Molecules and Covalent Bonding PDF

5.10 Molecules and Covalent Bonding Nitrous oxide, N2O, is a colourless, sweet-smelling gas that dentists sometimes use to relax nervous patien...

5.10 Molecules and Covalent Bonding Nitrous oxide, N2O, is a colourless, sweet-smelling gas that dentists sometimes use to relax nervous patients (Figure 1(a)). You would not, however, want to inhale nitrogen dioxide, NO2. This reddish-brown toxic gas is produced in the atmosphere from pollutants emitted in automobile exhaust. Nitrogen dioxide is a dangerous part of the smog that hangs over large cities in the summer (Figure 1(b)). People with asthma and other respiratory problems often find it more difficult to breathe on smoggy days. Figure 1 (a) Inhaling nitrous oxide, laughing gas, helps dental patients relax. (b) Nitrogen dioxide is the gas responsible for the reddish-brown colour of smog over large cities during the (a) (b) summer. Pollution control technology in modern cars helps reduce nitrogen dioxide emissions. A catalytic converter, attached to the car’s exhaust system, To find out more about the converts nitrogen dioxide into harmless nitrogen and oxygen. The Ontario Ontario Drive Clean Program, Drive Clean Program requires that most cars over five years old pass an GO TO NELSON SCIENCE emissions test every two years. This helps reduce pollution in Ontario. Nitrous oxide and nitrogen dioxide are both molecular compounds. As molecular compound a pure substance the name implies, molecular compounds are made up of individual particles formed from two or more non-metals called molecules (Figure 2(a)). (Ionic compounds, as Figure 2(b) shows, consist of many ions in a crystal.) The chemical formula of a molecular compound gives the exact numbers of atoms in each molecule. The elements that make up molecular compounds are all non-metals. Molecular compounds are all around you: in the air that you breathe and the substances that you eat and drink (Figure 3). A soft drink, for example, contains water molecules, H2O, and sugar molecules, C12H22O11, as well as (a) (b) flavouring and colouring molecules. Carbonated soft drinks, like colas, also Figure 2 (a) Each nitrogen dioxide contain a large quantity of dissolved carbon dioxide molecules, CO2. In particle is a molecule made up of one fact, the majority of all known compounds are molecular. Living organisms nitrogen atom and two oxygen atoms. make thousands of different kinds of molecular compounds. Sugars, fats, (b) An ionic compound does not exist and proteins are all molecular compounds. Some of them are very large, as individual particles. Instead, millions of ions are tightly held together in containing thousands of atoms in a single molecule! Like nitrogen dioxide, a crystal. many molecular compounds affect our environment. C05-F32B-UBOS10SB.ai Will need photo to complete. Figure 3 Most of the chemicals that we encounter are molecular C05-F32A-UBOS10SB.ai compounds, including (a) sugar, (b) water, and (c) acetylsalicylic acid, (a) (b) (c) or Aspirin. Will need photo to complete. 206 Chapter 5 Chemicals and Their Properties NEL Bonding in Molecules In Section 5.4, you learned that ions form when metallic elements lose electrons to non-metal elements. An electron transfer occurs because the metal’s hold on its outer electrons is weak the attraction of the non-metal for the metal’s electrons is strong a full outer electron orbit is very stable Remember that non-metals all have almost-full outer electron orbits with “spaces” available that attract other electrons. This allows non-metal atoms to get relatively close to each other. When two non-metals bond with each other, both nuclei form strong attractions for the other’s electrons. However, neither atom attracts the other’s electrons strongly enough to pull them away completely. What results is a “tug of war” for electrons that neither atom ever wins. The net effect is that the two atoms share each other’s electrons, resulting in a bond that holds the atoms together. A chemical bond that results from atoms sharing electrons is called a covalent bond. The bonded atoms form a covalent bond a bond that results from molecule. Molecules that consist of two atoms joined with a covalent bond the sharing of outer electrons between non-metal atoms are called diatomic molecules. Covalent bonds can form between two identical atoms or between atoms molecule a particle in which atoms are of different elements. We will start by looking at two identical hydrogen joined by covalent bonds atoms. A hydrogen atom has one outer electron. To achieve a stable outer diatomic molecule a molecule consisting orbit like that of the nearest noble gas (helium), hydrogen must acquire one of only two atoms of either the same or different elements more electron. When two hydrogen atoms collide, the proton of one atom attracts the electron of the other and vice versa (Figure 4). Since the atoms are identical, they