LECTURE                         CHEMICAL BONDS

In this lecture you will learn:

COVALENT
The bonds are due to electron sharing. They are the glue that holds living things together AND where energy is stored that runs all living things.

They are energetically stable, the simplest ones are H2, N2, O2

Main difference with other bonds is how closely the two atoms are in that they share the outer electron orbit.

Because covalent is much closer to each other, therefore more energy is released in the formation of naturally occurring covalent bonds.

The covalent bond is VERY STRONG. 

Covalent bonds are like joint custody in that they share the electrons very closely. 



 
IONIC
Next strongest after covalent
 
The bonds are due to electrostatic interaction

- They are formed between ions like sodium and chloride

- Those elements in the far left of the periodic table would rather GIVE UP an electron to become very stable. 

- Those elements on the far right of the periodic table would rather TAKE an electron to become very stable. 

- AFter giving up or taking an electron these elements become IONS. 

- NOW, in order to be energetically stable they must find an "opposite" charge and form an ionic bond which gives the two ions a net zero charge  which is energetically favorable. 

- The attraction of ionic bonds is like static electricity
- the minerals that make up all the rocks on earth are inorganic and ionic to one degree or another many of them forming with either oxygen or sulfur
- ionic bonds form crystal arrangements that are strong, but brittle
- some ionic bonds are soluble in water. Those formed from light elements like NaCl are very soluble. As the elements get heavier or more complex, like Calcium carbonate, CaCO3, the energy necessary to dissociate the bond becomes higher.
 

IONS are like giving up custody or getting custody of the electron.  They share the electrons in ionic bonds, but the atoms never get as close as in covalent bonds. 


ionic covalent
custody of electron given up custody of electron shared
ions attracted to close contact contact much closer 
little energy to form or break a great deal more energy to form or break
strong but brittle bond very strong bond and flexible

 
HYDROGEN BOND
Below are water molecules, H2O.
Hydrogen bonds are POLAR in that they are the attraction of positive and negative ions. 
 
The bonds are due to electrostatic interaction

H-bonds are individually weak, sorta like static cling.  However, when there are a lot of H-bonds they are cumulative and STRONG.  So, for example, the double strand of DNA is "zipped up" with  hydrogen bonds which need an enzyme or heat  to unzip.  Hydrogen bonds hold all kinds of organic molecules together so they are functional. 

WATER IS A SPECIAL MOLECULE.  The COVALENT BONDS between oxygen in the center and hydrogen on either side are at an angle of 105 degrees.  (The angle of CO2 is 180o, O-C-O for reference.)

The HYDROGEN BONDING between water, like above, are due to the fact that oxygen REALLY really wants the electrons and holds onto them much more than hydrogen, so the oxygen is negatively charged and the hydrogen is positively charged most of the time.  The oxygen then hydrogen bonds with hydrogen from OTHER molecules of water while the hydrogen. 

This interaction is what makes water HARD with a strong surface tension.  Puts the ouch in belly flops.

http://207.10.97.102/chemzone/lessons/03bonding/mleebonding/hydrogen_bonds.htm

 
VAN DER WAALS
A very, very weak non-polar attraction between non-polar molecules like clay, grease, oils. 
 
 It is due to the "random" TRANSIENT separation of charges in an otherwise non-polar molecule. 

It is the reason that pencils (which are carbon) can be dragged across a surface leaving some of the carbon behind.  The carbon is in thin sheets or layers bound together by Van der Vaals. 


It is also how geckos can walk up smooth surfaces.  Click here for more. 


"Panel A shows an actual gecko foot at moderate magnification, while panel B shows a scanning electron microscope image of the detailed structure of the filamentous setae lining the gecko's foot pads. The bottom two images show the carbon nanotube analogs developed in Dhinojwala's laboratory.
Credit: Ali Dhinojwala, the University of Akron"
http://www.nsf.gov/discoveries/disc_images.jsp?cntn_id=112442&org=NSF

There are no suction cups on the end of the filaments. 


"A high-magnification scanning electron microscope image of gecko setae shows the complex structure of these filaments that cover the animal's foot. Each filament is only microns (millionths of a meter) in diameter.
Credit: Ali Dhinojwala, The University of Akron"
Evidence for van der Waals adhesion in gecko setae 
Abstract

Geckos have evolved one of the most versatile and effective adhesives known. The mechanism of dry adhesion in the millions of setae on the toes of geckos has been the focus of scientific study for over a century. We provide the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, and reject the use of mechanisms relying on high surface polarity, including capillary adhesion. The toes of live Tokay geckos were highly hydrophobic, and adhered equally well to strongly hydrophobic and strongly hydrophilic, polarizable surfaces. Adhesion of a single isolated gecko seta was equally effective on the hydrophobic and hydrophilic surfaces of a microelectro-mechanical systems force sensor. A van der Waals mechanism implies that the remarkable adhesive properties of gecko setae are merely a result of the size and shape of the tips, and are not strongly affected by surface chemistry. Theory predicts greater adhesive forces simply from subdividing setae to increase surface density, and suggests a possible design principle underlying the repeated, convergent evolution of dry adhesive microstructures in gecko, anoles, skinks, and insects. Estimates using a standard adhesion model and our measured forces come remarkably close to predicting the tip size of Tokay gecko seta. We verified the dependence on size and not surface type by using physical models of setal tips nanofabricated from two different materials. Both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures.

Published online before print August 27, 2002, doi: 10.1073/pnas.192252799 

PNAS  September 17, 2002   vol. 99  no. 19  12252-12256
 

http://207.10.97.102/chemzone/lessons/03bonding/mleebonding/van_der_waals_forces.htm
http://www.pnas.org/content/99/19/12252.abstract?cited-by=yes&legid=pnas;99/19/12252#cited-by

BONDS ARE FORMED BETWEEN ATOMS MAINLY TO LOWER THEIR OVERALL ENERGY AND MAKE THE RESULTING COMPOUND MORE STABLE.  MOST elements are so energetically unstable that they chemically react with other elements to lower their energy levels.  Only a few elements are found PURE in nature, like gold, copper, silver.



SOME REVIEW QUESTIONS

Draw a picture of an ionic and covalent bond and explain what the difference is between them.
Fill in the following table

 
ionic covalent
custody     
closeness    
energy level of bond    
strength    
Give some characteristics of hydrogen bonds and van der Waals bonds.
Draw a hydrogen bond.  This questions WILL be on the exam and is not easy to answer as it isnt a covalent bond.
What are the two properties of covalent bond that are essential for life?