Chapter 6.2 : Covalent Bonding and Molecular Compounds
-
Upload
chris-foltz -
Category
Education
-
view
13.432 -
download
3
Transcript of Chapter 6.2 : Covalent Bonding and Molecular Compounds
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