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-

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?