Covalent Bonding Molecular Polarity

Shape and Geometry Review

Covalent Bonds involve sharing electrons But Just like in real life, not all sharing Is equal !!!

Electronegativity is the ability of an atom to Attract electrons to itself in a covalent bond Large Electronegavity Small Electronegativity

So, the atoms in a covalent bond are engaged in a tug-of-war over electrons

Results of Tug of War

The atom that “wins” will be the one that is more electronegative. Because this atom gets the electrons more of the time, it becomes slightly negative The atom that “loses” becomes slightly positive

The Periodic Table allows us To predict differences in electronegavity F is the most electronegative

+ - A. Dipole Moment H Cl Means the direction of the polar bond in a molecule. Arrow points toward the more electronegative atom. (The direction the electrons will flow H Cl + -

B. Determining Molecular Polarity Depends on: Electronegativity differences dipole moments molecular shape

B. Determining Molecular Polarity Polar Molecules Dipole moments are asymmetrical or have lone pairs,and don’t cancel . H2O H O net dipole moment

B. Determining Molecular Polarity Nonpolar Molecules Dipole moments are symmetrical and cancel out. BF3 F B

B. Determining Molecular Polarity Therefore, polar molecules have... asymmetrical shape (lone pairs) or asymmetrical atoms CHCl3 H Cl net dipole moment

Linear molecules can be nonpolar . . . Example: CO2 What makes the difference? No Net Dipole Moment Or Polar… Example: HCN Net Dipole Moment Towards N N has a pair of unshared electrons here

Trigonal Planar molecules are usually nonpolar Example: BF3 But, what makes this one polar? Example: BF2Cl

Trigonal Pyramidal Molecules are Polar. Example: NH3 Net Dipole Toward N

Explain this! Tetrahedral Molecules can be polar Example: CH3F Or non polar Example: CF4 Explain this!

Can a Molecule be Both Polar and NonPolar? Trigonal planar with Asymmetric atoms Tetrahedral with symmetric atoms Polar End Non Polar End

So, polarity can be more of a spectrum than a black and white determination Polar------------------------------------------------------nonpolar

Ionic, Polar, Nonpolar: A Continuum Electrons transferred E- shared unequally E- shared Huge charge separation no charge sep. some charge separation High MP/BP Low MP/BP Solids at STP Liquids at STP Gas at STP STP is Standard Temperature and Pressure (273 K and 1 atm)

More Nonpolar Molecules Share electrons the “best” Have the closest electronegativities (or smallest differences) Have lower boiling and melting points . . .so More likely to be gases at room temperature Like dissolves like . . . Use a nonpolar solvent to dissolve a nonpolar solute

More Polar Molecules Do not share well at all! Their electronegativity differences are great Are more like ionic compounds Have higher melting and boiling points Most likely to be liquids at room temperature Like dissolves like . . . Use a polar/ionic solvent to dissolve a polar/ionic solute