Sticky Tape Post LabThe Role of Charge in Chemistry

Charge on a Macroscopic Scale Opposite charges attract each other.

Similar charges repel each other.

Neutral objects are attracted to all charged objects, but exert no force on other neutral objects.

Charge on the Particle Scale

All particles contain both positive and negative charges that normally cancel each other out.

An object may become charged when some of its positive or negative charges are transferred to another object.

Charge on the Particle Scale

J.J. Thomson conducted experiments in the 1890’s with cathode rays to demonstrate which charges are able to be transferred.

Charge on the Particle Scale Some interesting facts about chocolate chips . . .

Positively-charged dough attract negative chips.

Some dough is better at attracting chips than other dough.

Chips can move from one bowl to another due to these differences in attraction.

**Since chips repel each other, the number of chips that can fit in dough is limited**

Charge on the Particle Scale We’ll call this the chocolate chip model

The positive cores are represented by scoops of dough The negative charges are represented by chocolate chips.

Positive Core

Negative Charges

Charge on the Particle Scale Some interesting facts about dough . . .

ChocoChips (-) fall out of some dough. +++++ Some dough can attract extra ChocoChips. A ball of dough with extra chips

(-) has a negative charge. ChocoChips (-) can move around in one ball of dough or be transferred

between balls of dough. ChocoChips (-) are repelled by other ChocoChips (-)

Cookies vs. TapeNormal dough has the perfect # of ChocoChips to

counteract the positive dough.Some dough looses chips and becomes “chip-lite” (+)Some dough gets extra chips and becomes “chip-

heavy” (-)

Charge on the Particle Scale What are we really talking about?

Thomson’s model proposed that what we once thought of as the smallest pieces of matter are actually composed of two parts: Large, massive, positively-charged core.

Tiny negatively-charged particles spread throughout the positive core = called electrons.

The positive core is immobile, the negative electrons can move from one particle to another.

Charge on the Particle Scale Charge is not a substance.

Charge is a property of particles (cores and electrons) that determines the strength of their electrical interactions.

Charge plays the role of mass in gravitational interactions.

Charges in the Sticky Tape

Consider which tape was negatively-charged after they were separated.

The bottom tape was negative, so the smooth side of the tape must have a stronger attraction to the negative electrons.

Charges in the Sticky Tape The particle-level transfer of electrons results in an imbalance of charge in

both pieces of tape. The positive cores remain the same, the top piece now has an excess of

(+) charge, the bottom piece has an excess of (-) charge.

top(+)

bottom (-)

Before Adhesion In Contact Separated

Charges in the Sticky Tape

top(+)

bottom (-)

Before Adhesion In Contact Separated

Bottom tape has same + core but extra – electrons. Each bottom tape is now NEGATIVE!

Top tape has same + core but fewer – electrons. Each top tape is now POSTIVE!

Charge and Electrical Conductivity What type of substances conduct electricity?

Metals Some Solutions

Electrical conductivity is the result of electrons moving from one core to another when subjected to an external electrical field.

Charge and Electrical Conductivity What is it about metals that allows electrons to move from

one core to another? Metal’s cores are not strong attractors of negative charge. The energy required to move an electron from one metallic core

to another is relatively low compared to that of a nonmetal.

Part II Sticky Tape Post-Lab

Interactions Between Charged and Neutral Objects Both the top and bottom tape attracted the foil and paper.

The tape-foil interaction was stronger than the tape-paper interaction.

What is different about the behavior of the negative charges in the two substances?

Charge in a neutral substance is evenly-distributed, this is true for both the foil and the paper.

Interactions Between Charged and Neutral Objects In the foil, the negative charges can move from one core to

another. We say the charge is delocalized. When a (+) object is placed close to the foil, the negative charges

move closer to the object and the electrons are no longer evenly distributed.

Interactions Between Charged and Neutral Objects In the paper, the negative charges are bound to their

positive cores. The paper is still polarized as the negative charges move closer

to the (+) charge. The polarization is less pronounced than in the foil because the

electron distribution is not as asymmetrical.

Definitions Metals – The positive core of metals do not have a

strong attraction to the negative electrons. Electrons tend to leave metals so the metals become positive. (+)

Nonmetals – The positive core of nonmetals have a strong attraction to the negative electrons. Extra electrons get pulled towards the nonmentals so they become negative. (-)

Label periodic table.

Interactions Between Charged and Neutral Objects

Electrical conductors are more-readily polarized than nonconductors because their electrons can move from one core to another.

Conductor Nonconductor

Interactions Between Charged and Neutral Objects Sketch the charged particle diagrams for:

1. The bottom tape held in between a piece of foil and a piece of paper.

2. The top tape held in between a piece of foil and the bottom tape.

3. A piece of foil held in between the bottom tape and a piece of paper.

4. A piece of foil held between the top tape and the bottom tape.

Charges in Compounds Electrical interactions resulting from charged particles result

in the formation of compounds. Different types of particles have different cores, and therefore

different abilities to attract negative electrons. Differences in core ability to attract electrons results in different

types of compound formation and different compound properties.