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Demonstrations Lecture 2 EIGHT IS ENOUGH: Periodic TableCombine pairs of elements

Key Idea The ratio of elements in a compound depends on the number of valence electrons. Where are the elements on the periodic table? What do you expect will happen? Chem 1A, UC Berkeley, Fall 2011 Chem 1A, UC Berkeley, Fall 2011

Main engine: Hydrogen plus oxygen

Stoichiometries of CompoundsBalance the equation.

Only certain stoichiometries are observed:

This is a hugely powerful reaction !Solid rocket booster Why dont we write 2H + O H 2O solids

gases

What patterns do you observe? Chem 1A, UC Berkeley, Fall 2011 Chem 1A, UC Berkeley, Fall 2011

Bonding PatternsWhat patterns do you see in the bonding of C, H, O, and N?H H C H H H C H H H C H H C H H C H O C O H N H C H H C H Chem 1A, UC Berkeley, Fall 2011

What structural formulas are possible for C2H6O?

H

H

N H

H

H

O H

H

H

Chem 1A, UC Berkeley, Fall 2011

Electron Pair BondsWhat are the lines in structural formulas? What do the dots represent?H

Shell Models of the Atoms1 2 3 4 5 6 7 8

H H C H H

H H C H HH

C

H

H

Notice that one electron comes from each atom to make an electron pair bond. Chem 1A, UC Berkeley, Fall 2011

What patterns do you notice? Use the shell models to explain why properties are periodic. Chem 1A, UC Berkeley, Fall 2011

Lewis Electron Dot SymbolsValence electrons are involved in bonding

Covalent Bonding

1 H Li Na

2 He Be Mg

3

4

5

6

7

8

B Al

C Si

N P

O S

F Cl

Ne Ar

H

HFor these molecules, each atom contributes one electron to form the covalent bond. Chem 1A, UC Berkeley, Fall 2011

Octet rule: Noble gas envy! Chem 1A, UC Berkeley, Fall 2011

Slide 10

O2

N2

Resonance StructuresMore than one Lewis structure satisfies the octet rule.

SO2H2 + N2

H3O+

OH!

Measurements indicate both bonds are the same Bond order refers to the number of bonds. The bond between each S and O is Chem 1A, UC Berkeley, Fall 2011 Chem 1A, UC Berkeley, Fall 2011

Formal ChargeBonded atoms do not contribute electrons equally.

O!S=O

Formal charge?

S and O each give 2 electrons to the double bond. S gives 2 electrons to the single bond. O gives none.Draw the Lewis structure of CO. Chem 1A, UC Berkeley, Fall 2011

Chem 1A, UC Berkeley, Fall 2011

..

Why is this molecule not observed?

..

..

..

.. .. ..

overall charge =

Illustrated by David R Dudley

Chem 1A, UC Berkeley, Fall 2011

H!O=C!O!H

.. .. ..

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