5.10 Molecules and Covalent Bonding

Transcription

1 510 Molecules and Covalent Bonding Nitrous oxide, N 2 O, is a colourless, sweet-smelling that dentists sometimes use to relax nervous patients (Figure 1(a)) You would not, however, want to inhale nitrogen dioxide, NO 2 This reddish-brown toxic 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, helps dental patients relax (b) Nitrogen dioxide is the responsible for the reddish-brown colour of smog over large cities during the summer (a) (b) To fi nd out more about the Ontario Drive Clean Program, molecular compound a pure substance formed from two or more non-metals (a) (b) Figure 2 (a) Each nitrogen dioxide particle is a molecule made up of one nitrogen atom and two oxygen atoms (b) An ionic compound does not exist as individual particles Instead, millions of ions are tightly held together in a crystal Pollution control technology in modern cars helps reduce nitrogen dioxide emissions A catalytic converter, attached to the car s exhaust system, converts nitrogen dioxide into harmless nitrogen and oxygen The Ontario Drive Clean Program requires that most cars over five years old pass an emissions test every two years This helps reduce pollution in Ontario Nitrous oxide and nitrogen dioxide are both molecular compounds As the name implies, molecular compounds are made up of individual particles 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, H 2 O, and sugar molecules, C 12 H 22 O 11, as well as flavouring and colouring molecules Carbonated soft drinks, like colas, also contain a large quantity of dissolved carbon dioxide molecules, CO 2 In fact, the majority of all known compounds are molecular Living organisms make thousands of different kinds of molecular compounds Sugars, fats, and proteins are all molecular compounds Some of them are very large, containing thousands of atoms in a single molecule! Like nitrogen dioxide, many molecular compounds affect our environment Figure 3 Most of the chemicals that we encounter are molecular compounds, including (a) sugar, (b) water, and (c) acetylsalicylic acid, or Aspirin (a) (b) (c) 206 Chapter 5 Chemicals and Their Properties NEL

2 Bonding in Molecules In Section 54, 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 molecule Molecules that consist of two atoms joined with a covalent bond are called diatomic molecules Covalent bonds can form between two identical atoms or between atoms of different elements We will start by looking at two identical hydrogen atoms A hydrogen atom has one outer electron To achieve a stable outer orbit like that of the nearest noble (helium), hydrogen must acquire one 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 H 2 A solid line linking the atoms, H H, represents a covalent bond covalent bond a bond that results from the sharing of outer electrons between non-metal atoms molecule a particle in which atoms are joined by covalent bonds diatomic molecule a molecule consisting of only two atoms of either the same or different elements H H H H Figure 4 A covalent bond results from the sharing of a pair of electrons 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 F 2 There are other kinds of diatomic molecules Some are made up of atoms 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) Other molecules are made up of three or more atoms A water molecule, for example, consists of one oxygen atom and two hydrogen atoms: H 2 O 9p + H F Figure 5 A hydrogen atom and a fluorine atom form a hydrogen fluoride molecule NEL 510 Molecules and Covalent Bonding 207

3 LEARNING TIP Who Is Hofbrincl? Hofbrincl, more correctly written as HOFBrINCl, is a name made up using the chemical symbols of the diatomic elements It might help you remember this list of elements Table 1 lists common elements that exist as diatomic molecules This list will be useful when you write chemical equations in Chapter 6 Table 1 Common Diatomic Elements Name of element Chemical symbol Formula of molecule hydrogen H H 2 State at room temperature oxygen O O 2 fl uorine F F 2 bromine Br Br 2 liquid iodine I I 2 solid nitrogen N N 2 chlorine Cl Cl 2 TRY THIS MOLECULAR MODELS SKILLS: Observing, Communicating Making models of molecules might help you understand how some elements form compounds Each sphere, representing an atom, has a number of connection sites This number represents the number of bonds that the atom can make with another atom Each different colour represents a different element: white = hydrogen; red = oxygen; green = a halogen (eg, chlorine); black = carbon Equipment and Materials: molecular model kit 1 Select two white spheres and connect them together to represent a hydrogen molecule Sketch your model C 2 Select two red spheres and connect them together to represent a molecule of oxygen Sketch your model C 3 Connect one black and four white spheres Sketch your model C 4 Build a model of a molecule with one oxygen atom and two hydrogen atoms Sketch your model C 5 Build a molecule of hydrogen chloride Sketch your model C 6 Build any other molecule using the molecular model kit Sketch your model C A Beside each sketch, write the chemical formula and, if possible, the name of the molecule K/U (a) Naming Molecular Compounds Unfortunately, the naming of molecular compounds is not as straightforward as the naming of ionic compounds Many molecular 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) Common name Chemical formula Use/Occurrence water H 2 O the most commonly available molecular compound on Earth; the universal solvent ammonia NH 3 used in window cleaners and in the production of fertilizers (c) Figure 6 Molecular models of (a) water, (b) ammonia, and (c) nitric oxide nitric oxide NO an air pollutant produced in the automobile engine when oline is burned hydrogen sulfi de H 2 S an invisible with a distinctive rotten eggs odour 208 Chapter 5 Chemicals and Their Properties NEL

