Observations in Science
2.8 Polyethylene
Once we know how to name both of the ions, an ionic compound is named sim- ply by combining the two names. The cation is listed fi rst in the formula unit and in the name. Example Problem 2.6 provides some examples of the way to determine the name of an ionic compound.
E X A M P L E P RO B L E M 2 . 6
Determine the name of the following ionic compounds: (a) Fe2O3, (b) Na2O, (c) Ca(NO3)2
Strategy We must determine the names of the constituent ions fi rst. The anions will provide a hint about the cation charges if we need it.
Solution
(a) Fe2O3: As noted in Table 2.6, oxygen is always a 2− ion in these compounds, so there is a total charge of 6− on the three oxide ions in the formula unit. Therefore, the two iron ions must have a total charge of 6+, requiring 3+ from each iron. So the name is iron(III) oxide.
(b) Na2O: Sodium from Group 1 always has a 1+ charge and oxygen always 2−. Therefore, the name is sodium oxide. No Roman numeral is needed for sodium because it has only one common ionic charge.
(c) Ca(NO3)2: The calcium ion is in Group 2 and always carries a charge of 2+. NO3− is a common polyatomic anion called nitrate. The name is calcium nitrate.
Check Your Understanding Name each of the following ionic compounds:
(a) CuSO4, (b) Ag3PO4, (c) V2O5
Occasionally throughout this text, we will encounter new classes of chemical compounds that will need more rules to determine their names. We will introduce these nomenclature systems when necessary.
INSIGHT INTO
that easily breaks down, producing highly reactive fragments called free radicals.
(The free radical is denoted as ‘R?’ below, where the ‘?’ represents an unshared elec- tron.) One of these free radicals attaches itself to a single ethylene molecule, opening its double bond and leaving one end unbonded.
C R C C
H H H
H
C? R?
H H
H H
The unbonded end of the ethylene now takes on the role of the free radical, attack- ing a second ethylene monomer and attaching it to the growing chain. As long as the number of available ethylene monomers remains large, the polymer can continue to grow in this fashion.
C C C
H H H
H
C? H H
H H C
C C? H H
H H R
H
H C R
H
H
Linking together thousands of monomers in this way would generate a single poly- ethylene molecule, whose structural formula would look like that below, only extend- ing over a much greater chain length.
C C H H
H H C
H
H C H
H C C H H
H H C
H
H C H
H C H
H C H
H
Eventually, the free radical end of the growing chain will meet up with another free radical, either from an initiator molecule or from another growing chain. When this happens, the chain will stop growing. When the monomer units grow end to end, as shown above, the result is known as linear polyethylene because all of the carbon atoms lie along a more or less straight backbone.
It is also possible to grow polyethylene under conditions that lead to branched chains, in which some of the hydrogen atoms along the backbone are themselves replaced by polyethylene-like chains. The contrasting forms of the linear and branched molecules are shown in Figure 2.17.
Although linear polyethylene is conceptually simpler, branched polyethylene is actually cheaper and easier to produce. Perhaps surprisingly, the linear and branched forms of polyethylene are actually polymers with signifi cantly different macroscopic properties. When arranged side by side, the long, linear polyethylene molecules can pack together very tightly, producing a relatively dense plastic. So linear polyethylene is also known as high-density polyethylene, or HDPE. It is a strong and hard mate- rial, used in bottles, kitchenware, and as a structural plastic in many children’s toys.
Because their chains cannot be stretched out straight, branched polyethylene mole- cules cannot be packed together as closely as those of linear polyethylene. This looser packing of the molecules produces a plastic with a much lower density, so branched polyethylene is usually referred to as LDPE, for low-density polyethylene. LDPE is commonly used in applications where relatively little strength is needed, such as plas- tic fi lms, sandwich bags, and squeeze bottles.
Recent advances in polymerization technology have made it possible to grow extremely long linear polyethylene chains, stretching to hundreds of thousands of monomer units. Because the individual molecules are relatively large and heavy, this has been called “ultra-high molecular weight polyethylene,” or UHMWPE. The very
long chains are so strong that this material is replacing Kevlar® (another polymer) as the standard fi lling for bulletproof vests. UHMWPE can also be formed into large sheets, and these have been used as ice substitutes for skating rinks.
This quick glimpse at some of the many forms and uses for polyethylene shows how the observable properties of polymers are closely linked to the chemical structure of the individual molecules. It also illustrates how the interplay between chemistry and engineering can lead to polymers that are selectively produced to meet certain design specifi cations.
