The bonds that hold atoms together in compounds
are called intramolecular forces.
The 3 main types of intramolecular forces
are ionic bonds, covalent bonds, and metallic
bonds.
This video will focus on covalent bonds: Ionic
bonds and Metallic bonds will be featured
in their own videos.
Covalent bonds are stable because the bonding
atoms achieve noble gas configuration by sharing
electrons.
The name, covalent, should suggest to you
that the atoms are sharing their valence electrons.
We can show this with a Lewis dot diagram.
Hydrogen fluoride (HF) is a molecule with
a single covalent bond formed between two
atoms.
Fluorine has 7 valence electrons, and hydrogen
has one.
By sharing 2 electrons in a bond, now hydrogen
has 2 valence electrons, and has the same
electron configuration as the noble gas helium.
Fluorine now has 8 valence electrons, and
has the same electron configuration as the
noble gas neon.
We can replace those 2 shared electrons in
the diagram by a single line, representing
the single covalent bond.
Sometimes, two atoms share more than 2 electrons,
in the case of a double or triple covalent
bond.
We can see an example of that in carbon dioxide,
CO2.
The Lewis dot structure looks like this: Carbon
has 4 valence electrons, and Oxygen has 6
valence electrons.
Carbon needs 4 more electrons to achieve noble
gas configuration.
Oxygen needs 2 more electrons to achieve noble
gas configuration.
This can be achieved if the carbon atom forms
2 double bonds with each oxygen atom.
We can replace the two shared pairs of electrons
in the diagram with 2 straight lines, representing
a double bond.
If the two atoms in a covalent bond are identical,
they have the exact same electronegativity
as each other.
(Click here to learn more about electronegativity).
The bond between these identical atoms is
called a non-polar covalent bond.
Hydrogen, for instance, exists in nature as
a diatomic molecule, H2.
The two hydrogen atoms pull equally on the
shared pair of electrons in the bond, so there
is no directionality, or POLARITY, of the
bond.
Compare that with the bonds in a polar molecule,
like water, H2O.
Oxygen is much more electronegative than hydrogen,
so the electrons in the covalent bonds spend
more time around the oxygen than around the
hydrogen.
We call this kind of uneven sharing of electrons
a polar covalent bond.
Notice that this results in the water molecule
being polar as a whole - one side of the molecule
is more negative than the other side.
A lowercase delta is used to show the partial
negative charge on the oxygen atom and the
partial positive charge on the hydrogen atoms.
We use this delta notation to distinguish
these partial charges from the full charges
carried by ions.
You might get confused between molecules which
contain polar covalent bonds and molecules
which are polar as a whole.
Water is both - it contains polar bonds, and
is a polar molecule (as a whole) because one
end of the molecule is slightly positive and
the other side is slightly negative.
That’s a result of the polar covalent bonds
that hold the water molecule together.
But consider the carbon tetrachloride molecule,
CCl4.
Chlorine is more electronegative than carbon,
so this molecule has 4 polar covalent bonds.
You might think, adding the 4 bonds together,
this molecule is going to be VERY polar as
a result.
But actually, when you look at the 3 dimensional
structure, you see that the 4 bonds point
in 4 opposite directions, so they cancel each
other out.
You can’t find one SIDE of CCl4 that is
more negative or positive than the other,
so carbon tetrachloride as a whole is a nonpolar
molecule.
Chemists generally measure the polarity of
a bond according to a scale established by
Linus Pauling {show table of values}.
If the relative electronegativities of the
two bonded atoms differ by less than 0.4 on
the Pauling scale, the bond is considered
nonpolar covalent.
If the difference in relative electronegativities
is between 0.4 and 1.7, we call it a polar
covalent bond.
And if the electronegativities differ by more
than 1.7, it’s an ionic bond.
Are covalent bonds, like many ionic bonds,
disrupted by water?
Some are, some are not.
For instance, Sucrose, C12H22O11 (that’s
table sugar), is a molecule with atoms held
together by covalent bonds.
If you put sucrose, or other sugars in water,
the covalent bonds stay intact and a sugar-water
solution does not conduct electricity as well
as a salt-water solution.
Acids, on the other hand, like HCl, hydrochloric
acid, are covalent compounds which readily
dissociate into H+ and Cl- ions, so they DO
conduct electricity.
We call these substances that ionize when
they dissolve “electrolytes.”
Most soluble salts, acids, and bases act this
way.
Even though some covalent bonds can come apart
in water, they are considered strong bonds,
as are ionic bonds.
We’ll compare their relative strengths in
another video.
