
    - the surrounding atoms are all the same element

A BOND is nonpolar (covalent) if…
    - it occurs between two atoms of the same element

IT IS POSSIBLE, EVEN LIKELY, THAT A NONPOLAR MOLECULE HAS POLAR COVALENT BONDS!

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