Molecules and Compounds

Unit Overview

Chemical bonds, the nomenclature system, and properties of compounds are essential concepts in chemistry which are highlighted in this unit.

This unit presents an opportunity to use a variety of instructional methods. For a description of the range of instructional methods, see Instructional Approaches: A Framework for Professional Practice, pages 15-19.

In order to understand how chemicals react, one must understand how molecules form.

The way that molecules aggregate determines the properties of the compound. Both these concepts can be developed through model making and through comparison of and experimentation with variety of compounds. These are two key instructional methods.

To a certain extent, this unit can be integrated with the units dealing with chemical reactions and stoichiometry, since the formation of molecules depends on the occurrence of a reaction, and elements combine in stoichiometric ratios.

Factors of scientific literacy which should be emphasized

• A2 .........historic
• A4 .........replicable
• A5 .........empirical
• A8 .........tentative

• B1 .........change
• B2 .........interaction
• B3 .........orderliness
• B8 .........quantification
• B9 .........reproducibility of results
• B10 .........cause-effect
• B13 .........energy-matter
• B15 .........model
• B16 .........system
• B20 .........theory
• B21 .........accuracy
• B22 .........fundamental entities
• B23 .........invariance
• B29 .........gradient

• C1 .........classifying
• C6 .........questioning
• C8 .........hypothesizing
• C9 .........inferring
• C10 .........predicting
• C11 .........controlling variables
• C12 .........interpreting data
• C13 .........formulating models
• C14 .........problem solving
• C15 .........analyzing
• C16 .........designing experiments
• C17 .........using mathematics
• C20 .........defining operationally

• D1 .........science and technology
• D4 .........science, technology, and the environment
• D10 .........technology controlled by society

• E1 .........using magnifying instruments
• E3 .........using equipment safely
• E5 .........computer interaction
• E7 .........manipulative ability
• E12 .........using electronic instruments
• E13 .........using quantitative relationships

• F2 .........questioning
• F5 .........respect for logic

• G2 .........confidence
• G3 .........continuous learner
• G5 .........avocation
• G6 .........response preference
• G7 .........vocation
• G8 .........explanation preference
• G9 .........valuing contributors

Foundational Objectives for Chemistry and the Common Essential Learnings

Use the formulas and names of compounds fluently.

• Recognize the component atoms, simple ions, or polyatomic ions in a molecule.
• Use both the periodic table and a table of common ions to aid in determining the formulas of binary and other simple compounds.
• Use a table of common ions in determining the formulas of compounds containing one or more polyatomic ions.
• Write the formula of an inorganic compound, given its name.
• Use the oxidation state notation to describe the valence of polyvalent elements, e.g. Fe(III).
• Write the name of an inorganic compound, given the formula.
• Recognize the pattern in the names of the alkane and alkene series.
• Apply the general formula C_nH_2n+2 for alkanes and C_nH_2n for alkenes to write formulas for members of those series.

Discuss the mechanics of bonding between atoms in a molecule.

• Recall the structure of the atom.
• Understand the importance of the interaction of electrons when two atoms or ions approach each other.
• Contrast the bonding produced by shared pairs of valence electrons and by transfer of electrons to form ions.
• Apply the octet rule to determine the number of covalent bonds which form or the charge of the ion which forms.
• Draw Lewis structures for molecules.
• Use VSEPR theory to predict shapes of simple molecules.

Examine the bonding between molecules or atoms in solid and liquid phases.

• Describe the physical properties of ionic, metallic, covalent (molecular), covalent (network), and van der Waals solids.
• Relate the properties of compounds to the uses which those compounds have.
• Compare the properties of some alkanes to the properties of their derivative alcohols, e.g. methane-methanol, propane-propanol, octane-octanol.
• Explore the relationship between the strength of the forces holding solids and liquids together and the magnitude of the melting and boiling points of those substances.

Investigate the factors which influence solubility.

• Recognize the importance of water as a solvent.
• Use the terminology related to solubility.
• Compare the solubilities of several solute/solvent combinations.
• Recognize how the addition of a solute changes the properties of a solvent.

