Although all covalent bonds involve a sharing of one or more pairs of electrons between bonding atoms, most of the time this sharing is not equal. One of the atoms is more electronegative than the other so it “hogs” the shared bonded pair of electrons more of the time. This type of covalent bond is known as a polar covalent bond.

Electronegativity Difference If the difference in electronegativities is between: 1.8 to 4.0: Ionic 0.5 to 1.7: Polar Covalent 0.0 to 0.4: Non-Polar Covalent Example: NaCl Na = 0.8, Cl = 3.0 Difference is 2.2, so this is an ionic bond!

Bond Polarity Check the polarity of HCl: 3.0 – 2.1 = 0.9 Because it is polar, it has positive and negative ends Cl has a greater share in bonding electrons than does H. Cl has slight negative charge (-d) and H has slight positive charge (+ d) called DIPOLES

MOLECULAR GEOMETRY

VSEPR MOLECULAR GEOMETRY Valence Shell Electron Pair Repulsion theory. Molecule adopts the shape that minimizes the electron pair repulsions, by keeping them as far apart as possible! VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.

VSEPR charts Use the Lewis structure to determine the geometry of the molecule How the electrons are arranged determines the bond angles and shapes. Arrangement focuses on the CENTRAL atom for all data! Think REGIONS WHERE ELECTRONS ARE LOCATED rather than bonds (for instance, a double bond would only be 1 region)

Some Common Geometries Linear Tetrahedral Trigonal Planar

Structure Determination by VSEPR The electron pair geometry is TETRAHEDRAL 1) Looking at the central atom, how many bonding regions are there? 2) How many bonded atoms? How many lone pairs? Water, H2O The molecular geometry is BENT. 2 bond pairs 2 lone pairs

Structure Determination by VSEPR Ammonia, NH3 Draw! The electron pair geometry is Tetrahedral , and the Geometry name is TRIGONAL PYRAMID.

Polarity of Whole Molecule Determining the polarity of a bond was easy, remember how? Electronegativity values! But what about the whole molecule? Strangely enough, you can have polar bonds but is still a nonpolar molecule! SO…. How do you determine if a molecule itself is going to be polar or nonpolar? Look at the Lewis Structure and check out the central atom!

Determining if a Molecule is Nonpolar or Polar Look at what is attached to the central atom -If every attached structure is the same -If the attached structures are different -The molecule is symmetrical -The molecule is asymmetrical -There are NO lone pairs on central atom -There are lone pairs on central atom And therefore NONPOLAR And therefore POLAR

Here’s why… If a molecule has polar BONDS in it, it has dipoles because one end of the bond is more negative than the other. If these dipoles are the same strength (meaning same eneg value), then they can cancel each other out if they are in exactly opposite directions. If the dipoles are NOT the same strength because of different elements, then the dipoles don’t cancel and the molecule is polar If the dipoles are NOT in exactly opposite directions because of lone pairs, then the dipoles don’t cancel and the molecule is polar

Important Points to Note: The shape of the molecule can only cancel out polarity, it cannot create it. So if the molecule only has nonpolar bonds, the entire molecule will always be nonpolar If there is only one bond in the whole molecule, and it’s polar, then there is no other bond that could cancel it out. So any 2-atom molecules will automatically be a polar molecule if the bond between the atoms is polar.

No other bond to cancel out polarity… Polar Bond δ - δ + H Cl No other bond to cancel out polarity… Polar Molecule

O = C = O The molecule is nonpolar So dipoles cancel… Polar Bonds δ - δ + δ + δ - O = C = O However, the shape is symmetrical So dipoles cancel… The molecule is nonpolar

Nonpolar Bond Br Br If only nonpolar bonds… Nonpolar Molecule

Polar Bond Polar Bond δ - δ - δ + δ + Polar bonds that aren’t in exact opposite directions due to lone pairs… Polar Molecule!

Molecular Polarity “Like Dissolves Like” Polar dissolves Polar Nonpolar dissolves Nonpolar

Molecular Polarity This is why oil and water will not mix! Oil is nonpolar, and water is polar. The two will repel each other, and so you can not dissolve one in the other