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Lecture 2 EIGHT IS ENOUGH: Periodic Table
Key Idea The ratio of elements in a compound depends on the number of valence electrons.
© Chem 1A, UC Berkeley, Fall 2011
Demonstrations
Combine pairs of elements
Where are the elements on the periodic table? What do you expect will happen?
© Chem 1A, UC Berkeley, Fall 2011
This is a hugely powerful reaction !
© Chem 1A, UC Berkeley, Fall 2011
Main engine: Hydrogen plus oxygen
Solid rocket booster
Why don’t we write 2H + O H2O
Balance the equation. Stoichiometries of Compounds
Only certain stoichiometries are observed:
What patterns do you observe? © Chem 1A, UC Berkeley, Fall 2011
solids gases
What patterns do you see in the bonding of C, H, O, and N?
Bonding Patterns
© Chem 1A, UC Berkeley, Fall 2011
C
H
H
H
H N H
H
H O
H
H
C
H
H
H C
H
H
N
H
C
H
H
H
C
H
H
H C
H
H
C
O
C
H
H
H
O
© Chem 1A, UC Berkeley, Fall 2011
What structural formulas are possible for C2H6O?
What are the lines in structural formulas? What do the dots represent?
CH
H
H
H
CH H
H
H
Electron Pair Bonds
Notice that one electron comes from each atom to make an electron pair bond.
© Chem 1A, UC Berkeley, Fall 2011
C
H
H
H
H
Shell Models of the Atoms
What patterns do you notice?
Use the shell models to explain why properties are periodic.
1 2 3 4 5 6 7 8
© Chem 1A, UC Berkeley, Fall 2011
Lewis Electron Dot Symbols
H He
Li
Na
Be B C N O F Ne
Mg Al Si P S Cl Ar
1 2 3 4 5 6 7 8
Octet rule: Noble gas envy!
Valence electrons are involved in bonding
© Chem 1A, UC Berkeley, Fall 2011
H H
Covalent Bonding
© Chem 1A, UC Berkeley, Fall 2011
For these molecules, each atom contributes one electron to form the covalent bond.
© Chem 1A, UC Berkeley, Fall 2011
O2 N2
H2 + N2
H3O+ OH!
© Chem 1A, UC Berkeley, Fall 2011
Slide 10
Resonance Structures
• Measurements indicate both bonds are the same
• Bond order refers to the number of bonds. The bond between each S and O is
More than one Lewis structure satisfies the octet rule.
SO2
© Chem 1A, UC Berkeley, Fall 2011
Formal Charge
overall charge =
• S and O each give 2 electrons to the double bond. • S gives 2 electrons to the single bond. O gives none.
O ! S = O .. .. ..
.. .. ..
Bonded atoms do not contribute electrons equally.
Formal charge?
© Chem 1A, UC Berkeley, Fall 2011
Draw the Lewis structure of CO.
Illustrated by David R Dudley
© Chem 1A, UC Berkeley, Fall 2011
H ! O = C ! O ! H .. .. ..
.. Why is this molecule not observed? Wrap-up points:
• The valence electrons are involved in bonding between atoms.
• Atoms have “noble gas envy.” They bond to get 8 valence electrons.
• Covalent bond: Atoms share electrons.
• A formal charge results if each atom in a bonded pair does not contribute equally to the bond.
© Chem 1A, UC Berkeley, Fall 2011
