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