Chemical Bonds

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Chemical Bonds Formation of Compounds from atoms
Preparation for College Chemistry Columbia
University Department of Chemistry
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Trends in the Periodic Table
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Atomic and Ionic Radii
Atomic Radii decrease going across a period from
left to right, increases going down group
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Ionization Energy
Minimum energy necessary to remove an electron
from a neutral gaseous atom in its ground state
(IE gt 0, ground state stable system)
X(g) e-
?E IE1
X(g)
X2(g) e-
?E IE2
X(g)
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Electron Affinity EA
Electron attachment energy. Energy released when
an atom in ground state gains a single electron.
X(g) e-
X-(g)
gt 0
EA
lt 0
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Lewis Structures of Atoms
Gilbert Lewis. American Chemist
F
F
2s2p5
P
2s2p3
P


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The Octect Rule


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In H2O and HF, as in most molecules and
polyatomic ions, nonmetal atoms except H are
surrounded by 8 electrons (an octet). Each atom
has a noble gas electronic configuration (ns2p6 )
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Ionic Bond. Electron Transfer
F
F-
(2s2p6)
-
Na
(2s2p6 )
Na

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Ionic Bond. Electron Transfer
Mg
Mg2
Mg2
O
Mg
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Ionicity vs. Covalency
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Covalent Bond. Sharing e-

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Collinear orbitals form s bond

Two p AO Two s MO
Only bonding MO shown
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Coplanar orbitals form p bond

Four p AO Two s MO, and two p MO
Only bonding MO shown
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Electronegativity
Linus Pauling
Dipole

0
3.3
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Lewis Structures of Compounds
Count valence electrons available. number of
valence electrons contributed by nonmetal atom is
equal to the last digit of its group number in
the periodic table. (H 1)
Add electrons to take into account negative
charge. Ex. OCl ion 6 (O) 7 (Cl) 1
(charge) 14 valence e CH3OH molecule 4 (C)
4(H) 6 (O) 14 valence e
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Lewis Structures of Compounds
Draw skeleton structure using single bonds Note
that carbon always forms four bonds. Central
atom is written first in formula. Terminal atoms
are most often H, O, or a halogen. Ex.
O Cl -
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Lewis Structures of Compounds
Subtract two electrons for each single
bond O-Cl ion 14 2 12 valence e-
left CH3 OH molecule 14 10 4 valence e-
left
Distribute remaining electrons to give each atom
a noble gas structure (if possible).
O Cl -
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Lewis Structures of Compounds
Too Few Electrons?
Form multiple bonds
Ex. What is the structure of the NO3 ion?
valence e 5(N) 18 (3O) 1(charge) 24 e
Skeleton
N
O
O
O
O
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Nitrate Ion (cont.)
valence e left 24 - 6 (3 single bonds) 18
e
Adding a double bond and rearranging
Resonance Structures
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Molecular Geometry
VESPR principle electron pairs around a central
atom tend to be oriented so as to be as far apart
as possible.
BeF2 linear (2 pairs of e-)
BF3 trigonal planar (3 pairs of e-)
CF4 tetrahedral (4 pairs of e-)
PF5 triangular bipyramid (5 pairs of e-)
SF6 octahedral (6 pairs of e-)
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VSEPR model