Regents Chemistry

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Regents Chemistry

• Agenda
• Start Chapter 12 - Chemical Bonding
• Bonding worksheet
• HW Finish worksheet

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What is bonding?

• Bonding is the glue that hold two or more
  elements together
• This glue is most likely formed as a result of
  a chemical reaction
• Bonding and molecular structure play a central
  role in determining the course of chemical
  reactions

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What is a bond?

• A bond can be thought of as a force that holds
  groups of two or more atoms together and makes
  them function as a unit
• Example water

O
Bonds require energy to break and release energy
when made
H
H
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Bonds and Energy

• BREAKING BONDS requires energy
• You Must Heat Wood to get it to react with oxygen
  (ENDOTHERMIC)
• MAKING BONDS releases energy
• Once the bonds are broken, they form new products
  and this process releases heat (EXOTHERMIC)

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Types of bonds

• Ionic bonds - typically formed between metals and
  nonmetals
• Covalent bonds - typically formed between
  nonmetals
• Metallic bonds - formed between metals

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Ionic Bonds

• Ionic substances are formed when an atom that
  loses electrons relatively easily reacts with an
  atom that wants electrons
• We call these substances ionic compounds and
  result when a metal reacts with a nonmetal
• Ionic bonds are strong

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Ionic Bonds

• Na and Cl
• Na is a metal and likes to lose one electron
• Cl is a nonmetal and likes to gain one electron
• the final ionic compounds is NaCl

Na
Cl-
NaCl

The electrostatic interaction keeps them
together!
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Ionic Bonds

• Na looses an electron and chlorine gains it!
• They do this to achieve an octet!

Na
Cl
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Covalent Bonds

• Covalent Bonds
• exist between nonmetals bonded together
• form when atoms of nonmetals share electrons
• electrons can be shared equally or unequally

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Covalent Bonds Cont

• Covalent bonds that have elements that share
  electrons equally are simply said to have a
  covalent type of bond
• Covalent bonds that share electrons unequally are
  said to have a polar covalent type of bond
• The unequal sharing results in a bond polarity

F
H
-

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Metallic Bonds

• Metallic bonds exist between metals
• Occur when two metals, usually the same metal,
  are bonded together

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Regents Chemistry

• Electronegativity

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How can we tell really tell which type of bond we
have?

• Electronegativity is the relative ability of an
  atom in a molecule to attract shared electrons to
  itself
• This tells us what type of bond we have
• Covalent, polar covalent or ionic
• Electronegativity values are determined by
  measuring the polarities of bonds between various
  elements to determine a specific value for each
  element

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Electronegativity

• Electronegativity values for each element are
  obtained by using the Periodic Table
• In fact, there is a general trend in
  electronegativity we observe in the Periodic
  Table
• Electronegativity values increase across and up
  the Periodic Table
• See table on pg. 332

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Electronegativity

• We take the difference between the
  electronegativity values to determine exactly
  what type of bond exists, in essence the polarity
  of the bond
• See table 12.1

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Determining Bond Polarity

• If the difference between the electronegativity
  values is
• 0.0 0.5 covalent bond (equal sharing)
• 0.6 1.6 polar covalent bond (unequal sharing)
• 1.7 up ionic bond (transferring electrons)

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Examples

• Use your Reference Tables to determine the
  difference in electronegativity values and the
  type of bond for each of the following
• H-H
• H-Cl
• H-O
• H-S

• H-F
• NaCl
• O2
• KBr

Worksheet
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Regents Chemistry

• Intro to valence electrons

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Electrons in an atom

• Electrons surround the nucleus of an atom in
  specific energy levels or shells
• Each level can hold only a certain amount of
  electrons
• It is an atoms ability to the lose, gain or share
  electrons from its outer shell that determine its
  reactivity

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The outer shell

• The outer shell in an atom contains the valence
  electrons
• Valence electrons can be lost, gained or shared
  to have eight electrons in the outer shell
• Each group on the table tells the number of
  valence electrons

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Periodic Table

• Groups 1, 2, 13, 14, 15, 16, 17, 18 have
  1,2,3,4,5,6,7,8 valence electrons, respectively
• We will not consider the transition metals
• See periodic table

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Sharing to reach the Octet Rule

• The octet rule states that an atom cannot have
  more than 8 electrons in its outer shell
• Valence electrons are lost, gained or shared with
  other atoms to attain 8 electrons in the outer
  shell
• Eight valence electrons means a filled and happy
  shell - like the noble gases

