VSEPR What shape are your molecules in?

Background you need…  Lewis structures  How many bonds do each element make?  What can expand?  Bonding (covalent)  Polarity  Electronegativity and determining bond type  Resonance v. Isomers  Formal charge Let’s review now…..

Lewis Structures  Remember that Lewis structures want a full outer shell  Remember that for a given Lewis structure, the number of electrons around the atoms must equal the total number of electrons individually assigned.  Ex: C has 4, H has 1, so CH 4 must have 8 total

Isomers  Same formula, different arrangement of atoms  Physically break bonds and MOVE atoms

Resonance Structures  Have the same alignment of atoms, but different bonding (electrons ONLY are moved, both in bonds and lone pairs)

Determining formal charge Formal charge can be determined by: Normal number of electrons in outer shell - [(1/2 the number of bonded electrons) + lone electrons] _____________________________________ = formal charge Example: N in NH 4 FC =5- [(1/2 of 8)+ 0]= +1

Formal charge and stability  The most “happy” molecules tend to have no formal charges  However, molecules may be “happy” if they have not NET charge on them (if there is 1+ and 1-, so a net of +1 + (- 1)=0)  Resonance structures that are the best have a minimal formal charge and a full octet around each atom

What is VSEPR?  Valence  Shell  Electron  Pair  Repulsion  Theory

Why?  The shape of molecules influences their characteristics:  Things like polarity which influence things like  boiling point, melting point, which dictate their nature (solid, liquid or gas at room temperature)

The parent geometries: all others come from these

Steric Number  The number of “things” sprouting off of an atom  These can be either  Bonds  Of any order (1, 2, or 3) Or  Lone pairs of electrons

Steric Number Examples  Ex #1: CH 4  There are 4 H’s branching off, so the steric number is 4  SN=4  Ex #2: H 2 O  SN= 4  Explain why  Ex #3: CO 2  SN= 2  Explain why

General Formulas  All molecules with a shared general formula have a shared geometry  we use them to help note shape  Formulas are typically written with A’s, X’s, and E’s The letters stand for:  A= the central atom  X *= the number of atoms attached to the central atom  E= the number of lone pairs of electrons attached to the central atom  *Some sources use A’s, B’s, and E’s

General Formula Examples  Ex #1: CH 4  AX 4  Ex #2: H 2 O  AX 2 E 2  Ex #3: CO 2  AX 2

Linear

Trigonal planar  AX 3

Tetrahedral  AX 4

Pyramidal (Trigonal or tetrahedral)  Tetrahedral parent shape  1 lone pair of electrons  AX 3 E

Bent  Tetrahedral parent shape  2 lone pair of electrons  AX 2 E 2

When determining polarity it is important to look at the dipole moments- do they cancel out?

Trigonal bipyramidal  AX 5

Seesaw a.k.a. Teeter-totter  Trigonal bipyramidal parent shape  1 lone pair of electrons  AX 4 E

T-shaped  Trigonal bipyramidal parent shape  2 lone pair of electrons  AX 3 E 2

Linear  Trigonal bipyramidal parent shape  3 lone pair of electrons  AX 2 E 3

Octahedral  AX 6

Square pyramidal  Octahedral parent shape  1 lone pair of electrons  AX 5 E

Square planar  Octahedral parent shape  2 lone pair of electrons  AX 4 E 2

T-shaped  Octahedral parent shape  1 lone pair of electrons  AX 3 E 3

Summary of shapes

ID these VSEPR shapes…

Sweet drill and practice web site  Given generic shapes to ID:  1.php?q=1 1.php?q=1 1.php?q=1  Given molecules to draw out:  Basic: drills.com/VSEPR-1.php?q=2 drills.com/VSEPR-1.php?q=2http:// drills.com/VSEPR-1.php?q=2  Advanced: drills.com/VSEPR-1.php?q=3 drills.com/VSEPR-1.php?q=3http:// drills.com/VSEPR-1.php?q=3