Looking for Patterns in Data
Scientific Process
• Based on observations
• Proposing a “rule” to organize the observations into patterns
(hypothesis)
• Gathering more information – is it consistent with the “rule”?
• If not, refine rule to accommodate the new information
Periodic Table
• List of all
elements
• Organized in a
table that
shows
repeating
nature
(“periodicity”)
of properties
Boiling Points of Hydrogen Containing
Compounds
What “rule” can you propose
about this data?
Boiling Points of Hydrogen Containing
Compounds
Does the rule hold true with
new data?
Boiling Points of Hydrogen Containing
Compounds
How about this data?
Boiling Points of Hydrogen Containing
Compounds
And this data?
Boiling Points of Hydrogen Containing
Compounds
Does your rule still work here?
What is missing from the data?
Boiling Points of Hydrogen Containing
Compounds
Does your rule still work here?
What is the problem?
We Need More Information!
We Need More Information!
Electronegativity:
chemical property that
describes the tendency of
an atom to attract
electrons towards itself.
The higher the
electronegativity, the
more “greedy” the atom
is for electrons.
Can You Create a Revised Rule?
Electronegativity Table
Polar Bonds
• You have uncovered an interesting property of oxygen (O), nitrogen
(N) and fluorine (F) atoms bonded to hydrogen (H).
• O, N and F are the most electronegative elements – they are very
greedy for electrons!
• Hydrogen has a single proton, which has a single positive charge. It
isn’t able to hold its electron as tightly as larger atoms with more
protons.
• As a result, O, N and F have a strong tug on the electron of the
bonded hydrogen atom, and the electron hangs around the larger
atom more than around the hydrogen atom. These bonds are said to
be polar.
Looking at Water
• The symbols δ+ and δ- (read “delta positive”
and “delta negative”) indicate partial
positive and partial negative charges,
respectively.
• The partial positive charge on one
molecule can interact with the partial
negative charge on another molecule.
δ+
δ+
δ-
Hydrogen Bonds
• Chemical interaction in which a hydrogen
atom covalently bonded to N, O, or F forms
an electrostatic link with another
electronegative atom (N, O or F) in the same
or another molecule.
• Hydrogen bonds are weak interactions.
• Yet a LOT of hydrogen bonds among
molecules can pack a powerful punch.
Hydrogen Bonding Raises Boiling Points
• Hydrogen bonding explains the
elevated boiling points of H2O, NH3’
and HF.
• What other properties of water can
be explained by hydrogen bonding?