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Nonmetals share

• Nonmetals share electrons to reach eight valence
  electrons
• Single, double and triple bonds can be formed by
  sharing electrons

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Metals non-metals lose/gain e-

• metals and nonmetals interact by losing and
  gaining electrons to reach 8 electrons (filled
  outer shell)
• The oxidation states on the periodic table
  represent this desire to move electrons
• ex K want to lose 1 electron to reach noble
  gas configuration of eight electrons

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Lewis structures your assignment

• The reading and problems focus on drawing Lewis
  structures
• Lewis structures are a means to represent bond
  formation between atoms
• Covalent bonded compounds have different Lewis
  structures than Ionic bonded compounds

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Example of a Lewis Structure
C
CH4 Covalent bonds
H
H
C
H
H
H
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Regents Chemistry

• Lewis Structures

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Lewis Structures

• The Lewis Structure is a representation of a
  molecule that shows how the valence electrons are
  arranged among the atoms in a molecule
• We used dots around the elemental symbol to
  represent the valence electrons

C
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Single Lewis Structure - Practice

• Draw four lone electrons first (if necessary)
  them pair them up
• Draw Lewis Structures for the following atoms

Br
Al
Na
Be
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Lewis Structures for Ionic Compounds

• For Lewis Structures of ionic bonds the atoms are
  not joined but draw next to each other

-
example KBr
Br
K
Bromine gains an electron to achieve the noble
gas configuration of Krypton
Potassium loses an electron to achieve the noble
gas configuration of Argon
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Lewis Structures Covalent Bonds

• Hydrogen forms stable molecules when it shares
  two electrons
• Two electrons fill Hydrogens valence shell
• Helium does not form bonds because its valence
  shell is already filled it is a noble gas
• Second row non-metals Carbon through Fluorine
  from stable molecules when surrounded by eight
  electrons the Octet Rule

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Lewis Structures Covalent Bonds

• Valence electrons in covalent bonds can either be
  bonding pairs, if involved directly in the bond
  or lone pairs if not involved in the bond

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Writing Lewis Structures - Rules

• Obtain the total sum of the valence electrons
  from all of the atoms
• Use one pair of electrons to form a bond between
  each pair of bound atoms. For convenience, a
  line (instead of a pair of dots) can be used to
  indicate each pair of bonding electrons
• Arrange the electrons to satisfy the duet rule
  for hydrogen and the octet rule for second row
• non metals

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Lewis Structures Covalent Bonds

• Examples

Step 1) 8 total valence e- total Step 2) Draw
one pair of electrons per bond
8-6 2 left Step 3) Arrange the remaining
electrons according to
octet rule
PH3
H l H P H
H
P
H
H
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Lewis Structures Covalent Bond Practice Examples
.. HBr
HBr
CF4
Worksheet
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Regents Chemistry

• Ionic Lewis Structures
• Multiple bonds in Lewis Structures
• Polyatomic ion Lewis Structures and Resonance

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Lewis Structures for Ionic Compounds

• For Lewis Structures of ionic bonds the atoms are
  not joined but draw next to each other

-
example KBr
Br
K
Bromine gains an electron to achieve the noble
gas configuration of Krypton
Potassium loses an electron to achieve the noble
gas configuration of Argon
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Examples of Ionic Lewis Structures

• Draw Lewis Structures for the following
• NaCl
• LiBr
• KI

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Multiple Bonds and Lewis Structuresreview first

• We have seen how to draw Lewis Structures for
  molecules with single bonds
• For example
• NH3

8 total valence e- 3 bonds x 2e- 6 bonding 2 e-
left over

• Sum the total
• valence e-
• Subtract number
• of bonding e-
• Place remaining
• valence e-

H
H
N
H
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Multiple Bonds

• Between atoms of the same element
• Example
• Oxygen
• O O

Also a Lewis Structure
O O
Just O O is called a structural model
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Example of Multiple Bonds
Nitrogen
N N
N N
We now meet the octet rule!
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Multiple Bonds

• Between atoms of different elements
• CO2

C
O
O
O C O
We must use double bonds to meet the octet rule!
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Lewis Structures for Polyatomic Ions and
Resonance Structures

• Read pg. 344 (bottom) to 349 and answer questions
  a-g in example
• 12.4 (pg. 347) and a-i in the Self Check
  exercise 12.4 (pg. 348)