Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Divide the “building blocks” from the molecular model set provided with the kit into eight sets for Parts B–E. Place each set into a separate resealable plastic bag and label each bag with a permanent marker. Make two identical sets for each part.

Part B: 2 H atoms, 2 O atoms, 2 N atoms, 1 single bond link and 3 double bond links
Part C: 2 C atoms, 6 H atoms, 1 O atom, 8 single bond links and 2 double bond links
Part D: 4 C atoms, 10 H atoms and 13 single bond links
Part E: 2 C atoms, 4 H atoms, 2 O atoms, 6 single bond links and 2 double bond links

2. Reserve 3 H atoms, 2 O atoms, 1 N atom, 1 C atom, 3 single bond links and 4 double bond links for Part A. The ammonia, water, and carbon dioxide molecules for Part A may be built as students watch to demonstrate how the atoms and links snap together or they may be built prior to the beginning of the class. See Sample Data Table A for structural formulas of ammonia, water and carbon dioxide.
3. Make a copy of the periodic table PDF for each group of students.

Lab Hints

• Enough materials are provided in this kit for 8 groups of students. Two class sessions are recommended for completion of this activity. After the Prelab Questions and Part A are completed, students may work on the other parts of the activity in any order. Rotate the model sets for Parts B–E among the groups in 10–12 minute intervals. Having students work collaboratively in groups of three or four will allow students to brainstorm and bounce ideas back and forth. This is the heart of the guided-inquiry process in which students use reasoning skills and critical thinking to actively “construct” their knowledge of the subject matter.
• Some of the links may be tight. Push the link partway into the hole of the first atom and then add the second atom to the other end of the link. Push the atoms together to secure the link.

Teacher Tips

• This activity is a good introduction to molecules and covalent bonding. Students should have a basic understanding of the structure of atoms and electron configuration.
• Guide students through Part A to familiarize them with the basic process of building molecules. Allow students to discover the need for double bonds with the carbon dioxide molecule.
• Remind students that the molecules they build are only stable if all the atoms have the correct number of bonds with no empty holes.
• In general, guided-inquiry activities are most successful if students understand that the activity replaces the lecture. Students are more likely to take responsibility for learning when they are actively engaged in the process of “constructing knowledge.” Guided-inquiry activities simulate the scientific method—students look at data, search for patterns or relationships, and try to identify guiding principles that will explain the data.
• Students may not always write the element symbols of chemical formulas in the correct sequence. Help them to see the patterns of the given chemical formulas, keeping in mind the spirit of the inquiry approach.
• Attaching the word organic to food items has become a mark of quality, to signify that something is natural and pure. The term reflects the historical roots of the study of carbon-based compounds. In the early 19th century, organic chemistry was a brand new science devoted to the study of compounds called natural products, which were isolated from plants and animals. Natural products included foods and medicines, soaps and perfumes, preservatives and cosmetics, spices and seasonings, etc. These compounds were originally called “organic” because it was thought that compounds obtained from living organisms required some sort of “vital” or animating force for their existence. Although this notion was discarded in 1828, when the first organic compound was synthesized in the lab, the exciting discoveries of the molecular basis of life in the 20th century more than justify the title “organic” chemistry.
• Demonstrate how hydrocarbon molecules can form rings by building the benzene molecule (C₆H₆) shown in Figure 4 or challenge students to build one. Benzene is the parent compound of a group called aromatic compounds.
  {12768_Tips_Figure_4_Benzene}
• Additional molecular model sets such as Catalog No. AP5453, Organic Small-Group Model Set, are available from Flinn Scientific.

Answers to Prelab Questions

1. What information does the chemical formula of a molecule provide? What additional information does the structural formula provide?

   The chemical formula identifies the elements that make up a molecule and indicates how many atoms of each element are present in one molecule. The structural formula indicates the number of shared electrons by each atom in a molecule as well as the arrangement of the atoms.

2. One exception to the octet rule is hydrogen. Explain why hydrogen only has two valence electrons around its nucleus, not eight, when it bonds to other atoms. Hint: Consider the number of valence electrons in an atom of the noble gas helium.

   Hydrogen has only one electron in the energy level closest to the nucleus. The maximum number of electrons the first energy level can hold is two. Helium is a noble gas with only two valence electrons because its outer energy level is full.

3. Of the 92 naturally occurring elements, only four make up over 95% of living things. These four elements are listed in the following chart. Use a periodic table and the octet rule to complete the chart by filling in the element symbols, number of valence electrons and number of bonds needed to gain stability.

