Transcript 5.3 Covalent Bonds

5.3 Covalent Bonds
• Key Concepts:
• What holds covalently bonded atoms together?
• What are the properties of molecular
compounds?
• How does unequal sharing of electrons occur, and
how does it affect molecules?
• Key terms:
– Covalent bond, molecule, double bond, triple bond,
molecular compound, polar bond, nonpolar bond
How Covalent Bonds form
• Covalent bond – the chemical bond created
when two atoms share electrons
• Electron sharing
– The force that holds atoms together in a covalent
bond is the attraction of each atom’s nucleus for
the shared pair of electrons.
– (molecule- neutral group of atoms joined by
covalent bonds)
Wait
• What’s the diff. between a covalent and an
ionic bond?
• In an ionic bond, atoms are attracted because
of ions’ opposite charges. Ions formed
because atoms lost or gained electrons.
• In a covalent bond, atoms are held together
because of shared electrons.
How many bonds?
• With few exceptions (like Hydrogen and
Helium), the number of covalent bonds that
nonmetal atoms can form equals the number
of electrons needed to make a total of 8.
(Hydrogen and Helium only need 2)
• Just like before, but now they’re sharing. Like
friends in kindergarten.
It’s like dis…
• Oxygen has six valence electrons.
• How do we know that? Well, it’s in group 16.
• So, it looks like….
It’s like dis…
• Hydrogen needs 2 (as we mentioned before)
valence electrons.
• It’s in group 1, so how many does it have? It
has…. Drum roll.. 1 electron.
• It looks like a dis….
So
• Oxygen would like 8 to be stable, and
Hydrogen has 1 and would like 2 to be stable.
• How would they come together in nature,
when both are present?
• Oxygen needs 2 more valence electrons to be
stable, so it bonds with two hydrogen atoms.
The hydrogen atoms are happy with 2 shared
electrons, and the oxygen atoms are happy
with 8 electrons. Everyone’s happy.
Happy because his electrons are happy
What about other stuff?
• Well, Carbon, for example, is in group 14.
• So how many valence electrons does it have?
• Oh well uh, 4. It’s in group 14 so it has 4. Tada.
• Nitrogen is in group 15 so it has… 5. Boom.
• Look at Nitrogen. It has 5 valence electrons.
How many covalent bonds can it form?
• Look at Boron, it has 3 valence electrons. It is
in group 13. How many covalent bonds can it
form?
Wait a minute!
• What’s that weird stuff in ethylene and
acetylene?
• That weird double set of electrons?
• It’s called a “double bond” – when atoms share
two pairs of electrons.
• A “triple bond” is when atoms share 3 pairs of
electrons.
Example of double bonds
Molecular compounds
• A molecular compound is a compound that is
composed of molecules. (LOL – this is a definition
from your book, you’re welcome)
• The atoms in a molecular compound are
covalently bonded (meaning they share
electrons). Molecular compounds usually have
low melting points and boiling points and they do
not conduct electricity when dissolved in water.
Why?
• Because the forces between molecules are
much weatker than the forces between ions in
an ionic solid. So less heat must be added to
molecular solids to separate the molecules.
Also, molecular solids
• Are poor conductors.
• Most molecular compounds do not conduct
electrical current
Unequal Sharing of electrons
• Atoms of some elements pull more strongly
on shared electrons than do atoms of other
elements. As a result, the electrons are pulled
more toward one atom, causing the bonded
atoms to have slight electrical charges.
Polar Bonds and nonpolar bonds
• The unequal sharing of electrons is enough to
make the atom with the stronger pull slightly
negative and the atom with the weaker pull
slightly positive.
• A covalent bond in which electrons are
shared unequally is called a polar bond.
• If they are pulled equally, it is a nonpolar
bond.
Polar Bonds in molecules
• A molecule may contain polar bonds and still
be nonpolar overall.
Attractions
• There is little attraction between nonpolar
molecules.
• The properties of polar and nonpolar
compounds differ because of their differences
in attractions.