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.