Chemical bonding
LA Charter School Science Partnership
10 Dec 2011
Nick Klein
Today’s Talk
• Intro & Biography
• Part 1: Chemical bonding on an atomic level
• Part 2: Lewis dot structures, ionic & metallic
bonding
• Part 3: Covalent bonding
Today’s Talk cont’d
• I’ve split this talk into three (roughly) even
parts. We will take a brief (1-2min) break
between sections. Please feel free to take
notes, handle the props, think of questions
for after the talk, etc.
About Me
• Third year PhD student in
Earth Sciences
• BA Biology and Chemistry
from Augustana College, SD
• I study global impacts of
marine trace chemistry
• Thesis research: production
of marine halocarbons
Field sampling on Lake Tahoe
My new toy!
Part 1: Bonding on an atomic level
• What is chemical bonding?
– Let’s define it
• A chemical bond is a strong attraction
between two or more atoms
– Electrons are given/taken or shared
– Can be ionic, metallic, or covalent in character
– When the chemical bonds in a molecule are
broken or rearranged, this is called a
chemical reaction
Part 1: Bonding on an atomic level
• Atoms are composed of a
nucleus and orbiting
electrons
• Nucleus contains protons
and neutrons
• Electrons are negatively
charged, protons positive.
Neutrons are not charged
(neutral).
Part 1: Bonding on an atomic level
• The story of bonding is the story of
electrons
• How electrons are exchanged determines
the type of bonds formed
• No matter how many electrons an atom
has, only the very outermost participate in
bonding; these are called valence
electrons
Part 1: Bonding on an atomic level
• You can easily determine the number of
valence electrons an element has by
counting the number of columns in the
periodic table from left to right (with one big
exception!)
Part 1: Bonding on an atomic level
8
1
2
Things get weird. Skip these.
3 4 5
The number of valence electrons
varies and there’s no real pattern.
6
7
Part 1: Bonding on an atomic level
• Each column is called a period. Elements
within the same period (column) have the
same number of valence electrons and
have similar chemical characteristics!
• When elements form chemical bonds, they
want to have a full valence of electrons to
be stable and happy. The magic number is
usually 8 – the Octet Rule
Part 1: Bonding on an atomic level
8
1
2
3 4 5
6
7
Break!
Part 2: Lewis Dot Structures
• Scientists write the number of valence
electrons using Lewis dot structures
• We write the symbol for the element and
then add a dot for each valence electron
• Let’s try a couple examples!
Part 2: Lewis Dot Structures
Part 2: Lewis Dot Structures
• Potassium is in period 1 and has 1
valence electron
K
Part 2: Lewis Dot Structures
• Oxygen has 6 valence electrons. This
gets a little trickier, though. There are four
orbitals and room for two electrons in
each... but same-charges repel!
O
Part 2: Lewis Dot Structures
• How might these two elements bind
chemically? Is K more likely to want to
take 7 electrons from another atom, or
give away 1? Is O more likely to take 2
electrons or give away 6?
• It’s also useful to look at electronegativity
- a measure of how badly an element
wants electrons (how “greedy” it is)
Part 2: Lewis Dot Structures
Part 2: Ionic bonding
• Oxygen has an electronegativity of around
3, which is one of the highest, and
potassium is maybe 0.5
• O wants electrons very badly, K wants to
get rid of them!
• They will undergo ionic bonding – a type
of chemical bonding where electrons are
completely removed from one atom and
given to another, meaning the atoms have
an electrical charge and are called ions
Part 2: Ionic bonding
+1
K
+1
K
O
-2
Part 2: Ionic bonding
• Even though these ions are not held tightly
to each other, opposites attract and many
ionic compounds form very orderly
arrangements of ions called crystals
Part 2: Ionic bonding
K2O
Potassium oxide
Part 2: Ionic bonding
Part 2: Ionic bonding
+1
Na
Cl
-1
Part 2: Ionic bonding
NaCl
Sodium chloride
(table salt)
Part 2: Ionic bonding
Part 2: Ionic bonding
+2
Ca
F
F
-1
-1
Part 2: Ionic bonding
CaF2
Calcium fluoride
Part 2: Ionic bonding
• Ionic compounds are very hard and brittle
with high melting points
• They are poor conductors of heat and
electricity as solids, but good at
conducting electricity when dissolved in a
liquid
• Most are very soluble in water (like
dissolves like!)
