Energy Levels, Electrons, and Ionic Bonding

Project the video Sodium and chlorine react. 

Before starting the video, tell students that chlorine is a greenish poisonous gas and sodium is a shiny, soft, and very reactive metal. But when they react, they form sodium chloride (table salt). Tell students that in the video, the drop of water helps expose the atoms at the surface of the sodium so that they can react with the chlorine. The formation of the salt crystals releases a lot of energy.

Note: If students ask if the salt they eat is made this way in salt factories, the answer is no. The salt on Earth was produced billions of years ago but probably not from pure chlorine gas and sodium metal. These days, we get salt from mining it from a mineral called halite or from evaporating sea water.

Give each student an activity sheet.

• Lesson 4.5 Student Activity Sheet PDF | DOCX | Google Doc
• Lesson 4.5 Activity Sheet Answers PDF | DOCX | Google Doc

Download the student activity sheet, and distribute one per student.

All Downloads

The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity sheets are formative assessments of student progress and understanding. A more formal summative assessment is included at the end of each chapter.

Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.

Project the animation Ionic bond in sodium chloride. 

Remind students that in covalent bonding, atoms share electrons. But there is another type of bonding where atoms don’t share, but instead either take or give up electrons. This is called ionic bonding. This animation shows a very simplified model of how sodium and chloride ions are formed.

Note: In order to simplify the model of ionic bonding, a single atom of sodium and chlorine are shown. In reality, the chlorine atom would be bonded to another chlorine atom as part of the gas Cl₂. The sodium atom would be one of billions of trillions of sodium atoms bonded together as a solid. The combination of these substances is a complex reaction between the atoms of the two substances. The animation shows single separated atoms to illustrate the idea of how ions and ionic bonds are formed.

Explain what happens during the animation.

Tell students that the attraction of the protons in the sodium and chlorine for the other atom’s electrons brings the atoms closer together. Chlorine has a stronger attraction for electrons than sodium (shown by the thicker arrow). At some point during this process, an electron from the sodium is transferred to the chlorine. The sodium loses an electron and the chlorine gains an electron.

Tell students that when an atom gains or loses an electron, it becomes an ion.

• Sodium loses an electron, leaving it with 11 protons, but only 10 electrons. Since it has 1 more proton than electrons, sodium has a charge of +1, making it a positive ion.
• Chlorine gains an electron, leaving it with 17 protons and 18 electrons. Since it has 1 more electron than protons, chlorine has a charge of –1, making it a negative ion.
• When ions form, atoms gain or lose electrons until their outer energy level is full.
  • For example, when sodium loses its one outer electron from the third energy level, the second level becomes the new outer energy level and is full. Since these electrons are closer to the nucleus, they are more tightly held and will not leave.
  • When chlorine gains an electron, its third energy level becomes full. An additional electron cannot join, because it would need to come in at the fourth energy level. This far from the nucleus, the electron would not feel enough attraction from the protons to be stable.
  • Then the positive sodium ion and negative chloride ion attract each other and form an ionic bond. The ions are more stable when they are bonded than they were as individual atoms.

Have students write a short caption under each picture to describe the process of covalent bonding and answer the first three questions. The rest of the activity sheet will either be completed as a class, in groups, or individually depending on your instructions.

Project the image Ionic bond in sodium chloride. 

Review with students the process of ionic bonding covered in the animation.

Help students write a short caption beside each picture to describe the process of ionic bonding in sodium and chloride ions. 

Project the image Sodium chloride crystal.

Review with students the process of ionic bonding covered in the animation so that students will understand why the sodium ions are positive and the chloride ions are negative. Remind students that the scale of any model of atoms, ions, or molecules is enormous compared to the actual size. In a single grain of salt there are billions of trillions of sodium and chloride ions.

Ask students:

• What ion is the larger ball with the negative charge?
  The chlorine ion.
• What made it negative?
  It gained an electron.
• What is the ion with the positive charge?
  The sodium ion.
• What made it positive?
  It lost an electron.

This two-part activity will help students see the relationship between the arrangement of ions in a model of a sodium chloride crystal and the cubic shape of real sodium chloride crystals.

Question to investigate:

Why are salt crystals cube-shaped?

Teacher preparation

The day before the lesson, dissolve about 10 grams of salt in 50 ml of water. Use Petri dishes or use scissors to cut down 5 or 6 clear plastic cups to make shallow plastic dishes. Pour enough saltwater to just cover the bottom of each dish (1 for each group). Leave the dishes overnight to evaporate so that new salt crystals will be produced.

Materials for each group

• Black paper
• Salt
• Cup with salt from evaporated saltwater
• Magnifier
• Permanent marker

Materials for each student

• 2 small Styrofoam spheres
• 2 large Styrofoam spheres
• 2 toothpicks

Procedure, Part 1

Observe sodium chloride crystals.

1. Place a few grains of salt on a piece of black paper. Use your magnifier to look closely at the salt.
2. Use your magnifier to look at the salt crystals in the cup.

Project the image Cubic sodium chloride. 

The image shows both a magnified view of ordinary table salt and a model of the sodium and chloride ions that make up a salt crystal.

Project the video Sodium chloride. 

The green spheres represent negatively charged chloride ions and the gray spheres represent positively charged sodium ions.

Ask students:

• What do the photograph, molecular model, and your observations of real salt crystals tell you about the structure of salt?
  In each case, the salt seems to be shaped like a cube.

Each student will make 1 unit of sodium chloride. Students in each group will put their sodium chloride units together.

You can help the groups combine their structures into a class model of a sodium chloride crystal.

Ask students

• Based on the way sodium and chloride ions bond together, why are salt crystals shaped like cubes?
  The size and arrangement of the ions forms a cube on the molecular level. Since the pattern repeats over and over again in the same way, the shape stays the same even when the crystal becomes the normal size that we can see.

Tell students that there is another common substance called calcium chloride (CaCl₂). It is the salt that is used on icy sidewalks and roads. Explain that when calcium and chlorine react, they produce ions, like sodium and chlorine, but the calcium ion is different from the sodium ion.

Ask students:

• What ions do you think CaCl₂ is made of?
  One calcium ion and two chloride ions.

Project the animation Calcium chloride Ionic Bond. 

Point out that the calcium loses two electrons, becoming a +2 ion. Each of the two chlorine atoms gains one of these electrons, making them each a –1 ion. Help students realize that 1 calcium ion bonds with 2 chloride ions to form calcium chloride (CaCl₂), which is neutral.

Some atoms gain or lose more than 1 electron. Calcium loses 2 electrons when it becomes an ion. When ions come together to form an ionic bond, they always join in numbers that exactly cancel out the positive and negative charge.

Project the image Calcium chloride Ionic Bond. 

Review with students the process of ionic bonding covered in the animation.

Have students write a short caption beneath each picture to describe the process of ionic bonding in sodium and chloride ions.