Hi everyone I'm Tiffany. Welcome to your lesson on polarity! By the end of today's lesson, you
will compare polar and nonpolar bonds, demonstrate how polar bonds affect the polarity of a molecule,
and compare the attractions between polar and nonpolar molecules.
Think about two atoms as if they are two people arm wrestling and their arms are covalent bonds.
As they arm wrestle, we would observe that their two hands spend most of the
time wavering somewhere in the middle or even slightly to the left or the right;
however, if one person or atom is significantly stronger, think Hercules, then Hercules would
pull his fist inward towards his own body as well as his opponent's hand down towards the table.
That is there would be a significant deviation from the mean position over the
middle of the table. Hercules would stretch the other person's arm, or covalent bond,
pulling their fist or shared electron closer to him. This is exactly what happens in some
covalent bonds which as you recall are chemical bonds between non-metal atoms that share valence
electrons. All non-metals have relatively high electronegativities. Electronegativity is the
tendency of an atom participating in a covalent bond to attract the bonding electrons. Therefore,
when they bond together the difference in their electronegativities is small.
A bond in which the electronegativity difference between the atoms is between 0.4 and 1.7
is considered a polar covalent bond. Atoms that are bonded with an electronegativity
difference of 0 to 0.3 are generally considered to be nonpolar covalent bonds.
Generally the smaller the difference, the more covalent character they will have. In general,
elements on the right side of the periodic table have a greater attraction for electrons than
elements on the left side of the table, except for noble gases. Typically the elements at the
top of a group have a greater attraction for electrons than elements at the bottom of a group.
For example, fluorine is on the far right and it is at the top of the halogen group,
thus it has the strongest attraction for electrons and it's the most non-reactive non-metal.
Take a look at hydrogen chloride. The compound that forms when hydrogen reacts with chlorine.
A chlorine atom has a greater attraction for electrons than the hydrogen atom. In a
hydrogen chloride molecule, the shared electrons are more attracted to the chlorine atom than the
hydrogen atom. The electrons in this covalent bond are not shared equally and are therefore
called polar covalent bonds. You can remember this because one meaning of the term polar is
opposite in character or direction, just like the north and south poles of a bar magnet
and earth's north and south magnetic poles are opposites in terms of their magnetic fields.
Some covalent bonds and chemical compounds have poles as well. But in the case of chemical bonds
and compounds, the poles are opposites in terms of their electric charge. These bonds
and compounds are described as polar. When atoms form a polar covalent bond,
the atom with the greater attraction for electrons has a partial negative charge and the other atom
has a partial positive charge. The symbol delta negative (δ-) and delta positive (δ+) are used
to show which atom has which charge. Delta is the lower case version of the Greek letter delta. In
other covalent bonds electrons are shared equally. These bonds are called nonpolar covalent bonds,
neither atom attracts the shared electrons more strongly. As a result, the atoms are happy
and remain neutral in charge. For example, the diatomic oxygen molecule has two nonpolar bonds.
The two oxygen nuclei have an equal force of attraction for their 4
shared electrons. Let me ask you a question. If a molecule has a polar covalent bond,
can you assume the molecule itself is polar too? Feel free to pause the video, scratch your head,
and think about this one! The answer to that question is a layered one. It turns out that
if a molecule has only 2 atoms, it will be polar much like our previous example of HCl,
however when molecules have more than 2 atoms, the answer is not as obvious.
The type of atoms in a molecule and its shape determine whether a molecule is polar or nonpolar.
Let's take a look at water and carbon dioxide. In carbon dioxide there are double bonds between each
oxygen atom and the central carbon atom. Because oxygen has a greater attraction for electrons than
carbon does, each double bond is polar; however, the molecule is linear which means that all 3
atoms are lined up in a row. The carbon-oxygen double bonds are directly opposite each other
and there is an equal pull on the electrons from opposite directions. The pulls cancel out
and the molecule as a whole is nonpolar. A water molecule on the other hand has two single bonds.
The bonds are polar because oxygen has a greater attraction for electrons than hydrogen. Because
the water molecule has a bent rather than linear shape, the polar bonds do not cancel out.
The two hydrogen atoms are located on the same side of the molecule opposite the oxygen atom.
The oxygen side of the molecule has a partial negative charge
and the hydrogen side of the molecule has a partial positive charge. Thus far, we've
learned about intramolecular forces which are the forces that hold atoms together within a molecule.
Let's now take that information, build upon it, and begin to explore some intermolecular
compounds which are forces that exist between molecules. In a covalent compound, also known as
a molecular compound, we see a variety of forces of attraction between molecules. These attractions
are not as strong as ionic or covalent bonds, but they are strong enough to hold molecules together
in a liquid or solid. It's important to note that the forces of attraction between polar molecules
are stronger than the attractions between non-polar molecules. Let's dive deeper
into this concept and get comfortable with it. Consider a water molecule and a methane molecule.
Water, H2O, is similar in mass to methane CH4; however, methane boils at -161.5°C water boils
at 100°C because methane molecules are nonpolar and water molecules are polar. Each dashed line
represents an attraction between a partially positive hydrogen atom in one water molecule
and a partially negative oxygen atom in another. Think of a glass of water. The water molecules
on the surface of the water are attracted to the molecules that lie below the surface
and are therefore pulled toward the center of the sample. These attractions increase the energy
required for water molecules to evaporate or enter the gaseous phase. They raise the temperature at
which vapor pressure equals atmospheric pressure which is also known as the boiling point. Although
the attractions among non-polar molecules are weaker than the attractions among polar molecules,
they still exist. These types of attractions explain why carbon dioxide can exist as a
solid dry ice and why nitrogen can be stored as a liquid at low temperatures and high pressures.
Because electrons are constantly in motion there are times when one part of a nitrogen
molecule has a small positive charge while another part has a small negative charge.
During those times, one nitrogen molecule may be weakly attracted to another nitrogen molecule.
Well that wraps up our lesson on polarity! If you were following along, you now know that when
atoms form a polar covalent bond the atom with a greater attraction 4 electrons has a partial
negative charge and the other atom has a partial positive charge. The type of atoms in a molecule
and its shape determine whether a molecule is a polar or nonpolar molecule. The forces of
attraction between polar molecules are stronger than the attractions between nonpolar molecules.
I look forward to our next lesson and remember in science as in life, you matter!