Sample Data

Build Models of Molecules Worksheet

Part A. Teacher Demonstration

Data Table A

Part B. Single, Double and Triple Bonds

Data Table B Part C. Hydrocarbons

Data Table C Part D. Isomers

Data Table D Part E. Organic Acids

Data Table E

Answers to Questions

Build Models of Molecules Worksheet 

Part A. Teacher Demonstration 

1. Observe the models of ammonia, water and carbon dioxide. How do the bonds in a water molecule fulfill the octet rule for hydrogen and oxygen?

   The oxygen atom needs two electrons to fill its outer energy level and each hydrogen atom needs one. Oxygen shares an electron with each hydrogen atom, therefore the oxygen atom has a complete valence level with 6 electrons plus two shared electrons, and each hydrogen atom has one electron and shares one, which completes its valence level.

2. How are the bonds in the carbon dioxide molecule different than the bonds in the water or ammonia molecules? Explain why carbon and oxygen bond this way.

   Carbon and oxygen form a double bond in a carbon dioxide molecule. The bonds in water and ammonia are all single. The carbon atom needs to share two electrons with each oxygen atom in order for each atom to have a complete valence level.

3. Complete Data Table A by filling in the chemical formula and structural formula for water and carbon dioxide.

   See Sample Data Table A.

Part B. Single, Double and Triple Bonds

1. Build models of hydrogen, nitrogen and oxygen molecules as found in air according to the chemical formulas in Data Table B. Refer to the chart in Prelab Question 3 for the number of bonds needed for each atom. Complete Data Table B by filling in the type of bond formed and the structural formula for each molecule.

   See Sample Data Table B.

2. How many total electrons are shared by the two nitrogen atoms?

   The two nitrogen atoms share three pairs or a total of six electrons.

Part C. Hydrocarbons

1. Hydrocarbons are compounds containing only carbon and hydrogen. The simplest hydrocarbon is methane, CH₄, the largest component of natural gas. Ethane has two carbon atoms linked together with a single bond; the rest of the bonded atoms are hydrogen. Build a molecule of ethane and determine its chemical formula. Record the chemical and structural formulas for ethane in Data Table C.

   See Sample Data Table C.

2. A substituted hydrocarbon has one or more of its hydrogen atoms replaced by an atom or group of atoms of other elements. Ethanol, or ethyl alcohol, is a compound in which one hydrogen atom has been replaced by an –OH (hydroxyl) group. Modify the model of ethane to build a model of ethanol. Fill in the information for ethanol in Data Table C.

   See Sample Data Table C.

3. Some hydrocarbons have double or triple carbon bonds. Ethylene (also called ethene) has two carbon atoms doublebonded together. Build a molecule of ethylene, adding the correct number of single hydrogen bonds. Complete Data Table C for ethylene.

   See Sample Data Table C.

Part D. Isomers

1. Butane, C₄H₁₀, is a hydrocarbon (a compound containing only carbon and hydrogen) with two possible structures. Build one model of butane with the carbon atoms in a long chain and draw its structural formula in Data Table D. Then rearrange the atoms to build a second model (isobutane) and draw its structure.

   See Sample Data Table D.

2. The two possible structural formulas for butane represent isomers. Write a definition of isomers that describes the relationship between these two molecules.

   Isomers are molecules or compounds that have the same chemical formula but different structural formulas.

Part E. Organic Acids

1. Organic, or carbon-based, acids called carboxylic acids are formed when a –CH₃ group of atoms is displaced by a –COOH (carboxyl) group. The simplest carboxylic acid is formic acid, also known as methanoic acid because it is based on a methane molecule. Build a formic acid molecule and draw its structural formula in Data Table E. Note: Chemical formulas may be written to show a group (such as a hydroxyl or carboxyl group) is part of the molecule.

   See Sample Data Table E.

2. Describe how the atoms in a carboxyl group are linked together.

   The carbon atom is double bonded to one oxygen atom and is also bonded to an –OH (hydroxyl) group.

3. Build a model of acetic (ethanoic) acid, CH₃COOH. Note that the carboxyl group stays intact. Draw the structural formula for acetic acid in Data Table E.

   See Sample Data Table E.

Post-Activity Questions

1. Hydrocarbons in which all of the C—C bonds are single follow a general formula. Based on the chemical formula for methane and ethane, determine the general formula for these hydrocarbons. Hint: The formula for propane is C₃H₈.

   C_nH_{2n +2} or for every n number of carbon atoms there are 2n + 2 hydrogen atoms (H = 2C + 2).