4 Chemists have established a system for naming molecular compounds that involves using prefixes to specify the number of atoms The prefix is attached to the name of the element to which it refers (Table 3) For example, the name dinitrogen pentoxide tells us that there are two nitrogen atoms (di means two) and five oxygen atoms (penta means five) in the compound The chemical formula for this compound, therefore, is N 2 O 5 The prefix mono is used only for the second element in the compound, so CO 2 is carbon dioxide Similarly, the name carbon monoxide states that there is one carbon and one oxygen atom in the molecule: CO LEARNING TIP Using Prefixes Note that mono is used only for the second element in the compound Further, the second o in mono is dropped when used with oxide to become monoxide rather than monooxide Table 3 Prefi xes Used for Molecular Compounds Prefix Number of atoms Sample molecular compound mon(o)- 1 carbon monoxide, CO di- 2 carbon dioxide, CO 2 tri- 3 sulfur trioxide, SO 3 tetra- 4 carbon tetrachloride, CCl 4 penta- 5 phosphorus pentafl uoride, PF 5 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 SAMPLE PROBLEM 1 Naming Molecular Compounds Name the molecular compound with chemical formula PCl 3 Step 1 Write the names of both elements in the same order as in the formula Replace the ending of the second element with ide phosphorus chlorine ide Step 2 Add prefi xes Remember that the prefi x mono is never used for the fi rst element phosphorus tri chloride The compound with chemical formula PCl 3 is called phosphorus trichloride Practice Name the compound with chemical formula N 2 O WRITING TIP Concluding Your Summary Write a closing sentence that connects the main idea and key points For example, The use of prefi xes helps students remember the number of atoms in a molecular compound sulfur dioxide Writing Chemical Formulas of Molecular Compounds Given its name, writing the formula of a molecular compound is relatively simple The prefixes in the name become the subscripts in the formula For example, the molecular compound called sulfur dioxide has the chemical formula SO 2 (Figure 7) SO 2 Figure 7 In the chemical name, the prefi x specifying the number of atoms comes before the element s name In the chemical formula, however, the number of atoms is specifi ed by a subscript after the element s chemical symbol NEL 510 Molecules and Covalent Bonding 209

5 Figure 8 Even something as ordinary as a backpack could contain hundreds of different molecular compounds, most of them made from fossil fuels 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 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, pharmaceuticals, and synthetic fabrics (Figures 8 and 9) Try to imagine life without the products from petrochemicals Half of your 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 Natural Gas Processing fuels and other products (helium, natural ) Petrochemical Plant agriculture fertilizers pesticides herbicides es methane ethane propane butane detergents synthetic fibres inorganic chemicals sulfuric acid ammonia nitric acid organic chemicals Oil Refinery es and liquids fuels oline kerosene jet fuel heating oils other products plastics rubber paints varnishes solvents adhesives explosives lubricants and other products greases waxes solvents asphalt sulfur Figure 9 Crude oil and natural are the raw materials for a vast range of chemical products 210 Chapter 5 Chemicals and Their Properties NEL