FO C U S O N P RO B L E M S O LV I N G
Question Boron is widely used in the production of enamels and glasses. Naturally occurring boron has an average atomic mass of 10.811 amu. If the only isotopes pres- ent are 10B and 11B, describe how you would determine their relative abundances.
Include in your description any information that you would need to look up.
Strategy This problem is the inverse of those in this chapter in which we used isotopic abundances to fi nd atomic masses. Here we must work from the atomic mass to fi nd the isotopic abundances. Since there are two isotopes, we have two unknowns. So we will need to write two equations that relate the percentages or fractions of 10B and 11B. We would also need to know the mass of each isotope, but presumably we could look those up.
Solution The abundances of the two isotopes must add to 100%. This gives us a fi rst equation:
(Fraction of 10B) + (fraction of 11B) = 1.00
The fact that the average atomic mass is 10.811 provides another equation:
(Fraction of 10B) × (mass of 10B) + (fraction of 11B) × (mass of 11B) = 10.811 amu Figure 2.17 ❚ Differences
between linear and branched polyethylene are illustrated. The left-hand panel shows linear, or high-density polyethylene, and the right-hand panel shows the branched, or low-density form.
In each case, the upper diagram illustrates the molecular structure for part of a polymer chain. The lower diagrams show the way that polymer chains would pack together to form the solid plastic. (Hydrogen atoms are omitted in these drawings for clarity.) Branched chains cannot approach one another as closely, so the resulting material has a much lower density.
Linear polyethylene (high-density)
Branched polyethylene (low-density)
Assuming that we are able to fi nd values for the masses of the two isotopes, we now have two equations in two unknowns, so the problem can be solved. (We could get a rough approximation by assuming that 10B has a mass of 10 amu and 11B has a mass of 11 amu, but that assumption would not lead to a very accurate result.)
S U M M A RY
The widely taught description of an atom as a massive, positively charged nucleus surrounded by lighter, fast moving electrons is based on the results of many ingenious experiments. Several details of this model are important in building our knowledge of chemistry. The number of protons in a nucleus determines the chemical identity of an atom, and must be equal to the number of electrons so that the atom is electrically neutral. If the num- bers of protons and electrons are not the same, the result is a charged particle, which is called an ion. Because of their electric charge, ions behave quite differently from neutral atoms. For example, the sodium in our diet is invariably sodium ions, not sodium atoms.
Electric charge plays a central role in determining the struc- ture of both atoms and molecules. The attraction of oppositely charged particles and the repulsion of like charged particles, as described by Coulomb’s law, is central to our understanding of the way atoms stick together, that is, chemical bonding. The vari- ous types of chemical bonds, including covalent, ionic, and me- tallic, can all be understood in terms of the interactions between negative and positive charges.
Once we establish that atoms will bond together to form chemical compounds, we are faced with the burden of summa- rizing a vast number of compounds. Many classifi cation schemes
have been developed over the years to assist us in organizing data on a wide range of chemical substances. The periodic table is the most common and important device for such a purpose.
It succinctly summarizes many properties of the elements, espe- cially their chemical tendencies. The periodic table also helps us learn chemistry by providing a template for trends. Thus we can remember that metallic elements are found toward the left and bottom of the table while nonmetals are found toward the upper right. Without the periodic table, remembering which elements are in each category would be much more challenging.
Other categorizations also help organize our study of chem- istry. In some cases, broad categories such as organic versus inor- ganic chemistry are helpful. For example, chemical nomenclature, the system we use to name compounds, is different for these two branches of chemistry. The way that we impart information about molecules symbolically also varies. In organic chemistry, because carbon is involved in all of the molecules, we use the fact that car- bon always forms four chemical bonds to simplify the depiction of molecules as line structures. For inorganic chemistry, in which we often encounter binary chemical compounds, we can devise a fairly broad system of nomenclature based on a relatively small number of rules. Learning this system is an important step in chemical communication.