have the same ability to attract electrons. As a result, both electrons are shared equally between the two atoms. This results in a covalent bond between the atoms. The resulting hydrogen molecule has the chemical formula H2. A solid line linking the atoms, H—H, represents a covalent bond. 1p+ 1p+ 1p+ 1p+ Figure 4 A covalent bond results from the sharing of a pair of electrons H H H–H represented by a dash. Fluorine is another example of a diatomic molecule. Fluorine has seven outer electrons—one electron short of a stable electron arrangement. When two fluorine atoms share a pair of electrons (one from each atom) to form a covalent bond, they form a relatively stable fluorine molecule with the chemical formula F2. There are other kinds of diatomic molecules. Some are made up of atoms 1p+ 9p+ that share two pairs of electrons. Oxygen is an example of this. There is a double covalent bond joining the two atoms: O==O. Other diatomic molecules are made up of two different elements (such as hydrogen fluoride, HF, shown in Figure 5). H–F Other molecules are made up of three or more atoms. A water molecule, Figure 5 A hydrogen atom and a fluorine for example, consists of one oxygen atom and two hydrogen atoms: H2O. atom form a hydrogen fluoride molecule. NEL 5.10 Molecules and Covalent Bonding 207  LEARNING TIP Table 1 lists common elements that exist as diatomic molecules. This list will Who Is Hofbrincl? be useful when you write chemical equations in Chapter 6. Hofbrincl, more correctly written Table 1 Common Diatomic Elements as HOFBrINCl, is a name made up State at room using the chemical symbols of the Name of element Chemical symbol Formula of molecule temperature diatomic elements. It might help you remember this list of elements. hydrogen H H2 gas oxygen O O2 gas fluorine F F2 gas bromine Br Br2 liquid iodine I I2 solid nitrogen N N2 gas chlorine Cl Cl2 gas T RY THIS MOLECULAR MODELS SKILLS: Observing, Communicating Making models of molecules might help you understand how some 3. Connect one black and four white spheres. Sketch your elements form compounds. Each sphere, representing an atom, model. C has a number of connection sites. This number represents the 4. Build a model of a molecule with one oxygen atom and number of bonds that the atom can make with another atom. Each two hydrogen atoms. Sketch your model. C different colour represents a different element: white = hydrogen; 5. Build a molecule of hydrogen chloride. Sketch your red = oxygen; green = a halogen (e.g., chlorine); black = carbon. model. C Equipment and Materials: molecular model kit 6. Build any other molecule using the molecular model kit. 1. Select two white spheres and connect them together to Sketch your model. C represent a hydrogen molecule. Sketch your model. C A. Beside each sketch, write the chemical formula and, if 2. Select two red spheres and connect them together to possible, the name of the molecule. K/U represent a molecule of oxygen. Sketch your model. C Naming Molecular Compounds Unfortunately, the naming of molecular compounds is not as straightforward as the naming of ionic compounds. Many molecular (a) compounds have been known for centuries and have common names that are still in use today (Figure 6). Some of these common names are given in Table 2. Table 2 Common Names of Some Molecular Compounds (b) Chemical Common name formula Use/Occurrence water H2O the most commonly available molecular compound on Earth; the “universal solvent” ammonia NH3 used in window cleaners and in the production of fertilizers nitric oxide NO an air pollutant produced in the automobile engine when (c) gasoline is burned Figure 6 Molecular models of (a) water, hydrogen sulfide H2S an invisible gas with a distinctive “rotten eggs” odour (b) ammonia, and (c) nitric oxide 208 Chapter 5 Chemicals and Their Properties NEL  Chemists have established a system for naming molecular compounds LEARNING TIP that involves using prefixes to specify the number of atoms. The prefix Using Prefixes is attached to the name of the element to which it refers (Table 3). For Note that mono is used only for the example, the name “dinitrogen pentoxide” tells us that there are two second element in the compound. nitrogen atoms (di means two) and five oxygen atoms (penta means five) in Further, the second “o” in mono is the compound. The chemical formula for this compound, therefore, is N2O5. dropped when used with oxide to become “monoxide” rather than The prefix mono is used only for the second element in the compound, so “monooxide.” CO2 is carbon dioxide. Similarly, the name “carbon monoxide” states that there is one carbon and one oxygen atom in the molecule: CO. Table 3 Prefixes Used for Molecular Compounds Prefix Number of atoms Sample molecular compound mon(o)- 1 carbon monoxide, CO di- 2 carbon dioxide, CO2 tri- 3 sulfur trioxide, SO3 tetra- 4 carbon tetrachloride, CCl4 penta- 5 phosphorus pentafluoride, PF5 When you