6 Spills and Leaks of Molecular Compounds The world s oil and natural deposits are concentrated in a few places, far from the places where they are sold and used Most of Canada s oil is in the west and north of the country, while most Canadian consumers live in the south and east As a result, huge quantities of oil and natural are carried across North America and around the world Oil is transported by rail, ship, or pipeline Natural is 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 found that some pollutants are food for microbes Dr Edwards has successfully used microbes to remove trichloroethene from soil As the microbes dine, they convert the pollutant into ethene, a relatively harmless These microbes are now being used to remove trichloroethene from polluted sites around the world DID YOU KNOW? Petrodollars Annual sales from the global petrochemical industry are estimated to be over $1 trillion With a global population of almost 7 billion, that is about $150 for every person in the world Figure 10 Spilled oil is a frequent environmental problem University professors are often involved in solving real-world problems To fi nd out more about Dr Edwards research, RESEARCH THIS SLICKS FROM SHIPS SKILLS: Researching, Identifying Alternatives, Analyzing the Issue, Defending a Decision, Evaluating Accidental spills of oil can be devastating for the environment Environmental engineers develop strategies to clean up oil spills, using their knowledge of oil s chemical and physical properties SKILLS HANDBOOK 4A, 4C 1 Research the role of chemical methods, physical methods, and biological agents in cleaning an oil spill Recently, biological agents have also been successfully used to clean up oil spills Naturally occurring micro-organisms such as algae and bacteria help break down the spilled oil This process is very slow but can be sped up by adding fertilizer to the contaminated area A Analyze the advantages and disadvantages of each clean-up strategy Which strategies are the most economically viable? Which strategies are the most environmentally friendly? T/I A B Which method can be used to clean up the majority of a spill on calm water? T/I NEL 510 Molecules and Covalent Bonding 211

7 IN SUMMARY Molecular compounds both occur naturally and are produced synthetically Some are beneficial; others are not Molecular compounds are made up of molecules A molecule is a group of two or more atoms joined by covalent bonds A covalent bond forms when two non-metallic atoms share electrons A variety of strategies are used to reduce the environmental damage resulting from spilled chemicals The name of a molecular compound includes prefixes to indicate how many atoms of each element are present (for example, dinitrogen pentoxide) If there is only one atom of the first element, the prefix mono is omitted The name of the last element ends in ide Fossil fuels provide valuable energy and petrochemicals Many important industrial chemicals and consumer products are made from petrochemicals CHECK YOUR LEARNING 1 (a) Name these compounds: NI 3, CCl 4, OF 2, P 2 O 5, and N 2 O 3 (b) Describe how each compound name indicates the ratio of elements K/U 2 Write the chemical formula for each of the following molecular compounds: K/U (a) carbon monoxide (d) nitrogen tribromide (b) sulfur tetrafl uoride (e) carbon disulfi de (c) dinitrogen tetroxide 3 For each of the following compounds, classify the elements as metal or non-metal, classify the compound as ionic or molecular, and name the compound K/U 7 (a) Why are fossil fuels a non-renewable resource? (b) What are the two main benefi ts that we get from fossil fuels? (c) What are two disadvantages of our dependence on fossil fuels? K/U A 8 Explain why chlorine occurs as diatomic molecules in nature, rather than individual atoms K/U 9 Hydrogen peroxide, H 2 O 2, is a molecular compound used to disinfect cuts (Figure 11) Why is the formula of this compound not written as HO? A (a) SO 2 (b) PbO 2 (e) KClO 3 (f) SnO 2 (c) AlCl 3 (d) N 2 O (g) FePO 4 (h) N 2 O 4 4 (a) How many electrons do atoms of hydrogen and oxygen have in their outer orbits? (b) How many electrons will these elements gain before they become stable? (c) Sketch a diagram to show how hydrogen and oxygen could bond to form a stable molecule K/U 5 Explain, with diagrams, why the term molecule is appropriate for hydrogen chloride but not for sodium chloride K/U 6 Contrast the way in which the elements in ionic and molecular compounds achieve stability K/U Figure How can molecular compounds be distinguished from ionic compounds (a) by looking at their chemical formulas? (b) by testing them in the lab? K/U 11 What effect could a disruption in the supply of oil have on the cost of goods you purchase? Why? A 212 Chapter 5 Chemicals and Their Properties NEL