Use language (listening, speaking, reading, writing) for different audiences and purposes. (COM)

• Share in own words ideas which are heard, read, viewed or discussed.
• Outline information for reporting, discussing or sharing.
• Clarify, refine, restate, adapt, change, give examples, make analogies, summarize a message when another does not understand.

Promote both intuitive, imaginative thought and the ability to evaluate ideas, processes, experiences and objects in meaningful contexts. (CCT)

• Compare similarities and differences in the properties and behaviours of compounds.
• Discover relationships and patterns.
• Apply conclusions and generalizations to new situations.
• Compare and evaluate what is being read, heard and viewed.
• Use the vocabulary of critical thinking, e.g. necessary condition, conclusion, evidence, argument.

Make positive contributions to society as individuals and as members of groups. (PSVS)

• Recognize that the behaviour of an individual can affect the quality of an experience for others.
• Appreciate the importance of respecting evidence, truth and the views of others when engaging in rational discussions.
• Recognize the value of school rules and norms which support the consistent and respectful treatment of all.

Suggested activities and ideas for research projects

• Methane oozes out of the mud in the bottom of swamps and escapes from the digestive tracts of animals. How is the methane formed in these situations? How does the methane we extract from the ground as natural gas form and accumulate?

• Methanol and ethanol can be used as fuels. Why are they considered to be cleaner burning than gasoline or diesel fuel? How are they produced for fuel use?

• Cyclohexane both freezes and boils somewhere within the range from 0°C to 95°C. Design a procedure to determine the freezing point and another to determine the boiling point of cyclohexane. Submit your written procedures to your teacher. Modify the procedures as suggested and then do them. How many trials must be done to ensure that your estimate is accurate?

  Note to teachers: If you are concerned about the fumes of cyclohexane (which are rated as less of a hazard than fumes of dichlorobenzene) consider suggesting a boiling point determination apparatus with a two-holed stopper venting vapours into an ethanol trap. Since cyclohexane is very flammable do not allow designs which involve heating a hot water bath with an open flame. Cyclohexane's boiling point is 81°C. Pose that as a problem to the students - to produce a hot bath which will still be above 90°C after five minutes.

• Oleic acid, the primary component of olive oil, will form a slick on the surface of water. Depending on conditions, the slick may be only one or two molecules thick. What are some of the conditions which may affect the thickness of the layer? To estimate the thickness of the slick is a good exercise in both indirect measurement and in helping to comprehend the dimensions of atoms and molecules.

  Get a large pan, fill it with water to a depth of about 2 cm and let the water become as still as possible. When still, sprinkle just enough talc or lycopodium powder (spores of a type of moss) on the surface so that the surface looks slightly hazy.

  The oleic acid has been diluted in alcohol so drop a drop of pure alcohol onto the surface of the water. Record the effect. Then put a drop of the oleic acid solution at the centre of the pan. measure the diameter of the slick that forms and estimate its area.

  By estimating the volume of oleic acid in one drop of the solution and the area of the slick, you can calculate the thickness of the slick.

  Note to teachers: Dilute the oleic acid to 0.5% by volume. Circular pizza pans with a diameter of 40 cm work well for this activity. Check around your storage spaces to see if any of this equipment remains from when it was used in the IPS Science program.

  Another exercise in indirect measurement is to calculate the thickness of aluminum foil or of stretchable plastic wrap.

• Microscale decomposition of water into its component gases can be done with a 9 volt battery in a 500 mL beaker.

  Half fill the beaker with a 0.10 M Na₂SO₄ or MgSO₄ solution and enough universal indicator solution to give a strong green colour. Fill two 13 by 100 mm test tubes with the solution and invert them in the beaker so that no air is trapped in them.

  Lower a 9 volt battery, with the terminals up, into the beaker and stand it on the bottom near one side. Ensure that there is enough solution in the beaker to cover the battery by 5-7 cm. Bring one test tube over and cover one of the terminals. Lean the tube against the wall of the beaker for support. Repeat this procedure to cover the other terminal with the other test tube. Collect the gases by downward displacement of water.

  Record observations.

