Unit Three: Chemical Bonding and Intermolecular Forces

Lesson 1: Introduction to Chemical Bonding

• Comptencies
• Starter Activities
• Main Activites

Competencies

• Define chemical bonding
• Explain why atoms form chemical bonds

Note for the teacher

Chemical bonding is one of the basic topics in chemistry. Since it is an abstract concept which cannot be applied to everyday life directly, many students aren’t able to comprehend this concept. They may have difficulty to relate a microscopic world to macroscopic world.

In addition, understanding chemical bonding requires some concepts from physics such as energy and force in which students already  have difficulties in understanding. As a result, they hold many misconceptions related to chemical bonding. Understanding chemical bonding is important in chemistry in order to comprehend the nature of the chemical reactions and some physical properties such as boiling point. Students usually have difficulties in understanding why and how bonding occurs.

Materials:

Chart for Lewis structures of the elements, periodic table chart

Starter activity (6 minutes)

Observing our environment

Let students observe their surroundings and identify substances they see. Ask them to tell whether most of the substances are compounds, mixtures or elements. The materials they listed may be:

Chalk board, chalk, the wall, the floor, air, pen, pencil, book, exercise book, desk, and wood, iron materials covered with paints, cloths and pupils.

1. Are most of the materials elements, compounds or mixtures?
2. How many of these materials are elements?
3. Why do we find many elements in our surroundings in a combined form?
4. Why some elements are found un combined in nature?

Let students give any idea they have. You may jot down the different ideas forwarded by the students. At this stage no correct information is expected from the students. The different guesses forwarded will help you   to know students’ perception of a chemical bond.

Now lead students to come to the conclusion, that among the listed substances, the elements are oxygen, nitrogen, and some noble gases like argon and helium, in the mixture of air, and iron materials covered by certain enamel (galvanized or painted). The elements oxygen and nitrogen are found as combined forming oxygen and nitrogen molecules respectively. As the result they are found as free molecules. However, iron is covered not to combine with other elements (remind them about rusting of iron). So, the only elements found as free atoms are the noble gases.

Evaluation

Ask the students about all the elements in the periodic table. Let them identify which ones are found as free atoms in nature?

Main activity (28 minutes)

• Why is the noble gas elements not combined?

Let the students make a research on the structure of these elements in comparison to the other elements. The following activity can help to make the investigation.

• Give the following atomic numbers of the elements: 1, 2, 11, 13, 6, 10, 15, 17, 18, 7 and 36
• Write down electron configuration in the main energy levels
• Identify those elements with similar electron configuration. What makes them similar?
• Where do you find the elements with similar configuration in the periodic table?
• Why do not these elements combine?
• What influences the reactivity of elements?

Check the students’ work and assist those with difficulty. They may identify elements with only 8 electrons in the outer shells   are similar elements.  But they may fail to categorize He as one member of the group. Explain that He is the member of noble gases because it has a complete first shell.

After some 10 minutes, ask the students to explain their findings.

Students should distinguish between noble and non-noble gases as follows

The difference between the two categories of elements is that those in the first category have 1 to 7 outer most electrons or incomplete shells where as elements in the second category have complete outer shells.

They should refer to the periodic table to find out that the elements with complete outer shells are found at the end of each period of the periodic table, or the right most group VIIIA.
They should also come to the conclusion that these elements do not combine because they have complete electron configuration. They can explain that electronic structure influences the combining capacity of the elements.

Help students to know that not only have noble gases complete outer shells but also lower energies.

Now students can be asked to suggest how the other elements can have similar electron configuration with the noble gases and achieve lower energy?

Let them discuss with their bench mates. Then wait them for some time, so that they can come up with some kind of solutions.

Some students may tell that the elements should gain or lose electrons in the outer most shells to have similar electron configuration as the noble gases.  If not, help them to come to this generalization. Explain that if elements have 8 electrons in their outer shell, they are said to have attained the octet rule.  .

Evaluation

Students may be asked to write the different electronic structure of noble gases and how it contributed to their stability.

Concluding activity

Now students should conclude why we don’t find most elements free in nature. All elements have the tendency of having the noble gas configuration. To attain this, the elements gain, lose or share electrons. In their last shell as the result, different types of bonds:, ionic covalent or metallic bonds are formed. That is why we don’t find elements in a free form.