K E Y T E R M S
actinides (2.5) addition reaction (2.6) alkali metals (2.5) alkaline earths (2.5) anion (2.3)
atomic mass unit (2.2) atomic number (2.2) binary compound (2.7) cation (2.3)
chemical bond (2.4) chemical compound (2.4) chemical formula (2.4) chemical nomenclature (2.7) Coulomb’s law (2.3) covalent bonds (2.4) electrons (2.2) empirical formula (2.4)
formula unit (2.4) free radicals (2.8) functional group (2.6) groups (2.5)
halogens (2.5) HDPE (2.8) hydrates (2.4) hydrocarbons (2.6) inorganic chemistry (2.6) ion (2.3)
ionic bonding (2.4) isotopes (2.2)
isotopic abundance (2.2) lanthanides (2.5) lattice (2.4) LDPE (2.8) line structure (2.6)
main group elements (2.5) mass number (2.2) metal (2.5)
metallic bonding (2.4) metalloid (2.5)
molecular formula (2.4) molecule (2.4)
monomer (2.1) neutrons (2.2) noble gases (2.5) nonmetal (2.5) nucleus (2.2)
organic chemistry (2.6) oxyanions (2.7) periodic law (2.5) periodicity (2.5) periods (2.5)
polyatomic ions (2.7) polymer backbone (2.1) protons (2.2)
representative elements (2.5) semimetals (2.5)
transition metals (2.5) UHMWPE (2.8)
P RO B L E M S A N D E X E RC I S E S
■ denotes problems assignable in OWL.
INSIGHT INTO Polymers
2.1 Defi ne the terms polymer and monomer in your own words.
2.2 How do polymers compare to their respective monomers?
2.3 Look around you and identify several objects that you think are probably made from polymers.
2.4 Which one element forms the backbone of nearly all com- mon polymers? Which other elements are also found in common household polymers?
2.5 The fact that a polymer’s physical properties depend on its atomic composition is very important in making these ma- terials so useful. Why do you think this would be so?
2.6 Use the web to research the amount of PVC polymer pro- duced annually in the United States. What are the three most common uses of this polymer?
2.7 Use the web to research the amount of polyethylene pro- duced annually in the United States. What are the three most common uses of this polymer?
Atomic Structure and Mass
2.8 In a typical illustration of the atom such as Figure 2.3, which features lead to misunderstandings about the struc- ture of atoms? Which ones give important insight?
2.9 Why is the number of protons called the atomic number?
2.10 ■ Which isotope in each pair contains more neutrons?
(a) chlorine-35 or sulfur-33, (b) fl uorine-19 or neon-19, (c) copper-63 or zinc-65, (d) iodine-126 or tellurium-127 2.11 Defi ne the term isotope.
2.12 ■ Write the complete atomic symbol for each of the following isotopes: (a) carbon-13, (b) phosphorus-31, (c) sodium-23, (d) boron-10
2.13 ■ How many electrons, protons, and neutrons are there in each of the following atoms? (a) magnesium-24, 24Mg, (b) tin-119, 119Sn, (c) thorium-232, 232Th, (d) carbon-13, 13C, (e) copper-63, 63Cu, (f) bismuth-205, 205Bi
2.14 ■ Consider the following nuclear symbols. How many protons, neutrons, and electrons does each element have?
What elements do R, T, and X represent?
(a) 30 14 R (b) 89 39 T (c) 133 55 X
2.15 Mercury is 16.716 times more massive than carbon-12.
What is the atomic mass of mercury? Remember to ex- press your answer with the correct number of signifi cant fi gures.
2.16 How can an element have an atomic mass that is not an integer?
2.17 Explain the concept of a “weighted” average in your own words.
2.18 ■ The element gallium, used in gallium arsenide semi- conductors, has an atomic mass of 69.72 amu. There are only two isotopes of gallium, 69Ga with a mass of 68.9257 amu and 71Ga with a mass of 70.9249 amu. What are the isotopic abundances of gallium?
Gallium melts just above room temperature.
2.19 ■ The atomic mass of copper is 63.55 amu. There are only two isotopes of copper, 63Cu with a mass of 62.93 amu and
65Cu with a mass of 64.93 amu. What is the percentage abundance of each of these two isotopes?
2.20 The following table presents the abundances and masses of the isotopes of zinc. What is the atomic mass of zinc?
Isotope Abundance Mass
64Zn 48.6% 63.9291 amu
66Zn 27.9% 65.9260 amu
67Zn 4.10% 66.9271 amu
68Zn 18.8% 67.9249 amu
70Zn 0.60% 69.9253 amu
2.21 ■ Naturally occurring uranium consists of two isotopes, whose masses and abundances are shown below.
