How are molecules depicted? Ch. 9, sections 3 & 4

Lewis electron-dot structures b 1. Electrons in the outermost energy level of an atom are involved in bonding, both ionic and covalent. b 2. These outer-level electrons are called VALENCE ELECTRONS. b 3. A LEWIS STRUCTURE represents the valence electrons in a molecule.

The diatomic hydrogen molecule (H 2 ) is the simplest model of a covalent bond, and is represented in Lewis structures as: b b The shared pair of electrons provides each hydrogen atom with two electrons in its valence shell (the 1s) orbital.   In a sense, it has the electron configuration of the noble gas helium

When two chlorine atoms covalently bond to form Cl 2, the following sharing of electrons occurs: b b Each chlorine atom shared the bonding pair of electrons and achieves the electron configuration of the noble gas argon.

b b In Lewis structures the bonding pair of electrons is usually displayed as a line, and the unshared electrons as dots:

The shared electrons are not located in a fixed position between the nuclei. In the case of the H 2 compound, the electron density is concentrated between the two nuclei: b b The two atoms are bound into the H 2 molecule mainly due to the attraction of the positively charged nuclei for the negatively charged electron cloud located between them

How do we represent more-complicated molecules with Lewis structures, such as CH 3 I ? b 1. Determine the total number of valence electrons in the compound. b 1 C atom with 4 electrons = 1 x 4 = 4 b 3 H atoms with 1 electron = 3 x 1 = 3 b 1 I atom with 7 electrons = 1 x 7 = 7 b Total # valence electrons =14

2. Determine which element will be the central atom. b The central atom is the element that *has the lowest electronegativity or *has the lowest electronegativity or *is the only single element or *is the only single element or *Hydrogen and the halides are usually NOT the central atom. *Hydrogen and the halides are usually NOT the central atom.

In the compound, CH 3 I, carbon has the lowest electronegativity so it will be in the middle. H H C I H H H C I H

3. Place two electrons in each bond. A BOND is the space between the central atom and the other atoms around it. (Electrons are found in pairs and by doing this, you are pairing them up.) b Look on the chalk board as I draw this for you. (I couldn’t figure out how to draw it on this powerpoint!!!!!)

4. Complete the octets of the atoms attached to the central atom by adding electrons in pairs. b See the chalk board again! (You should be writing these examples down as I write them on the white board.)

5. Place any remaining electrons on the central atom in pairs. b REMEMBER! The total electrons in the Lewis structure MUST equal the number of electrons in step #1!!!!!!!!! b See the chalk board again!

6. Now, replace each electron pair attaching the central atom to the outer atoms with a line. Each line represents where the electrons are shared. b For example: H : O : H H : O : H would be written as would be written as H - O - H H - O - H

Draw Lewis Structures for the following compounds: b 1. NF 3 b 2. SiCl 4 b 3. ClF b 4. CCl 2 F 2 b 5. HOCl

7. If the central atom does not have an octet after you have equaled out the dots and # of valence electrons, form double or triple bonds to give the central atom 8 electrons. b b Multiple bonds b b The sharing of a pair of electrons represents a single covalent bond, usually just referred to as a single bond b b In many molecules atoms attain complete octets by sharing more than one pair of electrons between them. b b Two electron pairs share a double bond b b Three electron pairs share a triple bond

Example of a TRIPLE BOND is below. b b Because each nitrogen contains 5 valence electrons, they need to share 3 pairs to each achieve a valence octet.

Resonance Structures b nding/Resonan/Bond07.htm nding/Resonan/Bond07.htm nding/Resonan/Bond07.htm

Draw Lewis Structures for the following compounds:  6. O 2 (multiple bonds)  7. CS 2 (multiple bonds)  8. HCN (multiple bonds)  9. SO 2 (resonance)  10. O 3 (resonance)

Exceptions to the Octet Rule 1. Molecules with an odd # of valence electrons EX: NO 2 ClO 2 ClO 2 NO NO 2. Compounds with fewer than 8 valence electrons present (this is very rare) EX: BH 3

3. Compounds in which the central atom has more than 8 valence electrons -- called an EXPANDED OCTET ; occurs in energy levels of elements in period 3 and up **Extra lone pairs are added to the central atom OR more than 4 bonding atoms are present. EX: IF 4 PCl 5 PCl 5

VSEPR Model : Valence Shell Electron Pair Repulsion model b Based on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom b READ pages 259 – 261. b Look at Table 9-3 as you read. b Complete #49 – 53 on pg. 262.

Electronegativity and Polarity b Electronegativity indicates the relative ability of an atom to attract electrons in a chemical bond. b It generally increases as the atomic number increases ACROSS A PERIOD and generally decreasesas you go DOWN A GROUP. b It generally increases as the atomic number increases ACROSS A PERIOD and generally decreases as you go DOWN A GROUP.

Polar or Nonpolar??????????? b Identical atoms, like N 2, have an electronegativity difference of zero, and the electrons in the bond are equally shared between the two atoms. This is a b Identical atoms, like N 2, have an electronegativity difference of zero, and the electrons in the bond are equally shared between the two atoms. This is a NONPOLAR COVALENT BOND, or a PURE COVALENT BOND. b A covalent bond between atoms of different elements does not have equal sharing of the electron pair, due to the difference in electronegativity.

b Unequal sharing results in a. The shared electrons are pulled toward one of the atoms and spend more time around that atom than the other atom. Partial charges occur at the ends of the bond. This bond is often referred to as a (two poles). b Unequal sharing results in a POLAR COVALENT BOND. The shared electrons are pulled toward one of the atoms and spend more time around that atom than the other atom. Partial charges occur at the ends of the bond. This bond is often referred to as a dipole (two poles). b To determine if the bond is or, you must look at the shape of the molecule. Draw the molecular structure, using what you know from Table 9-3. b To determine if the bond is polar or nonpolar, you must look at the shape of the molecule. Draw the molecular structure, using what you know from Table 9-3. b SYMMETRIC MOLECULES ARE USUALLY NONPOLAR AND ASYMMETRIC ARE POLAR AS LONG AS THE BOND TYPE IS POLAR.

Do #60 – 63 on page 266. Do #60 – 63 on page 266.