QOTD 1/29/14
Please get out your homework (p. 57-58) to be checked off.
Then answer the following question:
A stream of liquid X is falling straight down. A positive
charge attracts the stream.
1. Is liquid X polar or nonpolar?
2. Will a NEGATIVE charge attract liquid X? Why or why
not?
HW Questions?
• Please pick up a HW answer key and a
notes sheet.
• Go over your HW and ask questions if you
have them!
• TYPO ON PRELAB Q 1C! It should be
“no attraction”
Today’s Learning Target
• Be able to use electronegativity to
determine if a bond is polar covalent,
nonpolar covalent, or ionic.
Please read the comic strip on
pages 62-65 and answer
questions 1-12 on page 60!
(it helps if you read half way
and then start answering )
The Bare Essentials of Polarity
1. How does the comic book
define a “polar molecule”?
A molecule with a difference in electrical
charge between two ends.
2. Define electronegativity as you
understand it, after reading the 1st
2 pages of the comic book.
Electronegativity is when a “greedy” atom
attracts the electron cloud of the atom it has
bonded with toward itself.
3. Interpret the picture at the bottom of the
first page. Explain how the iceberg,
penguins & polar bears represent trends in
electronegativity.
Penguins do not attract bonded
electrons toward themselves (they
have a low electronegativity) and polar
bears do (have a higher
electronegativity).
4. What is the artist trying to represent
when there are two polar bears arm
wrestling together, or two penguins arm
wrestling together?
Bonded atoms with equal
electron-attracting strength will
have non-polar bonds (it’s a
tie!).
5. What three types of bonds are represented on the third
page of the comic book? What happens to the bonding
electrons in each type of bond?
1. Nonpolar covalent (the bonded electrons are
shared equally).
2. Polar covalent (the bonded electrons are
more attracted to one atom).
3. Ionic (The more electronegative atom seizes
all the bonding electrons and becomes a
negative ion, while the other atom becomes
positive).
6. Explain why there are four scoops of ice
cream in the illustration of O2 on page 3 of
the comic strip.
Oxygen has a double bond
(4 electrons are being
shared).
7. What do the 6 scoops of ice
cream represent in the illustration
of N2 on page 4 of the comic strip?
A triple bond (6
electrons being shared
by the nitrogen atoms).
8. Describe what you think is happening to
the penguin in the CO2 molecule in the
picture on page 4.
The penguin is being pulled in 2
directions equally (and so this
bond is nonpolar covalent).
9. Name 3 things that the picture of CO2 on
page 4 illustrates about the molecule.
It is linear, it has 2 very
electronegative atoms
(Oxygens) and the polar
bonds cancel out (so it is a
nonpolar bond).
10. Describe what you think is happening
to the penguins in the illustration of H2O on
page 4?
The oxygen is stealing
electron cloud from the
hydrogens (“winning!”)
11. Explain what you think the
crossed arrow represents in the
comic book.
It represents a pull
from one atom on
another’s electron
cloud.
12. What are the two definitions of
“dipole” given in the comic book?
A polar bond, a polar
molecule.
Notes time!
Please pick up notes sheet!
Who is going to win this game of tug-of-war?
How can we determine which atom will
get the shared electrons the most?
Electronegativity
The ability of an atom, while in a bond, to attract
electrons to itself.
***Basically, a measure of how hard the atom
“pulls” on the shared electrons (ice cream)
Increasing electronegativity
Most electronegative element = Fluorine
(F)
We ignore the noble gases because they don’t form bonds.
They already have an octet, so they never share electrons with
other atoms
Electronegativity is measured using an arbitrary scale of
0-4,
with 4 being the highest electronegativity (strongest attraction
for the electrons)
Using Electronegativity
• We can use electronegativity to determine
the type of bond formed:
NONPOLAR COVALENT
POLAR COVALENT
IONIC
NonPolar Covalent Bonds
• Electrons are shared
equally
• The two atoms have equal “pull” on the shared
electrons/ice cream
• The atoms have equal electronegativities
NonPolar Covalent Bonds
• Difference in electronegativity is between
0 and 0.5
– example: P (2.19) and H (2.10). Difference =
0.09  Nonpolar bond
YOUR TURN:
Identify 3 different combinations of atoms
that would be nonpolar using the table on
PAGE 53 OF YOUR PACKET.
Give 3 sets of atoms that will
make a nonpolar covalent bond
Polar Covalent Bonds
• Electrons are
shared unequally
• One atom “pulls” harder on the electrons than the
other
• Negative side = side with greater “pull” (larger
electronegativity)
• Positive side = side with weaker “pull” (smaller
electronegativity)
Polar Covalent Bonds
Difference in electronegativities is between
0.5 and 2.1
– Example: C (2.55) and Cl (3.16). Difference
= 0.61  POLAR BOND
YOUR TURN:
Identify 3 different combinations of atoms that
would form a polar bond.
Give 3 sets of atoms that will
make a polar covalent bond
Dipoles
• Polar molecules are also called dipoles
• Dipole: a molecule with two partially
charged ends, or poles
• Polar bonds are also referred to as dipoles
or having a “dipole moment”
Ionic Bond
• One atom takes electron(s) away from the
other atom (NO sharing)
– this happens when electronegativity
difference is very large!
• What holds the ions together is the +/attraction
Forming Ions
• Metals have low electronegativity, so they
have less of a “grip” on their electrons. They
tend to lose electrons to form positive ions
(cations).
– Non-metals are the opposite- they have
high electronegativities, they pull harder
on their electrons, and are more likely to
take electrons from other atoms, forming
negative ions (anions)
Ionic Bonds
• Difference in electronegativities is greater
than 2.1
– Example: K (0.82) and F (3.98). Difference =
3.16  IONIC BOND
YOUR TURN:
Identify 3 different combinations of atoms that
would create an ionic bond using the
electronegativity table.
Give 3 sets of atoms that will
make an ionic bond
2 ways in which a molecule can be NONpolar…
#1: the atoms in the molecule have equal
electronegativities (ie. same atom)
Or VERY close electronegativity
Example: the polar bears
are pulling with the same
amount of strength on the
ice cream
2 ways in which a molecule can be NONpolar…
#2: The pulls from the polar bonds “cancel out”
• Example: the bears are pulling equally in
opposite directions. There is no net movement
of the ice cream
Example: CO2
O
C
O
Dipoles can “cancel out”
In this case, the Oxygens are pulling equally
hard on the shared electrons from opposite
directions, so overall this is NON-POLAR
There is no net dipole:
there are no positive and negative ends
For a molecule to be
POLAR…
•There needs to be an “overall pull” of the
electrons in a certain direction
• example #1: two atoms, one atom (bear) pulls
harder than the other (penguin)
• example #2: 3 or more atoms, the individual
pulls add up to a general dipole in one direction
Shape Matters!
Wrap-Up
– Electronegativity measures how
strongly an atom will attract shared
electrons.
– The greater the difference in
electronegativity between two atoms,
the more polar the bond will be.
– In the case of an ionic bond, the
electronegativities between two atoms
are so greatly different that the
electron(s) of one atom is(are)
completely given up to the other atom.
Homework
• Answer questions #1-9 on bottom of p.70
• Answer all Qs on p. 71-72