The same number of protons and electrons exactly cancel one another in a neutral atom. Show animations and explain that protons and electrons have opposite charges and attract each other. Project the animation Hydrogen Atom. Give each student an activity sheet. Explore Do an activity to show that electrons and protons attract each other. Question to investigate What makes objects attract or repel each other? Materials for each group Plastic grocery bag Scissors Procedure, part 1 Charged plastic and charged skin Cut 2 strips from a plastic grocery bag so that each is about 2—4 cm wide and about 20 cm long.
Quickly pull your top hand up so that the plastic strip runs through your fingers. Do this three or four times. Allow the strip to hang down. Then bring your other hand near it. Expected results The plastic will be attracted to your hand and move toward it. Explain Show students models comparing the number of protons and electrons in the plastic and skin before and after rubbing them together.
Explore Have students investigate what happens when a rubbed plastic strip is held near a desk or chair. Procedure, part 2 Charged plastic and neutral desk Charge one strip of plastic the same way you did previously. This time, bring the plastic strip toward your desk or chair. Expected results The plastic moves toward the desk.
Have students charge two pieces of plastic and hold them near each other to see if electrons repel one other. Ask students to make a prediction: What do you think will happen if you charge two strips of plastic and bring them near each other? Procedure, part 3 2 pieces of charged plastic Charge two strips of plastic Slowly bring the two strips of plastic near each other.
Expected results The strips will move away or repel each other. Ask students: What happened when you brought the two pieces of plastic near each other? The ends of the strips moved away from each other. Use what you know about electrons and charges to explain why this happens.
Each strip has extra electrons so they are both negatively charged. Because like charges repel, the pieces of plastic repelled each other. Explore Have students apply their understanding of protons and electrons to explain what happens when a charged balloon is brought near pieces of paper.
Materials for each group Inflated balloon Small pieces of paper, confetti-size Procedure Rub a balloon on your hair or clothes. Bring the balloon slowly toward small pieces of paper.
Expected results The pieces of paper will jump up and stick on the balloon. Ask students: What did you observe when the charged balloon was held near the pieces of paper? The paper pieces moved up and stuck on the balloon. Use what you know about electrons, protons, and charges to explain why this happens.
When you rub the balloon on your hair or clothes it picks up extra electrons, giving the balloon a negative charge. When you bring the balloon near the paper, the electrons from the balloon repel the electrons in the paper. Since more protons are at the surface of the paper, it has a positive change.
The electrons are still on the paper, just not at the surface, so overall the paper is neutral. Opposites attract, so the paper moves up toward the balloon. Extra Extend Demonstrate how electrons can attract a stream of water. Materials for the demonstration Sink Balloon Procedure Rub a balloon on your shirt or pants to give it a static charge. Turn on the faucet so that there is a very thin stream of water.
Slowly bring the charged part of the balloon close to the stream of water. Expected results The stream of water should bend as it is attracted to the balloon. Ask students: What did you observe when the charged balloon was held near the stream of water?
The stream of water bent toward the balloon. When you bring the balloon near the stream of water, the electrons from the balloon repel the electrons in the water. Since more protons are at the surface of the water, it has a positive change.
Opposites attract, so the water moves toward the balloon. Downloads Lesson 4. Student Reading Use this related reading to extend student comprehension after completing the lesson.
Student Reading for chapter 4. Charge, meanwhile, can attract electrons to or repel them from a nucleus. Charge separation occurs often in the natural world. It can have an extreme effect if it reaches a critical level, whereat it becomes discharged.
Lightning is a common example. Dielectric polarization is the phenomenon that arises when positive and negative charges in a material are separated. The concept of polarity is very broad and can be applied to molecules, light, and electric fields. For the purposes of this atom, we focus on its meaning in the context of what is known as dielectric polarization—the separation of charges in materials.
A dielectric is an insulator that can be polarized by an electric field, meaning that it is a material in which charge does not flow freely, but in the presence of an electric field it can shift its charge distribution. Positive charge in a dielectric will migrate towards the applied field, while negative charges will shift away. This creates a weak local field within the material that opposes the applied field. Different materials will react differently to an induced field, depending on their dielectric constant.
This constant is the degree of their polarizability the extent to which they become polarized. The most basic view of dielectrics involves considering their charged components: protons and electrons.
If an electric field is applied to an atom, the electrons in the atom will migrate away from the applied field. The protons, however, remain relatively exposed to the field. This separation creates a dipole moment, as shown in. Reaction of an Atom to an Applied Electric Field : When an electric field E is applied, electrons drift away from the field.
On the molecular level, polarization can occur with both dipoles and ions. In polar bonds, electrons are more attracted to one nucleus than to the other. Water Molecule : Water is an example of a dipole molecule, which has a bent shape the H-O-H angle is When a dipolar molecule is exposed to an electric field, the molecule will align itself with the field, with the positive end towards the electric field and the negative end away from it.
