Unit 6 – Bond Polarity and Lewis Dots of Molecules Chapter 9

Valence Electrons Definition: The electrons in the highest energy level “s” and “p” orbitals All elements in the same group have same number of valence electrons. Only valence electrons participate in bonding. Electron dot structures tell us how an atom will bond. An atom will only bond if it has unpaired electrons.

Valence and Electron Dot Notation A way to show the valence electrons that an element has. Group# valence e-’sElectron configuration Dot Diagram 11s1s1 X. 22s2s2.X..X. 13/3A3s2p1s2p1.. X. 14/4A4s2p2s2p2.. X..

Group# valence e-’sElectron configuration Dot Diagram 15/5A5s2p3s2p3. :X.. 16/6A6s2p4s2p4. :X:. 17/7A7s2p5s2p5.. : X... 18/8A8s2p6s2p6.. :X:..

Octet Rule Bonds between atoms in compounds tend to form so each atom has the electron configuration of a noble gas. (That means 8 electrons in the highest energy level “s” and “p” orbitals) The atom does this by Gaining, Losing or Sharing electrons.

Covalent Bonds When two or more non-metals share a pair of electrons. Hydrogen is considered a non-metal. When illustrating covalent bonds: – A line indicates a shared pair in a molecular diagram. – Un-bonded electron pairs are shown as dots.

Examples of Covalent Molecules H2OH2O NH 3 CH 4

Electronegativity and Bonds Electronegativity – The ability of an atom to attract electrons to itself when bonded with another atom. – Based on a scale from 0 to 4.0 – Fluorine has the highest electronegativity at 4.0 – Every other atom is compared to Fluorine.

Bond Polarity Electronegativity Trend – Increases up and to the right.

Bond Polarity Most bonds are a blend of ionic and covalent characteristics. Difference in electronegativity determines bond type.

Electronegativity Difference Electrons are not necessarily always shared equally between 2 atoms. The difference in electronegativity values of 2 atoms involved in a bond tells us how equally atoms share the electrons between them.

Nonpolar Covalent Bond – e - are shared equally – symmetrical e - density – usually identical atoms – Electronegativity Difference – 0 to 0.4 Bond Polarity

++ -- Polar Covalent Bond – e - are shared unequally – asymmetrical e - density – results in partial charges (dipole) – Electronegativity difference = 0.5 to 1.7

Bond Polarity Non-Polar Covalent Polar Covalent Ionic ΔEN = 0 to 0.4 ΔEN = 0.5 to 1.7 ΔEN > 1.7

Calculating ΔEN & Classifying Bond Type Examples: Cl 2 HCl NaCl =0.0 Nonpolar =0.9 Polar =2.1 Ionic

Bond Polarity Only partial charges, much less than a true 1+ or 1- as in ionic bond Written as: H Cl the positive and minus signs (with the lower case delta: ) denote partial charges.    and  

Bond Polarity Can also be shown: – the arrow points to the more electronegative atom. HCl

Molecule Polarity Comparing electronegativities can help determine bond polarity, but in a molecule, there are several bonds, so molecule polarity has to be determined in a different way. To determine Molecular Polarity 3 things need to be considered 1) The molecules Lewis dot structure 2) The molecule’s symmetry 3) The molecule’s shape

Let’s start with LEWIS DOT STRUCTURES!  For an atom, it displays the # of valence electrons.  They can help us to predict bonds, atom arrangements in molecules, shapes of molecules and polarity of molecules. The key to finding Electron Dot Structures is to use “old world matchmaking” In the “old world” no mother wanted her child unmarried. To translate this to chemistry, no atom wants any of its’ electrons unpaired. It does not matter to the atom if the electron forms a bond or an unbonded pair, just as long as none of its electrons are single. Your job is to make sure that there are no single electrons!

To Draw Lewis Dot Structures for Covalent Molecules… 1)Draw the dot structure of the central atom. >The central atom is usually the atom you have only 1 of. >Carbon, oxygen, nitrogen, boron, silicon, phosphorus and sulfur are often the central atom because they have two or more unpaired electrons. --Note: If there are only two atoms in the molecule, then there is no central atom. Draw the dot structures of both, side by side.

2) Count how many unpaired electrons the central atom has. >This is how many bonds the central atom will form.  When determining this, put one electron on each of the four sides first, then start to double up. --How many unpaired electrons are in each of the drawings? C So C will form 4 bonds O So O will form 2 bonds

3) Place the remaining elements around the central atom and draw their dot structures (use x’s instead of dots to represent electrons). >When drawing the bonds, it is best to place single electrons on one atom next to single electrons on the central atom. >Go through your matchmaking process. Double check to make sure that each atom has a full eight electrons around it, including unbonded pairs. (except for H which will be happy with just 2 electrons!) --Remember: only unpaired electrons will form a bond.

Lets Draw the Lewis Dot for Methane (CH 4 ) 1)Place C in the middle since it is the solo atom and draw its valence electrons 2)Draw the Lewis Dots for the other atoms 3)Now place the other atoms around the C, Pairing up the single electrons. C H x H x H x H x So think of carbon as a family. The carbon family has four single children. They cannot pair up with each other cause that would be incest (which is illegal). These single electrons need to find a partner from another family or families. In this case, these electrons will hook up with single electrons from 4 Hydrogen families.

C H x H H H x x x Each line represents a pair of shared electrons

Example 2 – CO 2 C O O In this situation, the single electrons in this carbon family will hook up with the single electrons in two oxygen families. Each oxygen has 2 single electrons looking to hook up.

Example 2 – CO 2 C O O So this arrangement is like 2 siblings from one family hooking up with 2 siblings from another family (Carbon brothers marrying Oxygen sisters) The other 2 single carbon electrons hook up with 2 single electrons on another oxygen atom

The Triple Bond (triplet boys marrying triplet girls) P N 3 single electrons These single electrons can shift around to be able to align with the single electrons on the other atom P N P=N