CHEMISTRY - BURDGE-ATOMS FIRST 3E CH.6 - REPRESENTING MOLECULES.

Transcription

1 !!

2 CONCEPT: ELECTRON-DOT SYMBOLS Before we look at the first two bonding models, we have to figure out how to depict the valence electrons of bonding atoms. In the electron-dot symbol, the element symbol represents the nucleus and inner electrons, and the surrounding dots represent the electrons. EXAMPLE: Draw the electron-dot symbol for each of the following elements. 1A 2A 3A 4A 5A 6A 7A 8A Li Be B C N O F Ne It s easy to write the Lewis symbol for any Main-Group element: 1) Remember that Group Number equals Valence Electron Number. 2) Place one dot at a time on the four sides (top, right, bottom, left) of the element symbol. 3) Keep adding dots, pairing them up until you have reach the number of total valence electrons for that element. PRACTICE 1: Draw the electron-dot symbol for the following ion. Mg 2+ PRACTICE 2: Draw the electron-dot symbol for the following ion. N 3- PRACTICE 3: Draw the electron-dot symbol for the following ion. Cr 1+ Page 2

3 CONCEPT: CHEMICAL BONDING I Rules for Drawing 1. Least electronegative element goes into the center. Important Facts to Know: (a) Electronegativity increases across any Period going from left to right and up any Group going from bottom to top. (b) Hydrogen and Fluorine go in the center and they only make BOND. 2. Number of valence electrons equals group number. 3. Carbon must make bonds, except in rare occasions when it makes bonds. If the carbon atom were positive or negative then it would make bonds 4. Nitrogen likes to make bonds. 5. Oxygen likes to make bonds. 6. Halogens (Group 7A), when not in the center, make bond. 7. Expanded Valence Shell Theory: Nonmetals starting from Period to can have more than 8 valence electrons around them when in the center. Page 3

4 CONCEPT: INCOMPLETE OCTETS Nonmetals form covalent bonds to generally follow the rule, in which the element is surrounded by 8 valence electrons. Sometimes elements form compounds in which they have 8 valence electrons. These elements are said to have an incomplete octet or to be. EXAMPLE: Draw the following molecular compound. BH 3 PRACTICE: Draw the following molecular compound. BeCl 2 Page 4

5 CONCEPT: EXPANDED OCTETS Expanded Valence Shell Theory: Nonmetals starting from Period to can have more than 8 valence electrons around them when in the center. EXAMPLE: Draw each of the following molecular compounds. IF3 KrF5 + PRACTICE 1: Draw the following molecular compound. SBr4 PRACTICE 2: Draw the following molecular compound. I3 Page 5

6 CONCEPT: POLYATOMIC IONS Shortcut: If you have,,,, or connected to oxygen then the negative charge tells us how many oxygens are single bonded. The remaining oxygens are bonded to the central element. EXAMPLE: Draw each of the following molecular compounds. SO 4 2- PO 4 3- H2SO 4 PRACTICE 1: Draw the following molecular compound. SeO4 2- PRACTICE 2: Draw the following molecular compound. XeO6 4- Page 6

7 CONCEPT: FORMAL CHARGE Structures and polyatomic ions that break the octet rule often have Lewis Structures. The purpose of using the formal charge formula is to determine which Lewis structure is the best answer. Formal Charge = a) Use formal charge formula to check to see if you drew your compound correctly. b) Formal charges must be either,,. c) If you add up all the formula charges in your compound that will equal the overall charge of the compound. EXAMPLE: Calculate the formal charge for each of the following element designated for each of the following. a. The carbon atom in b. The sulfur atom in PRACTICE: Calculate the formal charge for each of the following element designated in the following compound. a. Both oxygen atoms in: A B! Page 7

8 CONCEPT: RESONANCE STRUCTURES Resonance structures are used to represent bonding in a molecule or ion when a single Lewis structure cannot correctly describe the Lewis structure. EXAMPLE: Determine all the possible Lewis structures possible for NO2. Determine its resonance hybrid. EXAMPLE: Determine the remaining resonance structures possible for the following compound, CO3 2-. O O C O Page 8

9 CONCEPT: DIPOLE ARROWS Before drawing covalent compounds we first need to understand the idea of polarity and its connection to electronegativity. Polarity arises whenever two elements are connected to each other and there is a significant difference in their electronegativities. Generally, electronegativity going from left to right of a period and going down a group. To show this difference in electronegativity we use a dipole arrow. The dipole arrow points towards the electronegative element. The Effect of Electronegativity Difference on Bond Classification Electronegativity Difference (ΔEN) Bond Classification Example Zero (0.0) Small ( ) Intermediate ( ) Large (Greater than 1.7) Pure Covalent Nonpolar Covalent Polar Covalent Ionic Page 9

10 PRACTICE: DIPOLE ARROWS EXAMPLE: Based on each of the given bonds determine the direction of the dipole arrow and the polarity that may arise. a. H Cl b. S O c. Br B Br PRACTICE 1: Based on the given bond determine the direction of the dipole arrow and the polarity that may arise. a. H C PRACTICE 2: Based on the given bond determine the direction of the dipole arrow and the polarity that may arise. a. N F PRACTICE 3: Based on the given bond determine the direction of the dipole arrow and the polarity that may arise. a. H N H Page 10

11 CONCEPT: CHEMICAL BOND IDENTIFICATION PRACTICE: Answer each of the following questions dealing with the following compounds. KBr NH3 F2 CaO NaClO a. Which of the following compound(s) contains a polar covalent bond? b. Which of the following compound(s) contains a pure covalent bond? c. Which of the following compound(s) contains a polar ionic bond? d. Which of the following compound(s) contains both a polar ionic bond and a polar covalent bond? Page 11