smells unit investigation iv: molecules in action

Smells Unit

Investigation IV: Molecules in Action

Lesson 4: Polar Bears and PenguinsLesson 5: Thinking (Electro)NegativelyLesson 6: I Can Relate

Smells Unit – Investigation IV

Lesson 4:Polar Bears and Penguins

Unit 2 • Investigation IV

ChemCatalyst

Consider the following illustration:

(cont.)

• Draw the Lewis dot structure for HCl.• 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?

• Would HCl be attracted to the charge wand? Explain your thinking.

(cont.)

The Big Question

• How can we explain partial charges on molecules?

You will be able to:

• Recognize and explain a polar 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 these topics and will introduce you to the concept of electronegativity, which helps us to understand partial charges.

Making Sense

• What does electronegativity have to do with polarity?

• This tendency of an atom to attract electrons shared between two atoms is called electronegativity.

• An atom that strongly attracts the shared electrons is considered highly electronegative. The atom with lower electronegativity will end up with a partial positive charge on it. The result is a polar bond.

• Chemists have a specific name for a molecule that has two poles—it is called a dipole. ("Di" means two.)

Notes (cont.)

(cont.)

• This illustration also uses a crossed arrow to show the direction of the dipole in HCl. The crossed end of the arrow indicates the positive (+) end of the polar bond and the arrow points in the direction of the negative (-) end.

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• Polar molecules are also called dipoles.The prefix di- means two. A dipole is a molecule with two partially charged ends, or poles. Chemists refer to polar molecules as dipoles and they also say that molecules with polar bonds havedipoles. This nomenclature can be a bit confusing with two related meanings for two closely-related meanings for the same word.

Notes (cont.)

• Nonpolar covalent bonds are the only bonds in which the electrons are truly shared equally.

• If the electronegativities between two atoms are even slightly different, they form what is called a polar covalent bond.

• 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.

Notes (cont.)

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.

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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 nonpolarcovalent, polar covalent, and ionic as the difference in electronegativity between the two atoms in the bond increases.

(cont.)

Lesson 5:Thinking (Electro)Negatively

ChemCatalyst • Explain how the illustration and the

table might relate to each other.

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The Big Question

• How does electronegativity relate to polarity and bonding?

• Determine whether a bond is polar, nonpolar, or ionic.

Activity

Purpose: This lesson explores electronegativity in a quantitative fashion—that is, it applies numbers to our investigation of polarity. Using the electronegativity scale it is possible to compare atoms and find out which ones will attract electrons more strongly in a bond.

(cont.)

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(cont.)

Making Sense

• Explain how you would determine both the direction and degree of polarity of a bond between two different atoms using the electronegativity scale.

(cont.)

• A molecule made from only two atoms is called a diatomic molecule.

(cont.)

• Nonpolar covalent bonds are really the only true covalent bonds.

• If the electronegativities between two atoms are even slightly different they form what is called a polar covalent bond. In polar covalent bonds the bonding electrons are located closer to the more electronegative atom.

Notes (cont.)

(cont.)

• 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.

Notes (cont.)

Check-In

• To what degree do the K and Cl atoms in KCl, potassium chloride, share electrons? Is the bond in potassium chloride nonpolar, polar, or ionic? Explain your thinking.

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.

Lesson 6:I Can Relate

ChemCatalyst

• HCl (hydrogen chloride) and NH3(ammonia) smell and they dissolve easily in water. O2, N2, and CH4(oxygen, nitrogen, and methane) do not smell and they do not dissolve easily in water. How can you explain these differences?

The Big Question

• How does polarity help to explain what is happening between smell molecules and the nose?

• Use polarity to predict whether a molecule will have a smell.

• Water molecules in liquid water orient so that the H atom from one water molecule is pointed toward the O atom of another water molecule. This type of interaction with an H atom between two electronegative atoms on two different molecules is called a hydrogen bond.

Activity

Purpose: The goal of this lesson is to give you practice in determining the polarity of small molecules with more than two atoms. In addition, you will explore how polarity and electronegativity relate back to smell.

(cont.)

CH4OCClF3CH2O

CF4H2SNH3HCl

(cont.)

YesAsH3YesH2SeNoCO2NoCH4

YesHBrYesPH3

YesNH3NoN2

Has a smell?MoleculeHas a smell?Molecule

(cont.)

Making Sense

• Explain why polarity might be an important concept to understand when trying to figure out the chemistry of smell.

(cont.)

Nonpolar molecules (cont.)

Notes (cont.)

Polar molecules(cont.)

Notes (cont.)

• Polar molecules tend to dissolve more easily in other polar molecules because of the intermolecular forcesbetween the molecules. Nonpolarmolecules tend not to dissolve in polar substances.

Notes (cont.)

Check-In

• Due to differences in electronegativity, we expect HCN, hydrogen cyanide, to be polar. Since water is polar as well, which way do you think water and hydrogen cyanide molecules would orient with each other? Explain your reasoning.

(cont.)

• Do you think HCN will have a smell? Explain.

(cont.)

Wrap-Up

• Differences in electronegativity values can be used to determine the direction of the dipole for an entire molecule.

• The polarity or nonpolarity of a molecule is responsible for a great many of its observable properties.

• Small polar molecules smell. Small nonpolar molecules do not smell.

smells unit investigation iv: molecules in action

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