Unit 4 Bonding Theories

O = O O = O VSEPR Theory Valence Shell Electron Pair Repulsion Predicts the shapes of covalent molecules The repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible. O = O O = O

- - Molecular Shapes 180o 120o 104.5o 109.5o 107o SHAPE DESCRIPTION ILLUSTRATION Linear Bond Angle = 180o One or two bonding pairs, no lone pairs - - X A 180o X A 104.5o Bent Bond Angle = 104.5o Two bonding pairs, two lone pairs X A 107o X A 120o Trigonal Planar Bond Angle = 120o 3 bonding pairs, no lone pairs X A 109.5o Trigonal Pyramidal Bond Angle = 107o 3 bonding pairs, one lone pair Tetrahedral Bond Angle = 109.5o Four bonding pairs, no lone pairs

Lone Pairs Take up MORE space SQUEEZE the bond angle X A X A

HF CH4 Practice: Determine the shape of each molecule: LINEAR TETRAHEDRAL

More Practice: BF3 NH3 SeCl2 TRIGONAL PLANAR TRIGONAL PYRAMIDAL BENT F

Bond Polarity A B Due to differences in ELECTRONEGATIVITY, Equal & Unequal Sharing of Electrons Shared Electron Pair A B  Due to differences in ELECTRONEGATIVITY, the sharing of electrons is not always equal between the atoms.

Electronegativity Electronegativity (EN) is the ability of an atom to attract electrons F=largest electronegativity Atoms with the largest electronegativity attract electrons more strongly than those with a small electronegativity.

Periodic Table of Electronegativities Why are the Noble Gases not listed?

Nonpolar Covalent Bonds Bonding electrons are shared equally. Examples: H2, CO2, BF3, CH4 GUIDELINES FOR IDENTIFYING A NONPOLAR BOND: If only two atoms involved, both atoms are identical. NO lone pairs are on the central atom. All atoms bonded to the central atom are the same. The bond types are identical around the central atom.

= = NONPOLAR COMPOUNDS F H B H C BOTH ATOMS ARE IDENTICAL ALL ATOMS BOUND TO CENTRAL ATOM ARE THE SAME BOTH ATOMS ARE IDENTICAL BOND TYPES ARE IDENTICAL AROUND CENTRAL ATOM NO LONE PAIRS ON CENTRAL ATOM H C = = O C BOND TYPES ARE IDENTICAL AROUND CENTRAL ATOM ALL ATOMS BOUND TO CENTRAL ATOM ARE THE SAME NO LONE PAIRS ON CENTRAL ATOM WHAT MAKES THIS A NONPOLAR COMPOUND?

Polar Covalent Bonds Bonding electrons are shared unequally. Examples: HCl, H2O, HCN, PBr3, CH3F GUIDELINES FOR IDENTIFYING A POLAR BOND: If only two atoms involved, both atoms are different. Lone pairs are on the central atom. At least one atom bonded to the central atom is different from the rest. There is more than one bond type around the central atom.

THERE ARE MORE POLAR COMPOUNDS THAN THERE ARE NONPOLAR! Br H Cl ATOMS ARE DIFFERENT THERE ARE MORE POLAR COMPOUNDS THAN THERE ARE NONPOLAR! LONE PAIRS ON CENTRAL ATOM O H H C F LONE PAIRS ON CENTRAL ATOM ATOMS BOUND TO CENTRAL ATOM ARE NOT THE SAME DIFFERENT BOND TYPES AROUND CENTRAL ATOM C H N WHAT MAKES THIS A NONPOLAR COMPOUND?

Electronegativity Differences Another way to determine the polarity of a bond is by determining the difference in electronegativity (ΔEN) of two atoms. ** You would have to be given a Periodic Table with these values on them Electronegativity Differences & Bond Types: IONIC: ΔEN > 1.7 POLAR COVALENT: 0.4 < ΔEN ≤1.7 NONPOLAR COVALENT: ΔEN ≤ 0.4 ΔEN = Ι EN atom 1 – EN atom 2 Ι

PRACTICE: H – F N – O C – H Si – P B – Cl Ι 2.1-4.0Ι = 1.9 IONIC Determine if the bond is IONIC, POLAR, or NONPOLAR: H – F N – O C – H Si – P B – Cl Ι 2.1-4.0Ι = 1.9 IONIC Ι 3.0 – 3.5 Ι = 0.5 POLAR Ι 2.5 – 2.1 Ι = 0.4 NONPOLAR Ι 2.1 – 1.8 Ι = 0.3 NONPOLAR Ι 2.0 – 3.0 Ι = 1.0 POLAR

So What Does It Mean To Be Polar? EXPLAINATION: EXAMPLE: In a polar bond, the more electronegative atom attracts electrons more strongly & gains a slightly negative charge. The less electronegative atom has a slightly positive charge. Chlorine is the more electronegative atom POSITIVE NEGATIVE H Cl

DIPOLES A molecule that has two poles is said to be a dipolar molecule or DIPOLE. Two poles have been created positive pole & negative pole When placed between oppositely charged plates, they tend to orientate with respect to the positive & negative plates. POSITIVE NEGATIVE H Cl

Intermolecular Forces Are weaker than either ionic or covalent bonds. Types of Intermolecular Forces: Hydrogen Bonds Van der Waals Forces Dipole Interactions Dispersion Forces

Hydrogen Bonds Attractive forces in which a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom.

Properties of Hydrogen Bonds Hydrogen bonds are very easy to break 5% the strength of a covalent bond Strongest of the intermolecular forces Water molecules are held together by hydrogen bonds. Hydrogen bonding accounts for many properties of water. Liquid at room temperature Cohesive properties

Van der Waals Forces Dipole Interactions Dispersion Forces Occur when polar molecules are attracted to one another Like Hydrogen bonds, but with other atoms Dispersion Forces Weakest of all molecular interactions Caused by the motion of electrons of a molecule when close to a neighboring molecule