Transcript
Chemical Bonds
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Intro
The Periodic Table lists all of the substances in the universe that have only one
type of atom in their makeup.
Countless millions of OTHER substances are made from combinations of these
atoms. When atoms come together, they form BONDS, and the resulting
molecule or compound has a whole new set of chemical and physical properties.
So lets look at chemical bonding, and start with the most abundant atom in the
universe -- hydrogen.
Covalent
The element hydrogen has one proton and one electron, making it quite simple.
The proton is positively charged in the center of the atom and the electron is
negatively charged and occupies a larger space outside the nucleus in the form
of an electron cloud.
This electron cloud surrounding hydrogen is often called an orbital. It is basically
all the places where an electron has a chance of being found.
Now lets bring another hydrogen atom into the picture. As we slowly start
moving them closer to one another, we will show their combined energy on this
graph. For a while nothing much changes, and then as they get close enough
together, their energy starts to drop and eventually hits a minimum….
…and this makes the combination stable. This minimum-energy-distance
between them is called the bond distance, and they are now sharing their
electrons. This is called a covalent bond, and instead of having two individual
hydrogen atoms, we now have an H2 molecule. Moving the atoms any closer
increases the energy greatly since the positive protons in each of the atoms are
repelling one another strongly
Polar-Covalent
Covalent bonds come in different lengths and strengths, depending on the atoms
involved.
These bonds are often shown as posts or lines or dots between atoms. But of
course these are simply representations of places where electrons spend their
time between atoms.
In a covalent bond, electrons are shared, but they aren’t necessarily shared
equally. Lets take one of the hydrogen atoms and replace it with a fluorine atom.
Watch what happens to the electrons throughout the atom.
Fluorine, by its very nature, has a stronger pull on the electrons in this molecule
than does the hydrogen. Fluorine shares the electrons… but not equally.
Instead, it pulls electrons closer to itself than to the hydrogen. This is called a
polar covalent bond.
Electronegativity
Each element in the periodic table has a tendency to pull electrons towards itself
in a bond. This property is called its electronegativity. When you alter the
periodic table to show electronegativity, it looks a little different.
Notice that elements on the left hand side of the table, metals, have very low
electonegativity values. They don’t pull electrons towards themselves at all. As
a matter of fact, they willingly give them up entirely.
NaCl
Table salt, as we know, is made up of Sodium and Chlorine, and so it is called
Sodium Chloride.
In forming table salt, the sodium atom gives up an electron and becomes
positively charged.
The chlorine atom, picks up the electron since it is more electronegative, and
becomes more negatively charged.
Now, a very old saying takes over: opposites attract. So here, the atoms are
drawn towards one another by their positive and negative charges. If there are
enough atoms, we can eventually wind up with something as big as a single salt
grain.
Ionic
This kind of bond is called an ionic bond. It is a very strong attraction between
positive and negative ions. Most people differentiate this from a covalent bond
by saying that in ionic bonds electrons are exchanged instead of shared.
Ionic bonds are very strong – stronger than covalent bonds. And you can easily
see this difference... Melting sugar – which is breaking a covalent bond -- is
simple and makes caramel. Melting table salt is very difficult, and requires
temperatures higher than 400 degrees Centigrade. Ionic bonds are very stable
bonds.
Hydrogen
Very important and much weaker than either ionic or covalent is the hydrogen
bond. When a hydrogen atom connects to a small, strongly electronegative atom
like fluorine, oxygen, or nitrogen – the resulting molecule pulls the electrons so
strongly to the larger atom that the hydrogen nucleus – its one proton – is
essentially exposed on one side. So it will bond with the negative side of any
similarly polar molecules. This bond is called a hydrogen bond.
DNA
The difference in strength of the different types of bonds is fundamental to
allowing the DNA molecule to replicate itself. The bonds that hold the opposite
sides of the double-helix together -- the A’s and C’s and G’s and T’s -- are
hydrogen bonds and are very weak compared to the bonds that hold each letter
to the backbone and the bonds that that hold the backbone together. This allows
the molecule to unzip easily to expose one side as a template for replication. Life
is all about bonds – chemical bonds…who knew?