Part 2: Metallic bonding
• The transition metals (the
middle group that we
skipped when talking
about valence!) can
participate in a special
form of bonding called
metallic bonding
• In metallic bonds, atoms share electrons with
all neighboring atoms and electrons are able to
flow freely throughout the material
Part 2: Metallic bonding
• Metallic bonding produces compounds
that are usually shiny, hard, very good
conductors of heat and electricity
• Ductile (can be drawn into wires) and
malleable
Break!
Part 3: Covalent bonding
• This is where things get complicated!
• We’ve looked at ionic bonding, where
electrons are given or taken
• In covalent bonding (think co-valence),
two electrons are shared, creating a tight
bond between two atoms
• Carbon especially likes to form covalent
bonds. Why might this be?
Part 3: Covalent bonding
Part 3: Covalent bonding
Part 3: Covalent bonding
• Carbon has 4 valence electrons. It could
give up 4... or it could lose 4. Or it could
share!
• Carbon doesn’t want electrons nearly as
badly as oxygen or nitrogen
• Carbon can actually form up to four
covalent bonds, which can lead to very
large and complex molecules... the basis
of all life!
Part 3: Covalent bonding
O
C
O
Part 3: Covalent bonding
O
C
O
Each line represents two electrons being shared
Part 3: Covalent bonding
• Why is water a “bent” molecule?
O
H
H
Part 3: Covalent bonding
• Why is water a “bent” molecule?
O
H
H
think 3D!
Part 3: Covalent bonding
• Think back to the last workshop. What
causes a molecule to be polar or
nonpolar?
• A bond can be more or less polar, too.
• Look back at CO2 for an example
Part 3: Covalent bonding
-
O
+
-
C
O
Part 3: Covalent bonding
• The bonds in CO2 are polar since oxygen
is more electronegative
• However, the molecule is not polar overall
since it is linear, and one end of it is not
more negative or positive than the other
• The carbon is “buried”
• Water has polar bonds and is polar overall
because it is “bent”
Part 3: Covalent bonding
• Often when chemists write the structure of
an organic compound (one containing
carbon), we save time and space by not
writing in the letters for carbon and
hydrogen, since we know those two
elements form the backbone of all organic
molecules
• This is called a skeletal formula
Part 3: Covalent bonding
propane
Part 3: Covalent bonding
acetic acid (vinegar)
Part 3: Covalent bonding
• When elements other than just carbon and
hydrogen are in an organic molecule,
these give the molecule special properties
and are called functional groups
• You’re not expected to know the functional
groups, but as we look at some of them,
think about the bonding and polarity of the
bonds. You’ll build many of them in the
activity!
Part 3: Covalent bonding
R
OH
Isopropyl alcohol (rubbing alcohol)
The “R” means “any chain of carbons.” Alcohols
are good at making other molecules inert and damage
proteins, so are often used as disinfectants.
Part 3: Covalent bonding
O
R
H
formaldeyhyde
Aldehydes are generally very fragrant. Many perfume
and other fragrances contain aldehydes.
Part 3: Covalent bonding
O
R
R
acetone (nail polish remover)
Ketones are important in many biological molecules.
They hydrogen bond like water and are part of many sugars.
Part 3: Covalent bonding
O
R
OH
acetic acid (vinegar)
The “R” means “any chain of carbons.” Organic acids
are weak acids, they usually taste sour.
Part 3: Covalent bonding
O
R
OR
butyl butyrate (pineapples)
Esters usually have very “fruity” odors and tastes.
They are present in many foods, especially fruits.
Part 3: Covalent bonding
R
NH2
Serine (an amino acid)
Amines are basic and very important in biology (think amino acids).
Gaseous amines smell like ammonia, liquid amines smell fishy.
Part 3: Covalent bonding
• Now you should be able to look at almost
any formula and build that molecule with
your models! You should also be able to
make some guesses about the functional
groups present and how that might affect
the chemical properties of the molecule.
Part 3: Covalent bonding
Questions?