HIGH SCHOOL SCIENCE Physical Science 11: Chemical Bonds
HIGH SCHOOL SCIENCEPhysicalScience 11:ChemicalBondsWILLMAR PUBLIC SCHOOL2013-2014 EDITION
C HAPTER 11ChemicalBondsIn this chapter you will:1. Recognize stable electron configurations.2.Predict an element’s chemical properties usingthe number of valence electrons and electrondot diagrams.3.Describe how ionic bonds form and relate theproperties of ionic compounds to the structureof crystal lattices.4.Describe how covalent bonds form and the attractions that keep atoms together in molecules.5. Compare polar and nonpolar bonds.6.Name and determine chemical formulas forionic and molecular compounds.7. Describe the structure and strength of bond inmetals.
S ECTION 11.1What are Chemical Bonds?O BJECTIVES :1. Recognize stable electron configurations.2. Predict an element’s chemical properties usingthe number of valence electrons and electrondot diagrams.Vocabulary:chemical bondelectron dot diagramchemical formulasubscriptThere is an amazing diversity of matter in the universe, butthere are only about 100 elements. How can this relativelysmall number of pure substances make up all kinds of matter?Elements can combine in many different ways.Did you ever make cupcakes from scratch, like the boypictured above? You mix together flour, sugar, eggs, and otheringredients to make the batter, put the batter into cupcakepapers, and then put them into the oven to bake. Thecupcakes that come out of the oven after baking are differentfrom any of the individual ingredients that went into thebatter. Like the ingredients that join together to makecupcakes, atoms of different elements can join together toform entirely different substances called compounds. Incupcakes, the eggs and other wet ingredients cause the dryingredients to stick together. What causes elements to sticktogether in compounds?Elements form compounds when they combine chemically.Their atoms join together to form molecules, crystals, or otherstructures. The atoms are held together by chemical bonds. Achemical bond is a force of attraction between atoms orions. It occurs when atoms share or transfer valence electrons.Valence electrons are the electrons in the outer energy level ofan atom.Water (H2O) is an example of a chemical compound. Watermolecules always consist of two atoms of hydrogen and oneatom of oxygen. Like water, all other chemical compoundsconsist of a fixed ratio of elements. It doesn’t matter how2
much or how little of a compound there is. It always has thesame composition.The same elements may combine in different ratios. If theydo, they form different compounds. Both water (H2O) andhydrogen peroxide (H2O2) consist of hydrogen and oxygen.However, they have different ratios of the two elements. As aresult, water and hydrogen peroxide are different compoundswith different properties. If you’ve ever used hydrogenperoxide to disinfect a cut, then you know that it is verydifferent from water! Both carbon dioxide (CO2) and carbonmonoxide (CO) consist of carbon and oxygen, but in differentratios.There are different types of compounds. They differ in thenature of the bonds that hold their atoms together. The typeof bonds in a compound determines many of its properties.Three types of bonds are ionic, covalent, and metallic bonds.An ionic bond is the force of attraction that holds togetheroppositely charged ions. Ionic bonds form crystals. Table saltcontains ionic bonds. A covalent bond is the force ofattraction that holds together two nonmetal atoms that sharea pair of electrons. One electron is provided by each atom,and the pair of electrons is attracted to the positive nuclei ofboth atoms. The water molecule represented above containscovalent bonds. A metallic bond is the force of attractionbetween a positive metal ion and the valence electrons thatsurround it—both its own valence electrons and those of otherions of the same metal. The ions and electrons form a latticelike structure. Only metals form metallic bonds.Chemical properties depend on an element’s electronconfiguration. When the highest occupied energy level of anatom is filled with electrons, the atom is stable and not likelyto react. The noble gases have stable electron configurationssince their outer shell is full. The chemical properties of anelement depend on the number of valence electrons.An electron dot diagram is a model of an atom in whicheach dot represents a valence electron. The symbol in thecenter represents the nucleus and all other electrons in theatom. Electron dot diagrams for carbon and chlorine areshown below. The show paired electrons and unpairedelectrons. These valence electrons are available for bonding.The unpaired electrons would like to be in a pair.3
