Covalent Bonds
Atoms can form molecules by sharing electrons in
the covalent bond. This is done only among nonmetal atoms.
Molecules
• Some elements in nature are found in the
form of molecules
• Diatomic = 2 atoms
BrINClHOF
• Compound composed
Of molecules is called a
Molecular compound
PROPERTIES
• Relatively low melting points and boiling
points
• Many are gases and liquids at room
temperature
• Composed of atoms of 2 or more nonmetals
Molecular Formulas
• Show how many atoms of each element a
molecule contains
• Ex. H2O has 2 hydrogen atoms and 1 oxygen
atom
• Doesn’t show shape of molecule or which
atoms are covalently bonded and how
Dot Structures-Octet Rule
(All atoms want 8 electrons around them.)
Valence electrons are those in the outermost orbitals. They
are the ones that can form bonds.
Electron sharing occurs in such a way so that atoms attain
the electron configuration of the noble gases.
SINGLE COVALENT BONDS
• Two atoms held together by sharing a pair of
electrons. Can be shown by lewis dot or a
dash
HH
UNSHARED PAIRS
In F2 each flourine contributes one electron
The electrons that are not shared are called
Unshared pairs, lone pairs or nonbonding pairs
Draw Lewis structures
• NH3
• PCl3
• Cl2
• SBr2
DOUBLE AND TRIPLE COVALENT
BONDS
Occurs when three pairs of electrons
Are shared
Occurs when two pairs of electrons
are shared
NASL Method for Lewis Dot Structures
• Helps to determine how to place electrons
around an atom ( double/triple bonds + lone
pairs)
• Exceptions to octet = H = 2, Be = 4, B= 6
STEPS
1. Write a skeleton molecule with the lone atom in
the middle (Hydrogen can never be in the middle)
2. Calculate (N) Needed which is the sum of
electrons needed for all atoms to obey the octet
rule.
NASL Method
Show structure for CO2
1. O C O
2. Needed electrons
C =1x8 =8
O = 2 x 8 = 16
N = 24
skeleton
NASL METHOD
• Step 3; Calculate A (Available) = sum of all
valence electrons. For anions and cations you
need to subtract or add electrons here.
• Step 4; Calculate S ( Shared) = Difference
between N and A
CO2
1 x 4 = 4 for
Carbon
16 A (available)
2 x 6 = 12 for Oxygen
Shared
Needed – Available =
24-16= 8
NASL Method
• Divide S by 2 to obtain the number of bonds
to be extended from the central atoms.
8/2=4
•
O::C::O
Calculate L ( Lone-pair electrons ) the difference
between A and S
16 – 8 = 8
..
..
O::C::O
●●
●●
Let's Try it!
1.S
2.N
3.A
4.S
5.L
6.E
H O H
Water H2O
2 x 2 = 4 for Hydrogen
1 x 8 = 8 for Oxygen
4+8=12 needed electrons
2 x 1 = 2 for Hydrogen
1 x 6 = 6 for Oxygen
You have 8 available electrons
12 – 8 = 4/2 = Bonding
12 N
- 8H
- 4B
4 NB
H:O:H
8 – 4 = 4 non-bonding electrons
..
H:O:H
●●
..
H:O:H
●●
1.S
2.N
3.A
H
HNH
Let's Try it!
3 x 2 = 6 for Hydrogen
1 x 8 = 8 for Nitrogen
6+8=14 needed electrons
3 x 1 = 3 for Hydrogen
1 x 5 = 5 for Nitrogen
You have 8 available electrons
4.S
14 – 8 = 6/2 = 3 bonding pairs
5.L
8-6 = 2 lone electrons
6.E
H
..
H:N:H
●●
Ammonia NH3
14 N
- 8H
- 6B
2 NB
..H
H:N:H
H
..
H:N:H
●●
REVIEW
• Draw lewis dot structures for the following
– PCL3
– CH4
Review Quiz #2
• Using NASL Show covalent bonding for the
following compounds. Show all work for
credit. ( 6 points )
SO2
Nomenclature
Naming of Binary Molecular Compounds
Binary Compounds: Composed of two different elements that are
nonmetals.
Ex. CO and CO2 (not carbon Oxide)
CO = Carbon monoxide = poisonous gas
CO2 = carbon dioxide waste product of breathing
Need to differentiate different types of compounds by using prefixes.
Tells you how many of each element are present
Mono = 1
Di
=2
Tri
=3
Tetra = 4
Penta = 5
Hexa = 6
Hepta = 7
Octa = 8
Naming Guidelines
• 1. make sure you have a binary molecular compound
composed of two nonmetals
• 2. Name must identify the elements and how many are
present
• 3. Name the elements in the order listed
• 4. Omit the prefix – mono when the first element only
has one
– Ex. SF6 is sulfur hexaflouride NOT monosulfur hexaflouride
• 5. The suffix for the second element is -ide
Writing Formulas
• Use the prefixes in the name to determine the
correct subscripts for the formula.
• Ex. Silicon Carbide ( no prefixes so must
contain one of each) SiC
• Dinitrogen Tetroxide = N2O4
Let’s Practice!
OF2
SO2
N2O4
• Phosphorus Pentachloride
• Iodine Heptaflouride
• Chlorine triflouride
Using a few sentences discuss what the
cartoon might mean in terms of how Cl and
H bond
POLARITY
• Covalent bonds involve atoms sharing
electrons (pulled in a tug of war)
• When electrons are pulled equally = nonpolar
Covalent bond
• When electrons are pulled unequally = polar
covalent bond
Electronegativity
• The magnitude of attraction for electrons is
called “Electronegativity”. The more
electronegative an atom is, the more it wants
the electrons.
The higher the electronegativity
Value the greater the ability to attract electrons
Bond types
WATER IS A POLAR MOLECULE
O = 3.5 ELECTRONEGATIVE
H = 2.1 ELECTRONEGATIVE
DIFFERENCE = 1.4 = POLAR COVALENT
OXYGEN PULLS HYDROGENS ELECTRONS CLOSER THAN HYDROGEN CAN PULL OXYGENS
ELECTRONS. OXYGEN GETS AN OVERALL PARTIAL NEGATIVE CHARGE AND HYDROGEN
GETS AN OVERALL PARTIAL POSTITIVE CHARGE DESIGNATED BY A GREEK DELTA SIGN
VSEPR Theory
• Electron dot structures fail to reflect the 3dimensional shape of molecules
Molecules in reality are 3 dimensional and can be explained by the VSEPR theory
VSEPR Theory
• Explains 3-D shape of molecules
• Based on the theory that repulsion occurs
between electron pairs and causes a variety of
molecular shapes.
• Electrons want to stay as far apart as possible
• Can be determined by first looking at the lewis
dot structure
• Counting how many electron clouds (pairs) are
radiating
LINEAR MOLECULES
The simplest structure has two groups around the central atom. Carbon
monoxide, CO2, is an example of this kind of shape. The two groups of
electrons in the two double bonds repel each other and assume a shape with
an angle of 180 degrees.
MOLECULAR SHAPES
Review
Pyramidal = 3 bonding pairs
And a lone pair
Trigonal planar = 3 bonding
No lone pairs