ch. 12 chemical bonding 12.1 ionic, covalent and polar covalent bonds. a bond is a force that holds...
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Ch. 12 Chemical Bonding
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12.1 Ionic, covalent and polar covalent bonds.
• A bond is a force that holds atoms together.
• Ionic Bondinga. When a metal reacts with a nonmetal,
electrons are transferred from a metal to a nonmetal and an ionic compound is made.
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Ionic Bonding
b. In ionic bonding, electrostatic attraction holds atoms together.
Na Cl
+1 -1
NaNaNaNa ClClClCl
+1 -1
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Covalent Bonding
a. Atoms make covalent bonds by sharing electrons.
b. Electrons are attracted to the nucleus of both atoms in the bond.
c. Nonmetals make covalent bonds.
H H
Let’s share!
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Electronegativity
• Electronegativity is a measure of how strongly an atom attracts electrons. Look at the electronegativity chart on p403.
– Fluorine has an electronegativity value of 4.0.
– Hydrogen has an electronegativity value of 2.1.
– Difference in electronegativity =
4.0 – 2.1 = ________
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Bond Types• Determined by difference in electronegativity
values—absolute value of DENPauling Electronegativity Values
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Electronegativity
d. At left is a picture of hydrofluoric acid (HF). At right is a picture of “The Blob.”
e. F has a higher electronegativity than H, so electrons are closer to F.
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• In a covalent bond, atoms have a difference of electronegativity of 0-0.2. Electrons are shared equally.
• In a polar covalent bond, atoms have a difference in electronegativity of 0.3-1.7. Electrons are not shared equally.
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Electronegativity
• Exampe: Si—C– Electronegativity of Si is ________.
Electronegativity of C is _______. – Difference in electronegativity is
____________.– Si – C bond is ( covalent / polar covalent ) [circle one].
• Which attracts more electrons? ( Si / C )
1.8
2.5
0.7
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• In the picture below, – Label each molecule or compound as ionic
bonding, covalent bonding or polar covalent bonding.
– For polar and ionic bonds, label the more electronegative atom.
Covalent
Polar covalent
Ionic
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Identify each of the following bonds as ionic, covalent or polar covalent.
BondIs there a metal and nonmetal?
Electronegativity difference
Bond Type
H – H No 2.1 – 2.1 = 0 Covalent
S – H No 2.5 – 2.1 = 0.4 Polar covalent
Na – I Yes ionic ionic
Rb – N
B – P
H – P
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Dipoles
• In a polar molecule, one side has a partial positive charge, and the other has a partial negative charge.
• A dipole moment is represented with an arrow pointing to the negative side and the Greek letter “delta” δ to show the partial positive and negative charges:
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Dipoles
• Write the partial charges and draw the dipole moment on Cl –I
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12.2 Ionic Bonding
• Ions– Metals ( lose / gain ) electrons.– Nonmetals ( lose / gain ) electrons.– Group 1 elements form ions with a charge of ____.– Group 2 elements form ions with a charge of ____.– Group 6 elements form ions with a charge of ____.– Group 7elements form ions with a charge of ____.
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Ions
• That’s interesting…but WHY??? – Atoms gain or lose electrons to get the
electron configuration of a noble gas. – Noble gases have completely filled
energy levels, so they are very stable.– He has a completely filled 2s sublevel.– Other noble gases have filled s and p
sublevels.
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Ions
• Example: Li– The electron configuration of Li is ______– Li loses one 2s electron and becomes Li+.– The electron configuration of Li+ is _____– Li + has the same electron configuration
as He.
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Ions
• The electron configuration of F-– The electron configuration of F is
– F gains one 2p electron and becomes F-
– The electron configuration of F- is
– F- has the same electron configuration as ______
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Ions
• What would happen if Li reacted with F?– Li gives an electron to F– Li + F Li+ + F-
– And they form _______ (write the formula of the compound)
• Ionic bonding and structures– LiF is packed together in a group in order to
maximize attractions of the cations and anions.– It makes a hard, tight crystal.
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The size of ions
• Which is larger, Na or Na+? Why?– Na. Na loses a 3s electron, and then only
has electrons in the n=2 level. n=2 orbitals are smaller than n=3 orbitals.
Na Na+
F F-
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The size of ions
• Which is larger, F or F-? Why?– F- because it gains electrons.
• Which is larger F- or Na+? Why?
• F-. They both have the electron configuration of Ne, but Na+ has more protons (a stronger + charge), which pulls electrons closer.
• Which is larger, Ca or Ca2+? Why?
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Ionic SizeTaken from: http://www.chem.umass.edu/people/botch/Chem121F06/Chapters/Ch15/IonicRadii.jpg
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12.3 Lewis Structures
• The octet rule: Sharing of electrons usually occurs so that atoms acquire the electron configurations of noble gases (1s2 or ns2np6)
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Lewis dot structures
– The element symbol
represents the core electrons.– Dots to show the
valence electrons.
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Lewis dot structures
• 1. First, write the symbol for the element.
• 2. Imagine the molecule has four sides (but don’t draw the “x”)
• 3. Draw one dot at a time in each empty section.
• 4. You should only have pairs of e- if there are no empty sections.
Cl
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Lewis dot structures• The “paired” electrons
cannot usually make bonds.
• The three “unpaired” electrons can make bonds.
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Arsenic
• Dot Diagram?
• In order to get the configuration of a noble gas, how many bonds will arsenic form?
• Dot Diagram for Hydrogen?
• In order to get the configuration of a noble gas, how many bonds will H form?
3
H
1
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Arsenic Trihydride, AsH3
As HHHH
HHAs HHHH
HH
Lewis dot structure Structural formula
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Drawing Lewis Dot Structures
1. Use the molecular formula to find the total number of valence electrons
• 1 + 1 + 1 + 5 = 8
H H H As
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Drawing Lewis Dot Structures2. Draw the symbols of each element.
Draw the backbone/skeleton structure
– Terminal atoms go around the central atoms.
– Least Electronegative is usually central
– Hydrogen is always terminal.– Carbon is usually central– Oxygen and Halogens are usually
terminal
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Drawing Lewis Dot Structures
3. Draw the valence electrons
4. Make single bonds with pairs
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Drawing Lewis Dot Structures
5. Each pair represents a bond.
6. Count electrons again and make sure you get the same number (8).
Single bond
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Structural Formula
• The structural formula is drawn with a “–“ line to represent the bond.
• Double and triple bonds– Double bonds can be
formed by sharing two pairs of electrons
– Triple bonds involve sharing three pairs of electrons.
OO
NN
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Try the Dot Diagrams for each of the following:
O2 H2O BCl3 SiI4
OO HOH BCl
ClCl SiI
III
Each pair of shared
electrons can be
represented by a line.H
OH
OO B
Cl
ClCl Si
I
II
I
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Structural formulas for polyatomic ions
• Example: NO3-
• The negative charge means there is one more electron
• How many total electrons are there?• 5 + 3(6) + 1 = 24 electrons.
• NO3-
• Draw:
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Resonance
• Resonance– Example: ozone– Ozone’s structure can be drawn like this:
– The actual bonding in ozone is not like either of these structures. The actual structure lies somewhere in between these two.
– These drawings are resonance structures of ozone
O O O O O O