Chemistry, Covalent Bonding

Chemistry, Covalent Bonding - Transcript

Chapter 8
Chemistry 1
Covalent Bonding

Why do atoms bond
 It’s all about stability
 Atoms want to achieve the stable octet of the
noble gases
 Ionic and metallic bonds achieve this octet
through an exchange of electrons
 The exchange occur between a metal ion
(cation) and a non-metal ion (anion)
Covalent bonds are different

What is a covalent bond
 A covalent bond occurs when atoms share
electrons
Covalent bonds occur between two non-
metals
 Valence electrons play a role in covalent
bonds

How to determine a covalent bond
 In the case of hydrogen combining to form H2
 Each hydrogen has 1 valence electron
 In order to achieve the noble gas configuration
of helium, they each need two valence
electrons
H2 achieves this by sharing their valence
electrons and forming a bond
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What happens in a covalent bond
 Electrons are shared between the two atoms
 Each atom “thinks” it has a full outer shell
containing 8 valence electrons
 The sharing of electrons is what makes the
bond and holds the atoms together
Water is a covalently bonded molecule

Water
Hydrogen needs
2 electrons to fill
its outermost
shell
Oxygen needs
two electrons to
fill its outermost
shell

Lewis Structure of Water
http://web.visionlearning.com/custom/chemistry/animations/CHE1.7-an-H2Obond.shtml

Lewis Structure of Carbon Dioxide:
CO2
 Carbon has four
valence electrons, and
oxygen has six.
 Each pair of shared
electrons forms a single
bond
 There are two bonds (a
double bond) between
the carbon and each
oxygen

Covalent and Polar Covalent
Compounds
 http://link.brightcove.com/services/player/bcpid9113583001?bctid=1396941780
Covalent Bonding
 http://www.saskschools.ca/curr_content/chem20/covmolec/covintro.html

Covalent Bonding Tutorial
 "C:\Program Files\Covalent Bonding\chemsim3.exe"

Determining a Lewis Structure
 Write the symbol for the central atom in the molecule
 Join the other atoms using dashes
 Two shared electrons is a single bond
 Calculate the number of bonds
 The formula to use is:

Determining Lewis Structure contd
Add dashes to your diagram to make
the number of bonds calculated in step
3 equal to the number of dashes
shown in the diagram
Add dots to obey the octet rule
Do a final count that all atoms obeyed
the octet rule

Practice
 http://www.saskschools.ca/curr_content/chem20/covmolec/covlewis.html

You Try It
What would the formula be for a compound
containing carbon and fluorine
What is the formula for a compound
containing nitrogen and oxygen
Carbon and hydrogen?
 Antimony and bromine?
Chlorine and oxygen?

Electronegativity Table

Determining a Bond From
Electronegativities
Calculate the difference between the
electronegativities of the atoms involved in the
bond
When differences are 1.7 or greater the bond
is ionic
When differences are between 0.3 and 1.7,
the bond is polar covalent
When the difference is less than 0.3 the bond
is non-polar covalent

Non Polar Covalent Bond
When two fluorine atoms bond
 Each atom has a high attraction for
bonding electrons
 Both attract them equally. (We've
got two well-matched Boston
Terriers pulling on this sock.)
 The electronegativity difference
between the atoms is zero, and the
bond is pure covalent.

Polar Covalent
 F2O for example results in a bond that is
polarized
 Bonding electrons spend more time around
the fluorine than the oxygen because
fluorine has the higher electronegativity
 Fluorine end of the bond has a partial
negative charge
 The oxygen end has a partial positive
charge
 The electronegativity difference is 0.5, so
the bond is best described as polar
covalent. (Think of a Boston Terrier pulling
against a German Shepherd).

Ionic
 The bond between fluorine and
lithium results in a bond with a
much larger electronegativity
difference
 4.0 for F minus 1.0 for Li is 3.0
 This bond is best described as an
ionic bond.

Calculating the Electronegativity
Difference
 Predict the bond that will form between H and
S
 Between C and H?
 Between Na and S?
 Identify each as polar or nonpolar
– SCl2
– CS2
– CF4

Resonance Structures
 A resonance structure occurs when there is
more than one valid Lewis structure for a
molecule or ion
Resonance structures differ only in the
position of the electron pairs, never in the
position of the atoms

Lewis Structure of Ozone (O3)
Calculate the number of bonds
What is the number of electrons each oxygen
wants to have
What is the number of electrons each oxygen
has
Calculate the number of bonds using the formula

Lewis Structure of Ozone (O3) contd
 Begin by drawing the basic
structure
 Then add the third bond
However, we cannot be sure if the
double bond is on the left or the
right
Draw the resonance structures for
ozone

Resonance Structures of the Nitrate
(NO3-) ion
Calculate the number of bonds

Resonance Structures of the Nitrate
(NO3-) ion
Draw the basic atom
Insert the valence
electrons
Do all the atoms have
an octet?

Resonance Structures of the Nitrate
(NO3-) ion
Add bonds so that each atom
has an octet
Are there any other ways we
can draw this atom?
 If so, these resonance
structures must also be
drawn

Resonance Structures of the Nitrate
(NO3-) ion
 Below are all three possible structures for this
ion
 These are the resonance structures of nitrate

You Try It
Draw resonance structures for
– SO2
– SO32-
– CO32-

Naming Covalent Compounds
 Simple covalent compounds are generally
named by using prefixes to indicate how
many atoms of each element are shown in the
formula
 The ending of the last (most negative) element
is changed to -ide.

Prefixes
 The prefixes used are mono-, di-, tri-, tetra-,
penta-, hexa-, and so forth
 The mono- prefix is not used for the first
element in the formula
 The "o" and "a" endings of these prefixes are
dropped when they are attached to "oxide."
– Monoxide
– Hexoxide
– Pentoxide

Table of Prefixes
1 Mono-
2 Di-
3 Tri-
4 Tetra-
5 Penta-
6 Hexa-

Which Element is First?
Generally, they are in
the same left-to-right
order that they have on
the periodic table
Except that you would
have to squeeze
hydrogen in between
nitrogen and oxygen

Name the following compounds
 PH3
– phosphorus trihydride
CO
– carbon monoxide
HI
– hydrogen monoiodide
N2O3
– dinitrogen trioxide

What is the Formula
Carbon Tetrachloride
– CCl4
Dinitrogen Pentoxide
– N2O5
 Silicon Tetrabromide
– SiBr4

Chemical Bonds: Ionic and Covalent
 The World of Chemistry: Everyday Chemistry
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