are asked to write the name of a compound, first check the formula to see if it includes a metal. If the first element is a metal, the substance is an ionic compound and should be named accordingly (with no prefixes). If the compound consists only of non-metals, it is a molecular compound, and you should follow these steps to name it. WRITING TIP SAMPLE PROBLEM 1 Naming Molecular Compounds Concluding Your Summary Name the molecular compound with chemical formula PCl3. Write a closing sentence that connects the main idea and key points. For Step 1 Write the names of both elements in the same order as in the formula. Replace example, “The use of prefixes helps the ending of the second element with “ide.” students remember the number of phosphorus chlorine ide atoms in a molecular compound.” Step 2 Add prefixes. Remember that the prefix “mono” is never used for the first element. phosphorus tri chloride The compound with chemical formula PCl3 is called phosphorus trichloride. Practice Name the compound with chemical formula N2O. sulfur dioxide SO2 Writing Chemical Formulas of Molecular Figure 7 In the chemical name, the Compounds prefix specifying the number of atoms Given its name, writing the formula of a molecular compound is relatively comes before the element’s name. In the chemical formula, however, the simple. The prefixes in the name become the subscripts in the formula. For number of atoms is specified by a example, the molecular compound called sulfur dioxide has the chemical subscript after the element’s chemical formula SO2 (Figure 7). symbol. NEL 5.10 Molecules and Covalent Bonding 209  Molecular Compounds from Fossil Fuels Most compounds are molecular. Living things make a huge variety of different molecular compounds. Another source of thousands of different molecules is fossil fuels. Coal, oil, and natural gas are the most common fossil fuels. These substances take millions of years to form from the partially decayed remains of ancient plants and animals. Fossil fuels are called a non-renewable resource because they are not formed as quickly as we are using them. Fossil fuels have become very important to our way of life. When we burn fossil fuels, the energy stored within them heats our homes, powers our vehicles, and can be harnessed to generate electricity. Without the energy released from fossil fuels, our lives would be very different. But there is more to fossil fuels than just their energy. Compounds extracted from fossil fuels are processed into petrochemicals. We use these compounds to make important consumer products and industrial chemicals, including plastics, Figure 8 Even something as ordinary as a backpack could contain hundreds pharmaceuticals, and synthetic fabrics (Figures 8 and 9). of different molecular compounds, most Try to imagine life without the products from petrochemicals. Half of your of them made from fossil fuels. clothing is now gone! Personal products like toiletries, cosmetics, and their containers no longer exist. The paint on the walls and the synthetic carpet on the floor—gone. Do you need medication regularly? Too bad! Many drugs are made using petrochemicals. And what about communication? Computers, phones, and all portable electronic devices cannot function without their plastic cases and the insulation covering their electrical parts. fuels and other agriculture fertilizers Natural Gas Petrochemical products (helium, pesticides Processing Plant natural gas) herbicides gases methane detergents ethane propane synthetic fibres butane inorganic sulfuric acid chemicals ammonia nitric acid organic chemicals other products paints varnishes solvents Oil Refinery gases and liquids adhesives explosives fuels gasoline kerosene plastics jet fuel heating oils rubber lubricants and greases other products waxes solvents asphalt sulfur Figure 9 Crude oil and natural gas are the raw materials for a vast range of chemical products. 210 Chapter 5 Chemicals and Their Properties NEL Spills and Leaks of Molecular Compounds DID YOU KNOW? The world’s oil and natural gas deposits are concentrated in a few places, Petrodollars far from the places where they are sold and used. Most of Canada’s oil is Annual sales from the global petrochemical industry are estimated in the west and north of the country, while most Canadian consumers to be over $1 trillion. With a global live in the south and east. As a result, huge quantities of oil and natural population of almost 7 billion, that is gas are carried across North America and around the world. Oil is about $150 for every person in the transported by rail, ship, or pipeline. Natural gas is world. transported by pipeline or in specially designed tankers. Accidents are inevitable. Loaded oil tankers travel across oceans and through the Great Lakes. Spills from oil tankers can have a devastating effect on local ecosystems, contaminating water and shorelines and killing birds and other aquatic organisms (Figure 10). A lot of research is done on the best way to deal with the leaked chemicals. Accidental spills and poor waste disposal methods leak toxins into the ground, where they pollute groundwater. One such toxin is a molecular compound called trichloroethene. Trichloroethene is widely used in industry as a degreaser for cleaning metal and glass. Spills of this compound are a serious problem in Canada because they can contaminate the groundwater. Nearly 9 million Canadians rely on groundwater for their drinking water. Canadian researchers are working on innovative ways to clean groundwater. Dr. Elizabeth Edwards, a professor of chemical engineering at the University of Toronto, has Figure 10 Spilled oil is a frequent environmental problem. found that some pollutants are food for microbes. Dr. Edwards has successfully used microbes to remove trichloroethene University professors are often involved in solving real-world from soil. As the microbes “dine,” they convert the pollutant into ethene, problems. To find out more about Dr. a relatively harmless gas. These microbes are now being used to remove Edwards’ research, trichloroethene from polluted sites around the world. GO TO NELSON SCIENCE RESEARCH THIS SLICKS FROM SHIPS SKILLS: Researching, Identifying Alternatives, Analyzing the Issue, Defending a Decision, Evaluating SKILLS HANDBOOK 4.A., 4.C. Accidental spills of oil can be devastating for the environment. 1. Research the role of chemical methods, physical methods, Environmental engineers develop strategies to clean up oil spills, and biological agents in cleaning an oil spill. using their knowledge of oil’s chemical and physical properties. GO TO NELSON SCIENCE GO TO NELSON SCIENCE A. Analyze the advantages and disadvantages of each clean-up strategy. Which strategies are the most Recently, biological agents have also been successfully economically viable? Which strategies are the most used to clean up oil spills. Naturally occurring micro-organisms environmentally friendly? T/I A such as algae and bacteria help break down the spilled oil. This B. Which method can be used to clean up the majority of process is very slow but can be sped up by adding fertilizer to a spill on calm water? T/I the contaminated area. NEL 5.10 Molecules and Covalent Bonding 211  IN SUMMARY Molecular compounds both occur naturally and The name of a molecular compound includes are produced synthetically. Some are beneficial; prefixes to indicate how many atoms of each others are not. element are present (for example, dinitrogen Molecular compounds are made up of molecules. pentoxide). If there is only one atom of the first A molecule is a group of two or more atoms element, the prefix “mono” is omitted. The name joined by covalent bonds. of the last element ends in “ide.” A covalent bond forms when two non-metallic Fossil fuels provide valuable energy and atoms share electrons. petrochemicals. Many important industrial chemicals and consumer products are made A variety of strategies are used to reduce the from petrochemicals. environmental damage resulting from spilled chemicals. CHECK YOUR LEARNING 1. (a) Name these compounds: NI3, CCl4, OF 2, P2O5, and N2O3. 7. (a) Why are fossil fuels a non-renewable resource? (b) Describe how each compound name indicates the ratio of (b) What are the two main benefits that we get from fossil elements. K/U fuels? 2. Write the chemical formula for each of the following (c) What are two disadvantages of our dependence on fossil molecular compounds: K/U fuels? K/U A (a) carbon monoxide (d) nitrogen tribromide 8. Explain why chlorine occurs as diatomic molecules in nature, (b) sulfur tetrafluoride (e) carbon disulfide rather than individual atoms. K/U (c) dinitrogen tetroxide 9. Hydrogen peroxide, H2O2, is a molecular compound used to disinfect cuts (Figure 11). Why is the formula of this 3. For each of the following compounds, classify the elements compound not written as HO? A as metal or non-metal, classify the compound as ionic or molecular, and name the compound. K/U (a) SO2 (e) KClO3 (b) PbO2 (f) SnO2 (c) AlCl3 (g) FePO4 (d) N2O (h) N2O4 4. (a) How many electrons do atoms of hydrogen and oxygen have in their outer orbits? Figure 11 (b) How many electrons will these elements gain before they become stable? 10. How can molecular compounds be distinguished from ionic (c) Sketch a diagram to show how hydrogen and oxygen compounds could bond to form a stable molecule. K/U (a) by looking at their chemical formulas? 5. Explain, with diagrams, why the term “molecule” is (b) by testing them in the lab? K/U appropriate for hydrogen chloride but not for 11. What effect could C05-F48-UBOS10SB.ai a disruption in the supply of oil have on the sodium chloride. K/U cost of goods you purchase? Why? A 6. Contrast the way in which the elements in ionic and molecular compounds achieve stability. K/U 212 Chapter 5 Chemicals and Their Properties NEL Ontario Science 10 SB