Chemical Bonding I: Basic Concepts, Lecture notes of Chemistry

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Al-Anbar University

College Of Engineering

CHEMISTRY

1st. stage

Prepared By:

Dr. Hamad Khaleefah Abdulkadir

Assist. Prof. In Chemistry

HAMAD KHALEEFAH ABDULKADIR 1 ST. STAGE, CHEMISTRY ASSIST. PROF. IN CHEMISTRY COLLEGE OF ENGINEERING / Al-ANBAR UNIVERSITY

LECTURE 9

Chemical Bonding I: Basic Concepts

9.1 lewis Dot Symbols

9.2 The Ionic bond

9. 3 The Covalent bond

9. 4 Electroegativity

9. 5 Writing Lewis Structures

9. 6 Formal Charge and Lewis Structures

9. 7 The Concept of Resonance

9. 8 The Exception of Octate Rules

Types of Bonds

9.2 The Ionic Bond

 ionic bond is the electrostatic force that hold ions together in an ionic compound.

 the resulting anions & cations attract each other in such a ratio that the charges cancel out.

Note: Do not show the charges in the final product.

Example: KI NOT K+I-

Example: Ba+2 & F- - Need two negatives to neutralize +2 charge on barium ion: Ba+2 F-1 F-1 = BaF

Example 9.

Use Lewis dot symbol to show formation of Al2O

9.3 The Covalent Bond

 A covalent bond is a chemical bond in which two or more electrons are shared by two atoms.

Lewis structure of F 2

Comparing of the properties of covalent and ionic

 Covalent compounds are usually gases, liquid and low melting solid.  Ionic compounds are solids at room temperature and high melting point.  Many ionic compounds are soluble in water , and the resulting aqueous solutions conduct electricity, because the compounds are strong electrolytes.

9.4 Electronegativity

 Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond.  High electronegativity →pick up electron easily.

 Electronegativity increase from left to right in period.  Electronegativity increase from bottom to up in group.  Transition metals don’t follow these trend.

 Nonmetals have high electronegativity, metals have low electronegativity.

 high difference in electronegativity (2 or more ), element tend to form ionic bond.(NaCl)  small difference in electronegativity, element tend to form polar covelent bond .(HCl)  Same electronegative of the same elements from pure covelent bond (H2).

The Electronegativities of Common Elements

 Electron Affinity (EA) and electronegativity are related but in different concept  (EA) refers to isolated atoms attraction for additional electron (experimental)  EA →measurable, Cl is highest

 Electronegativity signifies the ability of an atom in a chemical bond( with another atom) to attract the shared electrons (estimated )  Electronegativity - relative, F is highest

9.5 Writing Lewis structures

Writing Lewis Structures

NH4+

 Step 2 – A= 5X1 + 1X4 -1 = 8 valance electrons  Step 3 – B = 8X1+2X4 =16 electrons  Step 4 - C = 16-8 =8/2=4 bonds  Step 5 - D= 8-4 =4 non bonding electrons , 2 pair of electrons

Example 9.

Example 9.

Write the Lewis structure of formic acid (HCOOH ).

 Step 1 – put C in center ,surrounded by 2O atoms , H Step 2 – A= 4X1 + 6X2 +2x1 = 18 valance electrons  Step 3 – B = 8X1+8X2 +2 x2 =28 electrons  Step 4 - C = 28-18 =10/2=5 bonds  Step 5 - D= 18-10 =8 nonbonding electrons or 4 pair of electrons

Example 9.

Write the Lewis structure of carbon dioxide [CO3]-

 Step 1 – C is less electronegative than O, put C in center  Step 2 – A= 4X1 + 6X3 +2 = 24 valance electrons  Step 3 – B = 8X1+8X3 = 32 electrons  Step 4 - C = 32-24 =8/2=4 bonds  Step 5 - D= 24-8 =16 nonbonding electrons or 8 pair of electrons

Example 9.

Write the Lewis structure of carbon dioxide [NO2]-

 Step 1 – N is less electronegative than O, put N in center  Step 2 – A= 5X1 + 6X2 +1 = 18 valance electrons  Step 3 – B = 8X1+8X2 = 24 electrons  Step 4 - C = 24-18 =6/2=3 bonds  Step 5 - D= 18-6 =12 nonbonding electrons or 6 pair of electrons

9.6 formal charge and Lewis structures

formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure.

Example 9.

9.7 the concept of resonance

 A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure (after formal charge has been determined ).  More possible structures gives the overall structure more validity.

Example 9.

Draw three resonance structure for N2O (NNO),indicate formal charge rank the structures.

An expanded octet

 Usually occurs in element in 3rd period and beyond

More than 4 bonds

 Elements ≥ row 3 can use s, p & d orbitals and have > 8 VE  P: 8 OR 10  S: 8, 10, OR 12  Xe: 8, 10, OR 12

Examples

SF6 PF5 XeF

Example 9.

• Example 9.
• Example 9.