Experiment Regarding Types of Chemical Bonds

Elements form compounds through ionic and covalent bonding.

Introduction

Examination of the periodic table reveals that there are just over 100 elements. However, atoms of these elements can join in countless combinations to produce hundreds of chemically distinct substances. Atoms bond because the total energy of the combination is lower than the energy of the separated atoms. The compounds that result from bonded atoms are chemically and physically different than the elements from which they are formed.

Atoms can form two basic types of bonds: ionic bonds and covalent bonds. The type of bonding depends on the arrangement of electrons within the atoms involved. In ionic bonding, two elements join because electrons of one element are transferred to the other element. As a result, two ions, or charged particles, are formed. One ion contains extra electrons and has a negative charge, while the other has lost one or more electrons and has a positive charge. These two oppositely charged particles are attracted to each other.

For example, during the reaction of sodium with chlorine, an electron from sodium is transferred to chlorine (see Figure 1). As a result sodium loses one electron and becomes a positive ion. Chlorine gains an electron and becomes a negative ion. The two ions are attracted to each other and form a bond that creates sodium chloride, or table salt. Covalent bonds are formed when atoms share electrons.

Sharing creates strong, stable compounds. The simplest example of covalent bonding occurs between two hydrogen atoms when they combine. Each hydrogen atom is made up of one proton and one electron. However, this is an unstable electron arrangement because two electrons are needed to make a hydrogen atom stable and unreactive. When two hydrogen atoms share their electrons, they form diatomic hydrogen gas, or H2. In this experiment, you will conduct research on the Internet to learn more about bonding.

Figure 1

Time Required

45 minutes

Materials

2 access to Internet

2 science notebook

Please review and follow the safety guidelines at the beginning of this volume.

Procedure

1. Go to the Basic Chemistry Web site by Dr. June Steinberg at http:// www2.nl.edu/jste/atomic.htm. Read the introduction and the section titled “Energy Shells” to find out the role of outer shell electrons in chemical bonds.
2. Answer Analysis questions 1 through 6.
3. Go to the Chemical Bonds Web site by Dr. June Steinberg at http:// www2.nl.edu/jste/bonds.htm. Read the entire page and answer Analysis questions 7 through 16.
4. Goto Mr.Kent’s Web site at http://www.kentchemistry.com/links/ bonding/bondingflashes/bond_types.swf.
5. Click on each bond type to see an animation explaining how the bonds form. Answer Analysis question 17.

Analysis

1. What are the three basic components of atoms?

2. Where are electrons located?

3. Which energy shell contains the most energy, the innermost shell or the outermost shell?

4. Use the 2-8-8 rule to explain how 12 electrons will be distributed in an atom’s energy shells.

5. What are the three ways in which atoms can fill their outer shells with a stable number of electrons?

6. Define ion.

7. How are ionic bonds and covalent bonds formed?

8. Explain what happens in the animation that shows how sodium and chlorine ions are formed.

9. At the end of the animation, what is the condition of the outer shells of the sodium and chloride ions?

10. Why do sodium and chloride ions form bonds?

11. Explain what happens in the animation showing the formation of a covalent bond.

12. After the animation, how many electrons will each hydrogen atom have in its outermost shell?

13. What is the difference between a nonpolar and a polar covalent bond?

14. Water is a polar molecule. In water, the oxygen atom attracts electrons more strongly than the smaller, weaker hydrogen atoms. Draw a water molecule and show the slight negative and positive charges.

15. What type of bonds form between water molecules?

16. Arrange the three types of bonds, ionic, covalent, and hydrogen, in order from strongest to weakest.

17. After watching the animations at Mr. Kent’s Web site, summarize what you learned about each type of bond.

Stable or inert atoms are unreactive because they have a complete set of valence electrons, those in the outermost energy shells. Atoms that naturally have a stable electron configuration are the noble gases. Atoms of the noble gases have eight electrons in their outer energy levels, a configuration known as an octet. Other atoms, those that have incomplete sets of valence electrons, can reach a stable, noble gas configuration by losing or gaining electrons.

Atoms that have one, two, or three valence electrons are most likely to lose these to reach a stable octet. For example, sodium, located on the left side of the periodic table, has only one valence electron, which it can easily give up. Chlorine, on the opposite end of the period table, has seven valence electrons. The easiest way for chlorine to reach the noble gas configuration is to gain one electron. The transfer of an electron from sodium to chlorine creates two ions of opposite charges. The ions are held together by the attractive forces.

Atoms that have four valence electrons are likely to share with other atoms. Carbon, for example, has four valence electrons and can share these with four other carbon atoms, or with atoms of hydrogen and oxygen. Silicon also has four valence electrons, explaining why it easily forms covalent bonds with oxygen and with other silicon atoms.

The periodic table can be used to determine the number of valence electrons in an atom and therefore to predict the types of bonds that atoms will form. The data table shows that elements on the left side of the periodic table, groups 1, 2, and 3, have less than three electrons, which they are likely to give up. Groups 15, 16, and 17 have more than five electrons, so are likely to take electrons to fill their valence shells. Elements in group 4, which contains carbon and silicon, share electrons to make covalent compounds. The noble gases, group 18, are stable and unreactive.

Connections

Some covalent bonding produces polar molecules, those with a slight negative charge on one end and a slight positive charge on the other end. One such polar molecule is water. A water molecule is made of one oxygen atom and two hydrogen atoms. The nucleus of oxygen is much larger and stronger than the nuclei of the hydrogen atoms.

Even though oxygen and the two hydrogen atoms are sharing electrons, oxygen pulls on them with more force than the small hydrogen atoms. As a result, the shared electrons spend more time near the oxygen end of the molecule, giving it a slight negative (-) charge. Because the electrons are pulled away from the hydrogen atoms, they have slight positive charges (+). Therefore, each water molecule is polar (see Figure 2).

Figure 2

This polarity means that individual water molecules are attracted to each other. The attraction creates bonds that are weak, but significant. This attraction between polar molecules, called hydrogen bonding, is responsible for some of water’s unusual properties. For example, hydrogen bonding between water molecules creates surface tension, which causes the top layer of water to have a skinlike property. Hydrogen bonds also form in many organic molecules such as proteins. In the protein in Figure 3, hydrogen bonds hold the protein in a helical shape so are responsible for the three-dimensional structure of the molecule.