For example, the chemical formula for magnesium chloride isMgCl2. The 2 written to the right and slightly below thesymbol of the chlorine is a subscript. Subscripts are used toshow the relative numbers of atoms of the element present.If there is only one atom of an element in a formula, nosubscript is needed. From the formula, you can tell that thereis one magnesium ion for every two chlorine ions inmagnesium chlorine. A magnesium atom cannot reach astable electron configuration by reacting with just onechlorine atom since it has two valence electrons. It musttransfer electrons to two chlorine atoms.Elements that do not have complete sets of valence electronstend to react. By reacting, they achieve electronconfigurations similar to those of noble gases.You can make a simple salad dressing using just the twoingredients: oil and vinegar. Recipes for oil-and-vinegar saladdressing vary, but they typically include about three parts oilto one part vinegar, or a ratio of 3:1. For example, if youwanted to make a cup of salad dressing, you could mixtogether 3 cup of oil and 1 cup of vinegar. Chemicalcompounds also have “ingredients” in a certain ratio.However, unlike oil-and-vinegar salad dressing, a chemicalcompound always has exactly the same ratio of elements. Thisratio can be represented by a chemical formula. A chemicalformula is a notation that shows what elements a compoundcontains and the ratio of the atoms or ions of these elementsin the compound.4
Section Review:1.Describe when an atom is stable and not likely to react.2. What does each dot in an electron dot diagram represent?3. When do elements tend to react?4. What information is in a chemical formula?5. What are subscripts used to show?6. Why must magnesium react with two chlorine atoms toreach a stable electron configuration?7. The compound sodium sulfide consists of a ratio of onesodium ion (Na ) to two sulfide ions (S-2). Write thechemical formula for this compound.8. A molecule of sulfur dioxide consists of one sulfur atom(S) and two oxygen atoms (O). What is the chemical formula for this compound?9. Identify the ratio of atoms in the compound represented by the following chemical formula: N2O5.5
S ECTION 11.2Some elements achieve stable electron configurations throughthe transfer of electrons between atoms.Ionic BondsO BJECTIVES :1. Describe how ionic bonds form and relate theproperties of ionic compounds to the structureof crystal lattices.2. Predict an element’s chemical properties usingthe number of valence electrons and electrondot diagrams.Look at the electron dot diagram for chlorine. A chlorineatom is missing one valence electron for it to have a full outershell. Look at the electron dot diagram for sodium. A sodiumatom has one valence electron. If sodium were to lose thisvalence electron, its outer shell would be full and be a stableelectron configuration. When sodium reacts with chlorine, anelectron is transferred from the sodium atom to the chlorineatom. Each atom ends up with a more stable electronconfiguration after transferring the electrons than it hadbefore the transfer.3. Name and determine chemical formulas forionic and molecular compounds.Vocabulary:ionanioncationionic bondionization energyionic compoundcrystalsbinary compoundWhen an atom gains or loses an electron, the number ofelectrons is no longer equal to the number of protons. Thecharge is not balance and the charge is not neutral. An ion isan atom that has a positive or negative electric charge. An ionwith a negative charge (-) is an anion. An ion with a positivecharge ( ) is a cation.polyatomic ions6
When an anion and cation are close together, a chemical bondforms between the ions. A chemical bond is the force thatholds atoms together. An ionic bond is the force that holdscations and anions together.An electron can move to a higher energy level when an atomabsorbs energy. The energy allows electrons to overcome theattraction of the positive protons in the nucleus. Cations formwhen electrons gain enough energy to escape from atoms.It takes energy to remove valence electrons from an atombecause the force of attraction between the negative electronsand the positive nucleus must be overcome. The amount ofenergy needed depends on the element. The amount ofenergy used to remove an electron is called ionizationenergy. It varies from element to element. The lower theionization energy, the easier it is to remove an electron froman atom.Ionization energies tend to increase from left to right across aperiod. It takes more energy to remove an electron from anonmetal than from a metal in the same period. Ionizationenergies tend to increase from bottom to top in a group.Compounds that contain ionic bonds are ionic compounds.A chemical formula for an ionic compound tells you the ratioof the ions in the compound, but it does not tell you how theions are arranged in the compound.Solids whose particles are arranged in a lattice structure arecalled crystals. Lattice structures keep the ions in a fixedposition. The repeating pattern of ions in the lattice is like therepeating pattern of designs on wallpaper. Ionic crystalsdepend on the ratio of ions and their relative size. Crystals areclassified into groups based on the shape of their crystals.The properties of an ionic compound can be explained by thestrong attractions among ions within a crystal lattice. Ioniccompounds have high melting points, are poor conductors ofelectric current, and shatter when struck with a hammer.7