2. Without building models, draw out the possible structural formulas for three isomers of pentane, C₅H₁₂.

3. Complete the following sentence.

   “Carboxylic acids are substituted hydrocarbons because three hydrogen atoms have been displaced by an oxygen atom and an –OH group.”

4. Organic compounds are molecules that contain carbon. More than nine million organic compounds are known. Why do you think carbon is a part of so many different compounds?

   Carbon atoms form four covalent bonds to achieve a complete outer energy level. This allows for a large number of bonding possibilities.

Teacher Handouts

12768_Teacher1.pdf

References

Brown, Tom, Greg Rushton, and Marie Bencomo. 2008. Mighty molecule models. Science and Children 45(4): 33–37.

Introduction

Hydrogen, H₂, is a highly flammable gas. Oxygen, O₂, is a reactive gas that promotes combustion. Together they form water, H₂O, a liquid used to extinguish fire. H₂, O₂ and H₂O are formulas for molecules that are formed when two or more atoms bond together by sharing electrons. Build models of molecules to discover why atoms bond together and explore a great variety of compounds formed by just four different elements.

Concepts

• Chemical versus structural formulas
• Covalent bonding
• Molecules
• Valence electrons

Background

Atoms bond with other atoms to become more stable. Molecules are formed when atoms of nonmetals share electrons, forming a covalent bond. In general, only the outer energy level electrons, or valence electrons, are available for bonding. The number of valence electrons influences the number of bonds that an atom will form. The periodic table offers a convenient shortcut for determining the number of valence electrons in an atom. The columns in the periodic table represent groups or families of elements. Most periodic tables have the group numbers written above each column. These are written as Group 1 to 18 (see Figure 1). The number of valence electrons for any element, other than transition metals, is equal to the numeral in the ones place of the group number. Thus, potassium in Group 1 has one valence electron, carbon in Group 14 has four valence electrons, and chlorine in Group 17 has seven valence electrons.

The noble gases, Group 18, have eight valence electrons and are very stable. They do not readily combine with other elements because their outer energy levels are full. Other nonmetals may share electrons in order to achieve a valence electron “count” similar to that of the noble gases, thus becoming stable. This is known as the “rule of eight” or the “octet rule.” Consider the chlorine atom with seven valence electrons. Each chlorine atom needs only one more electron to form a stable molecule—two chlorine atoms come together and share one bonding pair of electrons (Equation 1). Even though the total number of valence electrons in a chlorine molecule is 14, since each atom shares an electron with the other atom, the octet rule is satisfied for each chlorine atom. The chemical formula for a chlorine molecule is Cl₂, where Cl is the symbol of the element and the subscript indicates the number of atoms of that element in the molecule. One shared pair of electrons is a single bond and can be represented by a single line between the element symbols. Figure 2 shows the structural formula of a chlorine molecule. The structural formula may also depict the arrangement of the atoms in a molecule. In Figure 3, the structural formula for ammonia, NH₃, shows how the hydrogen atoms are bonded to the nitrogen atom. Some atoms share two or even three pairs of electrons with another atom. Double bonds and triple bonds are represented by two and three lines, respectively.

Experiment Overview

The purpose of this activity is to discover the basic structures of compounds by building molecules with atoms of four different elements. The molecule models will be used to write chemical formulas and draw structural formulas of the compounds.

Materials

Prelab Questions

1. What information does the chemical formula of a molecule provide? What additional information does the structural formula provide?
2. One exception to the octet rule is hydrogen. Explain why hydrogen only has two valence electrons around its nucleus, not eight, when it bonds to other atoms. Hint: Consider the number of valence electrons in an atom of the noble gas helium.
3. Of the 92 naturally occurring elements, only four make up more than 95% of living things. These four elements are listed in the chart below. Use a periodic table and the octet rule to complete the chart by filling in the element symbols, number of valence electrons and number of bonds needed to gain stability.

Safety Precautions

The materials in this activity are considered safe. Please follow all classroom safety guidelines.

Procedure

1. Atoms are represented by colored spheres with holes. The number of holes varies with each atom, depending on the number of bonds needed.

Carbon — black
Hydrogen — white
Oxygen — red
Nitrogen — blue

2. Two different links are available to represent a pair of shared electrons. The shorter link is for single bonds and the longer flexible link is for molecules with double or triple bonds. Note: Use one link per pair of shared electrons; if a molecule has one double bond then two flexible links are needed to represent two shared pairs of electrons.
3. After building each molecule and drawing its structure, take apart the molecule and return the pieces to their respective bag for the next group to use.

Student Worksheet PDF

12768_Student1.pdf