COVALENT BONDING AND MOLECULAR COMPOUNDS

Chapter 6.2

Objectives:1. Define molecule and molecular formula.2. Explain the relationships between potential

energy, distance between approaching atoms, bond length, and bond energy.

3. State the octet rule4. List the six basic steps used in writing Lewis

structures.5. Explain how to determine Lewis structures

for molecules containing single bonds, multiple bonds, or both.

6. Explain why scientists use resonance structures to represent some molecules

Molecular compounds Molecule –

Neutral group of atoms that are held together by covalent bonds Compose most living things

Molecular compound– Chemical compound whose simplest

units are molecules

Molecular compounds Chemical formula –

Indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts H2O CO2 C12H22O11 Molecular formula –

Shows the types and numbers of atoms combined in a single molecule of a molecular compound H2O CO2 C12H22O11 Diatomic molecule –

Molecule containing only two atoms H2 O2 N2

Formation of a Covalent Bond Nature favors bonding because

Puts atoms at lower potential energy

Positive Nucleus

Electron Negative

Approaching nuclei and electrons Attracted to each

Decrease in potential energy

At the same time, both nuclei and two electrons repel each other

Increase in potential energy

Potential energy is minimized when attractive forces are equal to the repulsive forces

Characteristics of Covalent Bond Bond length –

The distance between two bonded atoms at their minimum potential energy, that is, the average distance between two bonded atoms Bond Energy –

The energy required to break a chemical bond and form neutral isolated atoms

As the bond energy increases- The bond length decreases

Bond length

Octet Rule Chemical compounds tend to form so

that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.

Hydrogen is an exception because it is stable with 2 electron in outer shell.

The eight electrons come from the main-group energy levels being filled. s2p6 totals 8 electrons

Bonding and octet ruleF __ __ __ __ __ 1s 2s 2pF __ __ __ __ __ 1s 2s 2p

Bonding electron pair in overlapping orbitals

Li __ __

1s 2s

F __ __ __ __ __ 1s 2s 2p

Exceptions to the Octet Rule Most main-group elements

Tend to form covalent bonds according to octet rule

Exceptions Hydrogen – forms bonds where it is surrounded by

only two electrons Boron - has just 3 valence electrons, so it tends

to form bonds in which it is surrounded by six electrons BH3

H

B

H H

Electron-Dot Notation Electron-configuration notation in which

only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the elements symbol Dots – valence electrons Symbol – nucleus and inner-shell electrons

I 7 Valence electrons

53 protons73 neutrons36 inner shell electrons

Electron-Dot Notation

First three rows of periodic table

Group 1 2 13 14 15 16 17 18

Lewis Structures Formulas in which atomic symbols represent

nuclei and inner-shell electrons, dot-pair or dashes and dots between two atomic symbols represent electron pairs in covalent bonds, and dots adjacent to only one atomic symbol represent unshared electrons. Unshared pair ( lone pair ) – Pair of electrons that is not involved in bonding

and that belongs exclusively to one atom

Structural formula – Indicates the kind, number, arrangement,

and bonds but not the unshared pairs of the atoms in a molecule

Single Bond – Covalent bond produced by the sharing of

one pair of electrons between two atoms.

B F

B – F

Draw the Lewis Structure of iodomethane, CH3I

1. Determine the type and number of atoms in the molecule1 C 3 H 1 I

2. Write the electron-dot notation for each type of atom in the molecule C H I

3. Determine the total number of valence electrons in the atoms to be combinedC 1 x 4e- = 4e- H 3 x 1e- = 3e- I 1 x 7e- = 7e-

14e-

Total valence electrons

4. Arrange the atoms to form a skeleton structure for the molecule. If Carbon is present, it is the central atom. Otherwise, the least-electronegative atom is central (except for hydrogen, which is never central). Then connect the atoms by electron-pair bonds.

HH C I H

5. Add unshared pairs of electrons so that each hydrogen atom shares a pair of electrons and each other nonmetal is surrounded by eight electrons

HH C I H

6. Count the electrons in the structure to be sure that the number of valence electrons used equals the number available. Be sure the central atom and other atoms besides hydrogen have an octet.

14 e- so this is correct!!!

Multiple Covalent Bonds Double bond –

Covalent bond produced by the sharing of two pairs of electrons between two atoms

H H C CH H

H H C CH H

OR

Triple bond – Covalent bond produced by the sharing of

three pairs of electrons between two atoms

N N N NOR

Draw the Lewis Structure for methanal, CH2O, which is also known as formaldehyde.1. Determine the type and number of atoms

in the molecule

2. Write the electron-dot notation for each type of atom in the molecule

3. Determine the total number of valence electrons in the atoms to be combined

1 C 2 H 1 O

C H O

C 1 x 4e- = 4e- H 2 x 1e- = 2e- O 1 x 6e- = 6e-

12e-

4. Arrange the atoms to form a skeleton structure for the molecule. If Carbon is present, it is the central atom. Otherwise, the least-electronegative atom is central (except for hydrogen, which is never central). Then connect the atoms by electron-pair bonds.

5. Add unshared pairs of electrons so that each hydrogen atom shares a pair of electrons and each other nonmetal is surrounded by eight electrons

6. Count the electrons in the structure to be sure that the number of valence electrons used equals the number available. Be sure the central atom and other atoms besides hydrogen have an octet.

HH C O

HH C O

14 e- = 12 e-

SO

HH C O

OR

HH C O

Resonance Structures Bonding in molecules or ion that

cannot be correctly represented by a single Lewis structure.

O O O

O O O

Or