QOTD 1/29/14Please 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 COVALENTPOLAR COVALENTIONIC

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

O C O

Example: CO2

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