Isotope Abundance Mass
235U 0.720% 235.044 amu
238U 99.275% 238.051 amu
Only 235U can be used as fuel in a nuclear reactor, so ura- nium for use in the nuclear industry must be enriched in this isotope. If a sample of enriched uranium has an aver- age atomic mass of 235.684 amu, what percentage of 235U is present?
© Cengage Learning/Charles D. Winters© Cengage Learning/Charles D. Winters
Ions
2.22 Defi ne the term ion in your own words.
2.23 What is the difference between cations and anions?
2.24 ■ Provide the symbol of the following monatomic ions, given the number of protons and electrons in each:
(a) 8 protons, 10 electrons, (b) 20 protons, 18 electrons, (c) 53 protons, 54 electrons, (d) 26 protons, 24 electrons 2.25 ■ How many protons and electrons are in each of the
following ions? (a) Na+, (b) Al3+, (c) S2−, (d) Br−
2.26 Identify each of the following species as an anion, a cation, or a molecule: (a) CO32−, (b) CO2, (c) NH4+, (d) N3−, (e) CH3COO−
2.27 ■ Write the atomic symbol for the element whose ion has a 2– charge, has 20 more neutrons than electrons, and has a mass number of 126.
2.28 In what region of the periodic table are you likely to fi nd elements that form more than one stable ion?
2.29 ■ Give the symbol, including the correct charge, for each of the following ions. (a) barium ion, (b) titanium(IV) ion, (c) phosphate ion, (d) hydrogen carbonate ion, (e) sulfi de ion, (f) perchlorate ion, (g) cobalt(II) ion, (h) sulfate ion 2.30 An engineer is designing a water softening unit based on
ion exchange. Use the web to learn what ions typically are “exchanged” in such a system. Given that the ion ex- changer cannot build up a large positive charge, what can you conclude about the relative numbers of the various ions involved?
2.31 Use the web to fi nd a catalyst for a polymerization reac- tion that uses an ion. What are the apparent advantages of using this catalyst for creating the polymer?
2.32 Using Coulomb’s law, explain how the difference between attractive and repulsive interactions between ions is ex- pressed mathematically.
Compounds and Chemical Bonds
2.33 How many atoms of each element are represented in the formula Ba(OH)2?
2.34 Which of the following formulas contains the most hydro- gen atoms? C2H6, (NH4)2CO3, H2SO4, or Fe(OH)3. 2.35 In general, how are electrons involved in chemical
bonding?
2.36 What is the difference between an ionic bond and a cova- lent bond?
2.37 When talking about the formula of an ionic compound, why do we typically refer to a formula unit rather than a molecule?
2.38 Which formula below is correct for an ionic compound?
What is incorrect about each of the others? (a) AlO3/2, (b) Ca2Br4, (c) Mg(PO4)3/2, (d) BaCO3
2.39 Explain the differences between ionic and metallic bonding.
2.40 Conduction of electricity usually involves the movement of electrons. Based on the concept of metallic bonding, ex- plain why metals are good conductors of electricity.
2.41 Describe how a covalently bonded molecule is different from compounds that are either ionic or metallic.
2.42 Explain the difference between a molecular formula and an empirical formula.
2.43 Why are empirical formulas preferred for describing poly- mer molecules?
2.44 The molecular formula for the ethylene monomer is C2H4. What is its empirical formula?
2.45 Polybutadiene is a synthetic elastomer, or rubber. The corresponding monomer is butadiene, which has the mo- lecular formula C4H6. What is the empirical formula of butadiene?
The Periodic Table
2.46 What distinguished the work of Mendeleev that caused scientists to accept his concept of the periodic table when others before him were not believed?
2.47 How does the periodic table help to make the study of chemistry more systematic?
2.48 What is a period in the periodic table? From what does it derive its name?
2.49 How do binary compounds with hydrogen illustrate the concept of periodicity?
2.50 ■ Of the following elements, which two would you expect to exhibit the greatest similarity in physical and chemical properties? Cl, P, S, Se, Ti. Explain your choice.
2.51 Name the group to which each of the following elements belongs: (a) K, (b) Mg, (c) Ar, (d) Br
2.52 What are some of the physical properties that distinguish metals from nonmetals?
2.53 ■ Identify the area of the periodic table in which you would expect to find each of the following types of elements.
(a) a metal, (b) a nonmetal, (c) a metalloid
2.54 Why are nonmetals important even though they account for only a very small fraction of the elements in the peri- odic table?
