Structure and Bonding
The Periodic Table
The nucleus contains positively charged protons and
uncharged neutrons.
The electron cloud is composed of negatively charged
electrons. 1
A periodic table of the common elements seen in organic chemistry
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Review of Bonding
• Bonding is the joining of two atoms in a stable
arrangement.
• Through bonding, atoms attain a complete outer shell of
valence electrons.
• Through bonding, atoms attain a stable noble gas
configuration.
• Ionic bonds result from the transfer of electrons from one
element to another.
• Covalent bonds result from the sharing of electrons
between two nuclei.
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Summary: The usual number of bonds of common neutral atoms
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Label the following as either ionic or covalent bonded
molecules.
F2 CH3CH3
LiBr NaNH2
How many covalent bonds are predicted for each atom
O Al Br
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Review of Lewis Structures
There are three general rules for drawing Lewis structures:
1. Draw only the valence electrons.
2. Give every second-row element an octet of electrons, if
possible.
3. Give each hydrogen two electrons.
In a Lewis structure, a solid
line indicates a two-electron
covalent bond. 6
Formal Charge
• Formal charge is the charge assigned to individual atoms in a
Lewis structure.
• Formal charge is calculated as follows:
• The number of electrons “owned” by an atom is determined by its
number of bonds and lone pairs.
• An atom “owns” all of its unshared electrons and half of its shared
electrons.
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Resonance
Resonance structures are equivalent Lewis structures
having the same placement of atoms but a different
arrangement of electrons.
Examples: O3, CO32-
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Determining Molecular Shape
Two variables define a molecule’s structure: bond length and
bond angle.
• Bond length decreases across a row of the periodic
table as the size of the atom decreases.
• Bond length increases down a column of the
periodic table as the size of an atom increases.
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Drawing Organic Molecules—Condensed Structures
• All atoms are drawn in, but the two-electron bond lines are generally
omitted.
• Atoms are usually drawn next to the atoms to which they are bonded.
• Parentheses are used around similar groups bonded to the same atom.
• Lone pairs are omitted.
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Examples of Condensed Structures
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Examples of Condensed Structures Containing a C-O Double Bond
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Skeletal Structures
• Assume there is a carbon atom at the junction of any two lines or
at the end of any line.
• Assume there are enough hydrogens around each carbon to make
it tetravalent.
• Draw in all heteroatoms and hydrogens directly bonded to them.
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Examples of Skeletal Structures
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Words of Caution Regarding Interpretation of Skeletal Structure:
• A charge on a carbon atom takes the place of one hydrogen atom.
• The charge determines the number of lone pairs. Negatively
charged carbon atoms have one lone pair and positively charged
carbon atoms have none.
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Summary of Covalent Bonding Seen in Carbon Compounds
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Bond Length and Bond Strength
• As the number of electrons between two nuclei increases,
bonds become shorter and stronger.
• Thus, triple bonds are shorter and stronger than
double bonds, which are shorter and stronger than
single bonds.
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Electronegativity and Bond Polarity
Electronegativity is a measure of an atom’s attraction for bonding
electrons.
Electronegativity values for some common elements:
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Bonding between atoms of different electronegativity values results in
unequal sharing of electrons.
Example: In the C—O bond, the electrons are pulled away from C towards
O, the element of higher electronegativity. The bond is polar, or polar
covalent. The bond is said to have dipole; that is, separation of charge.
d+ means the
indicated atom is
electron deficient.
d- means the
indicated atom is
electron rich.
The direction of polarity in a bond is indicated by an arrow with the head of
the arrow pointing towards the more electronegative element. The tail of the
arrow, with a perpendicular line drawn through it, is drawn at the less
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electronegative element.
A polar molecule has either one polar bond, or two or more bond
dipoles that reinforce each other. An example is water:
A nonpolar molecule has either no polar bonds, or two or more
bond dipoles that cancel. An example is carbon dioxide:
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