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Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
Chemical Bonds
• ionic bond
• covalent bond
• metallic bond
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
The formation of chemical bonds involves valence electrons
Lewis Symbols: “shorthand” for depicting valence electrons
Gilbert Lewis (1875-1946)
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
Lewis dot structures:
H
Li
He
Be B C N O F Ne
Na Mg Al Si P S Cl Ar
For representative (main group) elements:group number = number of valence electrons
1A 2A 3A 4A 5A 6A 7A 8A
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
Noble gases, except helium, have
When atoms react, they tend to lose, gain, or share the number of electrons required to achieve eight valence
electrons (an “octet” of electrons)
K Cl+ K+ + Cl
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
Na+ Cl-
Cl- Na+
Na+ Cl-
Formation of an ionic lattice
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
Na+ (s) + Cl- (g) NaCl (s) very exothermicH<0
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
Na+ Cl-
Na+ Cl-
H
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
dQQ
Eel
charge on ions
distance between charges
Strength of ionic bond depends on Eel
• the larger Eel, the stronger the bond
Chapter 8: Periodic Properties of the ElementsChapter 8: Periodic Properties of the Elements
The stronger the ionic bond the the melting point
SrF2 +2, -2
66, 133
66, 140
113, 133
1261oC
r1 r2
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Covalent Bonds …
H H+ H H
F F+ F F
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Covalent Bonds
H H+ H H
F F+ F F
• A shared electron pair is drawn as a dash (two electrons!)
• Unshared electrons are drawn as dots
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Single and Multiple Bonds
F F+ F F
O O+ C + O C O
N NN + N
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Single and Multiple Bonds
X X
X X
X X
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Polarity and Electronegativity
Cl Cl non-polar covalent bond:equal sharing of electrons
• When both atoms attract bond electrons equally, electrons are shared equally
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Polarity and Electronegativity
H Cl polar covalent bond:unequal sharing of electrons
+
• If one of the atoms attracts bond electrons more strongly, electrons are shared unequally
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Polarity and Electronegativity
H Cl polar covalent bond:unequal sharing of electrons
+
For comparison:
ionic bond:electrons are not sharedNa+ Cl -
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
How do we know when a non-polar, polar, or ionic bond is formed?
The tendency of atom to attract electrons when forming a molecule is
summarized in the concept of
electronegativity
Incr
ease
in E
lectro
nega
tivity
Linus Carl Pauling (1901-1994)
• developed concept of electronegativity
• the electronegativity scale assigns avalue of 4 to Fluorine and 0.7 to Cesium, the least electronegative element.
• least electronegative atoms are thosethat give up electrons most easily.
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
How do we know when a non-polar, polar, or ionic bond is formed?
If the difference in Electronegativity, EN, is…
…smaller than 0.5EN < 0.5 => nonpolar bond
…greater than or equal to 0.5 AND smaller than 2.00.5 EN < 2.0 => polar bond
…greater than or equal to 2.0EN 2.0 => ionic bond
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Among the following examples, which bond is most polar?
S
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Among the following examples, which bond is shortest?
S
Bond length depends on (a) radii of the bonded atoms
(b) the number of bonds between atoms
remember that the atomic radii decrease along a period in the P.T.
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
If you had to guess, which of the following bonds do you think would be most difficult (require the largest energy) to break?
S
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Drawing Lewis Structures of Molecules
If the compound contains more than 2 atoms:
• how are the atoms bonded and,
• if there are nonbonding electron, where are they?
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Molecules with a central atom : NH3, PCl3, CHCl3
central atom is generally the first in the molecular formula
NH H
HC
H
Cl Cl
Cl
PCl
Cl
Cl
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
…unless the first element is Hydrogen :
H2OO
H H
HCN C NH
(same order as in formula)
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
NH3
8
NH H
H
6
n/a
+ 2
n/a
Rules for Drawing Lewis Structures
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
CO
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
SF2
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
Ions
NH4+
for ions, the charge is generally indicated by
square brackets and the sign
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
Ions
ClO2-
for ions, the charge is generally indicated by
square brackets and the sign
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule
On occasion, an atom in a molecule does not have an octet of valence electrons:
• If the molecule has an odd number of valence electrons
• an atom may have less than an octet [mainly Be, B]
• an atom may have more than an octet [period (n) 3]
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule: odd number of electrons
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
NO2
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule: less than an octet
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
BF3
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule: more than an octet
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
BrF5
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Resonance
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
SO3
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Which of the following molecules exhibits resonance?
NO3- HCN SO2
Draw Lewis structures!
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
If you had to guess, which of the following bonds do you think would be most difficult (require the largest energy) to break?
S
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Enthalpy… …is the energy required to break a covalent bond…is always positive
C
H
H H
H (g)
C (g) + 4 H (g) H = 1660 kJ/mol
C H (g) C (g) + H (g) D (C-H) = 1660/4 kJ/mol = 415 kJ/mol
per C-H bond:
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Enthalpy is the energy required to break a covalent bond
… and it offers yet another way to estimate the Horxn
Horxn = Σ bond enthalpies of bonds broken – Σ bond enthalpies of bonds formed
Horxn = Σ n x Dbroken – Σ m x Dformed
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Estimate Ho for the following reaction:
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)
C
H
H H
H
O O
O O
OH H
OH H
C OO
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Estimate Ho for the following reaction:
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)
Horxn = Σ n x Dbroken – Σ m x Dformed
Horxn = [4 x 414 + 2 x 498] – [2 x 803 + 4 x 463] = - 806 kJ/mol