Ionic compounds are those that are formed from permanently charge-separated ions. Ions are still free from one another and will naturally move at random.
If they happen to move in a way that is asymmetrical, and results in a greater concentration of positive ions in one area and a greater concentration of negative ions in another, the sample of ionic compound will be polarized—a phenomenon is known as ionic polarization. Electric charge is a physical property that is perpetually conserved in amount; it can build up in matter, which creates static electricity.
Electric charge is a physical property of matter. The matter is positively charged if it contains more protons than electrons, and it is negatively charged if it contains more electrons than protons. In both instances, charged particles will experience a force when in the presence of other charged matter.
Charges of like sign positive and positive, or negative and negative will repel each other, whereas charges of opposite sign positive and negative will attract each another, as shown in. Charge Repulsion and Attraction : Charges of like sign positive and positive, or negative and negative will repel each other, whereas charges of opposite sign positive and negative will attract each other.
The SI unit for charge is the Coulomb C , which is approximately equal to [latex]6. An elementary charge is the magnitude of charge of a proton or electron. Charge, like matter, is essentially constant throughout the universe and over time. In physics, charge conservation is the principle that electric charge can neither be created nor destroyed. The net quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved.
For any finite volume, the law of conservation of charge Q can be written as a continuity equation:. This does not mean that individual positive and negative charges cannot be created or destroyed.
Electric charge is carried by subatomic particles such as electrons and protons, which can be created and destroyed. For example, when particles are destroyed, equal numbers of positive and negative charges are destroyed, keeping the net amount of charge unchanged. It can be created through contact between materials, a buildup of pressure or heat, or the presence of a charge. Static electricity can also be created through friction between a balloon or another object and human hair see.
It can be observed in storm clouds as a result of pressure buildup; lightning see is the discharge that occurs after the charge exceeds a critical concentration. Lightning : Lightning is a dramatic natural example of static discharge. All materials can be categorized as either insulators or conductors based on a physical property known as resistivity. An insulator is a material in which, when exposed to an electric field, the electric charges do not flow freely—it has a high resistivity.
Conversely, a conductor is a material that permits the flow of electric charges in one or more directions—its resistivity is low. All conductors contain electric charges that, when exposed to a potential difference, move towards one pole or the other. The positive charges in a conductor will migrate towards the negative end of the potential difference; the negative charges in the material will move towards the positive end of the potential difference. This flow of charge is electric current.
Ionic substances and solutions can conduct electricity, but the most common and effective conductors are metals. Copper is commonly used in wires due to its high conductivity and relatively inexpensive price. However, gold-plated wires are sometimes used in instances in which especially high conductivity is necessary. Every conductor has a limit to its ampacity, or amount of current it can carry. This usually is the current at which the heat released due to resistance melts the material.
Insulators are materials in which the internal charge cannot flow freely, and thus cannot conduct electric current to an appreciable degree when exposed to an electric field. While there is no perfect insulator with infinite resistivity, materials like glass, paper and Teflon have very high resistivity and can effectively serve as insulators in most instances.
Just as conductors are used to carry electrical current through wires, insulators are commonly used as coating for the wires. Insulators, like conductors, have their physical limits. When exposed to enough voltage, an insulator will experience what is known as electrical breakdown, in which current suddenly spikes through the material as it becomes a conductor.
Conductor and Insulator in a Wire : This wire consists of a core of copper a conductor and a coating of polyethylene an insulator. The copper allows current to flow through the wire, while the polyethylene ensures that the current does not escape. On the other hand, the masses of protons and neutrons are fairly similar, although technically, the mass of a neutron is slightly larger than the mass of a proton.
Because protons and neutrons are so much more massive than electrons, almost all of the mass of any atom comes from the nucleus, which contains all of the neutrons and protons. The third column shows the masses of the three subatomic particles in "atomic mass units. Negative and positive charges of equal magnitude cancel each other out. This means that the negative charge on an electron perfectly balances the positive charge on the proton.
In other words, a neutral atom must have exactly one electron for every proton. If a neutral atom has 1 proton, it must have 1 electron. If a neutral atom has 2 protons, it must have 2 electrons. If a neutral atom has 10 protons, it must have 10 electrons. You get the idea.
In order to be neutral, an atom must have the same number of electrons and protons. This page was constructed from content via the following contributor s and edited topically or extensively by the LibreTexts development team to meet platform style, presentation, and quality:.
Learning Objectives Describe the locations, charges, and masses of the three main subatomic particles. Determine the number of protons and electrons in an atom. Define atomic mass unit amu.
Electrons Electrons are one of three main types of particles that make up atoms. If an electron was the mass of a penny, a proton or a neutron would have the mass of a large bowling ball!
Protons A proton is one of three main particles that make up the atom. Neutrons Atoms of all elements—except for most atoms of hydrogen—have neutrons in their nucleus. Summary Electrons are a type of subatomic particle with a negative charge.
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