CH 9: Ionic and Covalent Bonding Renee Y. Becker Valencia Community College CHM 1045

Covalent bonds are formed by sharing at least one pair of electrons. The attraction (nucleus/electrons) outweighs the repulsions (electron/electron & nucleus/nucleus)

Covalent Bonds Every covalent bond has a characteristic length that leads to maximum stability. bond length

Strength of Covalent Bonds Energy required to break a covalent bond in an isolated gaseous molecule is called the bond dissociation energy. Same amount of energy released when the bond forms

Which of the following is correct? Energy is absorbed to form a bond Example 1: Which of the following is correct? Energy is absorbed to form a bond Energy is released when a bond is formed

Bond polarity is due to electronegativity differences between atoms. Polar Covalent Bonds Bond polarity is due to electronegativity differences between atoms. Pauling Electronegativity: is expressed on a scale where F = 4.0

Pauling Electronegativities

Electron-Dot Structures Using electron-dot (Lewis) structures, the valence electrons in an element are represented by dots. Lewis symbols Valence electrons are those electrons with the highest principal quantum number (n). .

Electron-Dot Structures The electron-dot structures provide a simple, but useful, way of representing chemical reactions. Ionic: Covalent:

Electron-Dot Structures Single Bonds: Double Bonds: Triple Bonds:

Drawing Lewis-Dot Structures Rule 1: Count the total valence electrons. Rule 2: Draw the structure using single bonds. Rule 3: Distribute the remaining electron pairs around the peripheral atoms. Rule 4: Put remaining pairs on central atom. Rule 5: Share lone pairs between bonded atoms to create multiple bonds.

Drawing Lewis-Dot Structures NH2F Amino Fluoride: In this molecule, nitrogen is the central atom. Rule 1: Number of electrons = 5 + (2 x 1) + 7 = 14 = 7 pairs Rule 2 Rule 3 Rule 4

Drawing Lewis-Dot Structures

Drawing Lewis-Dot Structures Polyatomic molecules with central atoms below the second row ten: In this compound there are 10 valence electrons on bromine; this is called an expanded octet. The extra pairs go into unfilled d orbitals.

Example 2: Drawing Lewis-Dot Structures Draw electron-dot structures for: C3H8 H2O2 CO2 N2H4 CH5N C2H4 C2H2 Cl2CO H3S+ HCO3–

How is the double bond formed in O3? Resonance Structures How is the double bond formed in O3? The correct answer is that both are correct, but neither is correct by itself.

Resonance Structures When multiple structures can be drawn, the actual structure is an average of all possibilities. The average is called a resonance hybrid. A straight double-headed arrow indicates resonance.   O O O O O O

Resonance Structures The nitrate ion, NO3–, has three equivalent oxygen atoms, and its electronic structure is a resonance hybrid of three electron-dot structures. Draw them.

Formal Charge: Determines the best resonance structure. We determine formal charge and estimate the more accurate representation.

Example 3: Formal Charge Calculate the formal charge and determine the most favorable of the following electron dot structures: SO2 NO3– NCO– N2O O3 CO32–

What is the overall formal charge of the following structure? Example 4: What is the overall formal charge of the following structure? -2 -3 -1

Example 5: Ionic Radii of Ions Compare ionic radii Fe & Fe3+ Cl & Cl-