2.55 What is a metalloid?
2.56 A materials engineer has fi led for a patent for a new alloy to be used in golf club heads. The composition by mass ranges from 25 to 31% manganese, 6.3 to 7.8% aluminum, 0.65 to 0.85% carbon, and 5.5 to 9.0% chromium, with the re- mainder being iron. What are the maximum and minimum percentages of iron possible in this alloy? Use Figure 2.12 to make a prediction about how the density of this alloy would compare with that of iron; justify your prediction.
2.57 ■ Classify the following elements as metals, metalloids, or nonmetals: (a) Si, (b) Zn, (c) B, (d) N, (e) K, (f) S
2.58 A materials engineer wants to make a new material by tak- ing pure silicon and replacing some fraction of the silicon atoms with other atoms that have similar chemical proper- ties. Based on the periodic table, what elements probably should be tried fi rst?
Inorganic and Organic Chemistry
2.59 The chemistry of main group elements is generally sim- pler than that of transition metals. Why is this so?
2.60 Calcium and fl uorine combine to produce ionic calcium fl uoride, CaF2. Use the periodic table to predict at least
two other compounds that you would expect to have struc- tures similar to that of CaF2.
2.61 What is meant by the phrase organic chemistry?
2.62 Based on what you’ve learned in this chapter, would you clas- sify the chemistry of polymers as organic or inorganic? Why?
2.63 What is a functional group? How does the concept of the functional group help to make the study of organic chem- istry more systematic?
2.64 The molecule shown below is responsible for the smell of popcorn. Write the correct molecular formula for this compound.
O N
2.65 ■ Not all polymers are formed by simply linking identi- cal monomers together. Polyurethane, for example, can be formed by reacting the two compounds shown below with one another. Write molecular and empirical formulas for each of these two substances.
N N
C O
OH HO
C O
2.66 ■ The compound shown below forms an amorphous solid (a glass) at room temperature and has been used as a me- dium for storing information holographically. Write the correct molecular formula for this molecule.
C
O N O
N
2.67 The accompanying figure shows the structure gamma- aminobutanoic acid, or GABA. This molecule is a neu- rotransmitter. Some of the effects of alcohol consumption
are due to the interaction between ethanol and GABA.
Write the correct molecular formula for this compound.
H2N
O OH
2.68 The figure below shows the structure of the adrenaline molecule. Write the correct molecular formula for this substance.
H OH N
OH
OH
Chemical Nomenclature
2.69 Why are there different rules for naming covalent and ionic binary compounds?
2.70 Which binary combinations of elements are most likely to give ionic substances?
2.71 ■ Name the following covalent compounds: (a) N2O5, (b) S2Cl2, (c) NBr3, (d) P4O10
2.72 ■ Give the formula for each of the following compounds:
(a) sulfur dichloride, (b) dinitrogen pentaoxide, (c) silicon tetrachloride, (d) diboron trioxide (commonly called boric oxide)
2.73 ■ Write the molecular formula for each of the following covalent compounds: (a) sulfur hexafl uoride, (b) bromine pentafluoride, (c) disulfur dichloride, (d) tetrasulfur tetranitride
2.74 ■ Name each of the following ionic compounds: (a) K2S, (b) CoSO4, (c) (NH4)3PO4, (d) Ca(ClO)2
2.75 ■ Name each of the following compounds: (a) MgCl2, (b) Fe(NO3)2, (c) Na2SO4, (d) Ca(OH)2, (e) FeSO4
2.76 ■ Give the formula for each of the following ionic com- pounds: (a) ammonium carbonate, (b) calcium iodide, (c) copper(II) bromide, (d) aluminum phosphate, (e) silver(I) acetate
2.77 ■ Name the following compounds: (a) PCl5, (b) Na2SO4, (c) Ca3N2, (d) Fe(NO3)3, (e) SO2, (f ) Br2O5
INSIGHT INTO Polyethylene
2.78 What is a free radical? How are free radicals important in the formation of polyethylene?
2.79 How do molecules of low-density polyethylene and high- density polyethylene differ? How do these molecular scale differences explain the differences in the macroscopic properties of these materials?
2.80 Why do you think an initiator molecule is needed to in- duce the polymerization of ethylene?
2.81 Use the web to determine the amount of low-density poly- ethylene and high-density polyethylene produced annually