In this lecture you will learn:
There are 4 major bonds that hold atoms
Van der waals
|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.
|Next strongest after covalent
- They are formed between ions like sodium
- 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
- The attraction of ionic bonds is like
- 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.
|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
|Below are water molecules, H2O.
|Hydrogen bonds are POLAR in that they
are the attraction of positive and negative ions.
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
|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
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.
VAN DER WAALS
|A very, very weak non-polar attraction
between non-polar molecules like clay, grease, oils.
is due to the "random" TRANSIENT separation of charges in an otherwise
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
"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
There are no suction cups on the end of
"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
|Evidence for van der Waals adhesion in
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
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
Give some characteristics of hydrogen bonds
and van der Waals bonds.
|energy level of bond
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?