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Alchemy Unit – Investigation V

Lesson 7: Polar Bears and Penguins

ChemCatalyst

•  Describe what happens when a plastic spoon was brought near a falling stream of water.(H2O)

•  Describe what happens when a plastic spoon was brought near a falling stream of hexane. (C6H12)

•  What do you think caused the observed difference in behavior?

•  What kind of bonding holds water atoms together> Hexane atoms? Explain.

Unit 2 • Investigation IV


ChemCatalyst Consider the following illustration:

(cont.)


• If the penguin represents a hydrogen atom and the polar bear represents a chlorine atom, what does the ice cream represent in the drawing? What do you think the picture is trying to illustrate?

(cont.)


The Big Question

• How can we explain why some pairs of atoms will form a covalent bond while other will form an ionic bond?


You will be able to:

• Recognize and explain why atoms forms a polar bond versus an ionic bond.


Activity

Purpose: In this lesson you will be exploring polarity and bonding between atoms in greater detail. A comic book will provide new information about valence electrons in bonding and will introduce you to the concept of electronegativity. Electronegativity helps us understand why some molecules stick to each other and why some substances forms ions and some don’t.


Making Sense

• The greater the difference in electronegativity between two atoms sharing electrons, the more likely their bond will be ionic.

• The closer the difference in the electronegativity, the more likely the bond will be (non-polar) covalent?

Notes

•  Electronegativity measures the tendency of an atom to attract the electrons in a chemical bond

•  It is a value that is calculated from several different measurements and cannot be measures directly.


Electronegativity scale


Electronegativity scale


Electronegativity is a measure of how well two atoms attract shared electrons to themselves. The difference between them represents the relative “strengths” of each atom. When you calculate the difference, you can get a rough indication of the kind of bond they will have.


• Nonpolar covalent bonds are the only bonds in which the electrons are truly shared equally. (difference < 0.5)

• If the electronegativities between two atoms are even slightly different, they form what is called a polar covalent bond. (0.5 ≤ difference < 2.1)

• When the electronegativities between two atoms are greatly different, the bond is called an ionic bond. In the case of an ionic bond the electron of one atom is completely given up to the other atom. (difference ≥ 2.1)

Notes (cont.)

Using the electronegativity scale Calculate the difference in electronegativity. Then decide what kind of bond the following combination of atoms will make.


Elements Difference in electronegativity

Type of bond

Sodium and fluorine

Sulfur and oxygen

Carbon and bromine

Phosphorus and oxygen

3.05 ionic

0.86 Polar covalent

0.41 Non-polar covalent

1.25 Polar covalent


• When one of the two atoms share electrons in a bond, the atom that is more electronegative …

•  will attract the shared electrons a greater percentage of time to itself.

• This will give that atom a partial negative charge.

Notes

This symbol means this end of the molecule has a “partially negative charge”


• The atom with lower electronegativity will end up with a partial positive charge on it.

• That means one end of the bond is partially negative and the other end is partially positive.

• The bond forms a dipole (two poles), resulting in a polar bond.

Notes


• This illustration uses a crossed arrow to show the direction of the dipole formed by the molecule of HCl .

• The crossed end of the arrow indicates the positive (+) end of the polar bond

•  The arrow points in the direction of the negative (-) end.

Notes


• Imagine that the Chlorine atoms is the polar bear dragging the penguin (hydrogen atom) to the left.

• The bear-penguin “molecule” moves together. • That makes the molecule of HCl also a dipole.

Notes


• Chemists refer to polar molecules as dipoles and they also say that molecules with polar bonds have dipoles.

• These two definitions can be a bit confusing with two related meanings for two closely-related meanings for the same word – one for bonds and the other for the entire molecule.

• Molecules that form a dipole (polar molecule) must have polar bonds (dipole bonds) but …

•  just because a molecule has dipole bonds does not mean that the molecule will form a dipole. Why?

Notes (cont.)


O C O

Notes (cont.)

•  What does this diagram suggest about the bond between carbon and oxygen?

•  What does the diagram suggest about the how polar this molecule is? (think: 2 bears and a penguin)


Check-In • Is the bond between these atoms

polar? Explain your reasoning. • How would the atoms be portrayed in

the comic book—as polar bears, penguins, or both? Explain.


Wrap-Up

• Anytime there are two different types of atoms sharing electrons, there will be a partial negative charge on one atom and a partial positive charge on the other atom.

• Electronegativity measures the tendency of an atom to attract the electrons in a bond.

(cont.)


• The bonds are labeled nonpolar covalent, polar covalent, and ionic as the difference in electronegativity between the two atoms in the bond increases.

(cont.)

alchemy unit – investigation v lesson 7: polar bears … · alchemy unit – investigation v...

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