  Repeat the activity using different salt solutions (NaCl, KI, NaBr) or different indicators (bromothymol blue, red cabbage juice). What would happen if H₂SO₄ solution were used instead of a salt solution? (This activity was adapted from CHEM13 NEWS, #201, February 1991, page 7, based on an idea contributed by Marie C. Sherman of St. Louis, MO)

• List the melting points of the metals. Are there any generalizations you can make or trends you can deduce?

• What are some examples of alloys? Are alloys compounds? How do the properties of alloys differ from the properties of their components? Is steel classified as an alloy? Where did the term karat to describe the alloys of gold come from? Why is 24 karat gold pure gold? (Why not 100 karat gold for pure gold?)

  Pick a chemical compound used in some aspect of your life: food; clothing; medicines; housing; fuels; etc. Create a poster or video clip report of the chemical. Information about the name and formula, the properties and uses, the sources, how it is produced from its raw materials, and the value to society of the chemical should be included. Adapted from an article "Worldly Chemicals" by Larry Mossing, in The Accelerator (June 1990).

• What characteristics of solid carbon dioxide (dry ice) and solid paradichlorobenzene (moth crystals) permit them to go directly from the solid phase to the gaseous phase. Do other solids such as ice or iron do this?

• Below is the ingredients list taken from the back of a package of eye shadow. Determine the chemical formulas and the purpose in the eye shadow for as many of the chemicals as possible.

  Contains: talc, mineral oil, zinc stearate, lanolin, alcohol, urea/formaldehyde resin, octyl palmitate, calcium silicate, ozokerite, jojoba oil, methylparaben, imidiazolidinyl urea, propylparaben, aloe, BHA. May contain: iron oxides, mica, titanium dioxide, carmine, bismuth oxychloride, manganese violet, ultramarine blue, ultramarine pink, ultramarine violet, chromium hydroxide greens, bronze powder, aluminum powder, ferric ferrocyanide, ferric ammonium ferrocyanide.

• Some students enjoy making and using flash cards. Put a formula for a molecule or ion on one side of the card and the name of the species on the other.

• Make 3-D models of molecules or ions to hang from the classroom ceiling. These models can be made from commercial kits or be homemade. A label including the name and formula of the species can be strung on the string which supports the model. A poster for each model, with a 2-D sketch of the species and some information (Lewis structure, chemical properties, where it is found) about it can be made for the classroom walls. It is useful to have students start to associate the 2-D representations with the 3-D representations of the species. The posters could be colour-coded to indicate correspondence between poster and model or they could be left for the viewers to figure out.

• Select and promote a "Molecule (or Compound) of the Day". See the "Element of the Day" activity in the previous unit for ideas.

• SCI-TEC Instruments in Saskatoon produces an instrument called the Brewer Ozone Spectrophotometer. It is used in more than 20 countries to measure the levels of ozone, sulphur oxides and nitrogen oxides in the atmosphere. How do such machines detect what molecules are in the air?

Sample ideas for evaluation and for encouraging thinking

• How is separation of whole milk into skim milk and cream different than the separation of water into hydrogen and oxygen?

• For each of the substances in the list, find the chemical formula.

  hematite                         muriatic acid ethanoic acid galena

  limestone table salt quartz cellulose

• The following are formulas of gems. Match the gem to the formula.

  emerald	(AlF)2SiO4
  lapis lazuli	Al2O3
  ruby	Be3Al2(SiO3)6
  spinel	Mg(Al2O4)
  topaz	Na4(NaS·Al)Al2(SiO4)3
  zircon	ZrSiO4

  What are some formulas of other gems? Memorize the formula for diamond as a test of the ability of your memory.

• Using only a periodic table for aid, write the formulas for the molecules in the list.

  carbon dioxide                         rubidium bromide hydrogen sulfide

  calcium carbide silicon tetrachloride water

• Write instructions so that someone who knew nothing about chemistry could come into this lab and do each of the tasks below. (You may advise the person to use tables or charts on the walls or in the reference books, give the person page references for the books or any help short of giving the answer.)
  • Find the symbol for beryllium and silver.
  • Find the weight of one mole of NaCl.
  • Determine the names of the elements in CaCO₃.
  • Find the chemical formula for sucrose.

  Give your instructions to a partner for testing. When you are testing instructions, pretend that you don't know a lot of chemistry and see if the instructions alone are enough.