What shape are your molecules in? VSEPR What shape are your molecules in?

Background you need… Lewis structures Bonding (covalent) 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 CH4 must have 8 total

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 NH4 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: 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

Linear AX2

Trigonal planar AX3

Tetrahedral AX4

Pyramidal (Trigonal or tetrahedral) Tetrahedral parent shape 1 lone pair of electrons AX3E

Bent Tetrahedral parent shape 2 lone pair of electrons AX2E2

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