CHM 233 Lecture Notes - Lecture 3: Octet Rule, Chemical Formula, Formal Charge

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Published on 22 Sep 2015
School
ASU
Department
Chemistry
Course
CHM 233
CHM233 Lecture #3: Chapter 1: Remembering General Chemistry Electronic Structure and
Bonding
Lewis Structure (continuation)
Atom
# Valence e-
+1
0
-1
B
3
2 B / 0 N.B.P.
3 B / 0 N.B.P.
4 B / 0 N.B.P.
C
4
3 B / 0 N.B.P.
4 B / 0 N.B.P.
3 B / 1 N.B.P.
N
5
4 B / 0 N.B.P.
3 B / 1 N.B.P.
2 B / 2 N.B.P.
O
6
3 B / 1 N.B.P.
2 B / 2 N.B.P.
1 B / 3 N.B.P.
F
7
2 B / 2 N.B.P.
1 B / 3 N.B.P.
0 B / 4 N.B.P.
B = Bonds
N.B.P. = Nonbonding Pair(s)
This table shows a pattern to use instead of the formal charge calculations
Neutral state is known from the Lewis structure for the element
o Unpaired electrons represent the number of bonds typically made (valence e-)
Isomers
Molecules that have the same electronic configuration are isoelectronic
Molecules that have the same molecular formula are isomers
o “iso” – same
o “mer” – unit
Isomers have different types including constitutional, conformational and stereo
o Constitutional same molecular formula, but different physical attachments of the atoms
o Conformational same molecular formula, but have different rotations around bonds
o Stereo same molecular formulas, same physical attachments, different 3D orientations
C2H6O C2H6O
These are different molecules regardless of the chemical formula
Octet Rule Exceptions
Groups 1, 2; 3 too few electrons
o Some ions as well
Periods 1; 2 elements cannot have expanded octets
o Because second energy shells do not have ‘d’ orbitals
o These elements are usually involved in/with organic molecules
Periods 3, 4, 5, 6; 7 too many electrons
o May have expanded octets
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CH3+ AlH3
4+3=7 3+3=6
7-1=6 6-6=0
6-6=0 Aluminum
Carbon has a formal charge of +1 (carbon cation) is neutral because of its normal bonding patterns
(CH3)2SO Oxygen-sulfur double bond is created to make the
(4+3)2+6+6 molecule neutral. Oxygen has two bonding pairs;
8+6+12=26 two nonbonding pairs. Sulfur breaks the octet rule,
but meets its neutral status requirement with 4 bonding
pairs and 1 nonbonding pair. This sulfur bonding is not
one of the normal bonding patterns, and it requires calculations for the formal charge:
F.C. = valence e- - (1/2 * bonding e-) nonbonding e-
F.C. = 6 (1/2 * 8) 2 = 0, neutral molecular state
Drawing Other Organic Structures
Molecular formulas do not show how the atoms are attached
Two ways of drawing in organic chemistry are
o Condensed structures
Omits presenting electrons (bonds and lone pairs)
Keeps atoms attached to one another in a group
Indicates physical attachment of the molecule(s)
Represent all of the atoms, electrons, nonbonding pairs
Implied connectivity for bonding pairs
Rings aren’t present in these structures
It is important to know the valence of atoms
Hydrogen: monovalent
Carbon: tetravalent
Parenthesis indicates attachment to the central atom and group bonding
o Skeletal structures (line-angle structures)
Represents covalent bonds through zigzag lines
Carbons and hydrogens are implied
Hydrogen is shown when it’s attached to a heteroatom
Heteroatoms are any different atoms from H; C
Heteroatoms must be drawn
Carbons are present on terminuses and vertices of the zigzag lines
Octets are filled with Hydrogen unless another atom is present
Lone pairs are included, but may be left out
therefore need to know and understand bonding patterns
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Document Summary

Chm233 lecture #3: chapter 1: remembering general chemistry electronic structure and. This table shows a pattern to use instead of the formal charge calculations. Neutral state is known from the lewis structure for the element: unpaired electrons represent the number of bonds typically made (valence e-) Molecules that have the same electronic configuration are isoelectronic. Molecules that have the same molecular formula are isomers: iso same, mer unit. These are different molecules regardless of the chemical formula. Groups 1, 2; 3 too few electrons: some ions as well. Periods 1; 2 elements cannot have expanded octets: because second energy shells do not have d" orbitals, these elements are usually involved in/with organic molecules. Periods 3, 4, 5, 6; 7 too many electrons: may have expanded octets. Carbon has a formal charge of +1 (carbon cation) is neutral because of its normal bonding patterns (ch3)2so (4+3)2+6+6. Oxygen-sulfur double bond is created to make the molecule neutral.

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