The stronger the attraction among the particles, the morekinetic energy the particles must have before they canseparate.For an electric current to flow, charge particles must be ableto move from one location to another. The ions in a solidcrystal lattice have fixed position; however, when solid melts,the lattice breaks apart and the ions are free to flow.Cu1 ions to balance an O2ions. CuO is copper (II) oxidebecause it takes one Cu2 ions to balance an O2- ions.Polyatomic ions are a covalently bonded group of atomsthat has a positive or negative charge and acts as a unit.When a crystal is struck, negative ions are pushed intopositions near negative ions, and positive ions are pushed intoposition near positive ions. Ions with the same charge repelon another and cause the crystals to shatter.The name of an ionic compound must distinguish thecompound from other ionic compounds containing the sameelements. The formula of an ionic compound describes theratio of the ions in the compound.A compound made from only two elements is a binarycompound. The naming binary compounds are easy. Thename of the cation followed by the name of the anion. Thename of the cation is the name of the metal. The name of theanion uses part of the nonmetal with the suffix –ide. Forexample, if you have sodium and chlorine in a compound, itwould be sodium chloride.If you know the name of the ionic compound you can write itsformula. Place the symbol for the cation first, followed bysymbol for the anion. Use subscripts to show the ratio of theions in the compound. Because all compounds are neutral,the total charge on the cations and anions must add up tozero.Many transition metals form more than one type of ion.When a metal forms more than one ion, the name of the ioncontains a Roman numeral to indicate the charge of the ion.For example, Cu2O is copper (I) oxide because it takes two8
Section Review:1. How can an atom end up with a more stable electron configuration?2. What happens when sodium reacts with chlorine?3. When is it easier to remove an electron from an atom?4. Across a period, how does the ionization energies increase?5. In a group, how does the ionization energies increase?6. Use ionization energy to explain why metals lose electrons more easily than nonmetals.7. What does the shape of an ionic crystal depend on?8. How would the properties of an ionic compound be explained?9. Why do ionic crystals to shatter when struck?10.What does the name for an ionic compound distinguish?11.What does the formula for an ionic compound describe?12.How are binary ionic compounds named?13.How are transition metals distinguished from one another?14. How do you write a formula for ionic compound?9
S ECTION 11.3Covalent BondsIn a tennis match, two players keep hitting the ball back andforth. The ball bounces from one player to the other, over andover again. The ball keeps the players moving together on thecourt. What if the two players represented the nuclei of twoatoms and the ball represented valence electrons? Whatwould the back and forth movement of the ball represent? Theanswer is a covalent bond.O BJECTIVES :1. Describe how covalent bonds form and theattractions that keep atoms together inmolecules.2. Compare polar and nonpolar bonds.3. Name and determine chemical formulas forionic and molecular compounds.Vocabulary:covalent bondmoleculepolar covalent bondnonpolarA covalent bond is a chemical bond in which two atomsshare a pair of valence electrons. Covalent bonds shareelectrons while ionic bonds transfer electrons. The two atomsthat are held together by a covalent bond may be atoms of thesame element or different elements. When atoms of differentelements form covalent bonds, a new substance, called acovalent compound, results. Water is an example of a covalentcompound. A molecule is a neutral group of atoms that arejoined together by one or more covalent bonds. The10
attractions between the shared electrons and the protons ineach nucleus hold the atoms together in a covalent bond.In a molecule of an element, the atoms that form covalentbonds have the same ability to attract an electron. Sharedelectrons are attracted equally to the nuclei of both atoms. Ina molecule of a compound, electrons may not be sharedequally. A covalent bond in which electrons are not sharedequally is called a polar covalent bond.When atoms form a polar covalent bond, the atom with thegreater attraction for electrons has a partial negative charge.The other atom has a partial positive charge.When two atoms share one pairs of electrons, the bond iscalled a single bond. When two atoms share two pairs ofelectrons, the bond is called a double bond. When two atomsshare three pairs of electrons, the bond is called a triple bond.In general, elements on the right of the periodic table have agreater attraction for electrons than elements on the left have(except for Noble Gases). In general, elements at the top of agroup have greater attraction for electrons than elements atthe bottom of a group. Fluorine is on the far right and is atthe top of its group, so it has the strongest attraction forelectrons and is the most reactive metal.Covalent bonds form because the shared electrons fill eachatom’s outer energy level and this is the most stablearrangement of electrons.Not all polar bonds create a polar molecule. The type ofatoms in a molecule and its shape are factors that determinewhether a molecule is polar or nonpolar. If the molecule isin a straight line, or linear, with polar bonds, the molecule isnon-polar. However, it the molecule is at an angle to eachother with polar bonds, the molecule is polar.11
In a molecule compound, there are forces of attractionbetween molecules. Attractions between polar molecules arestronger than attractions between nonpolar molecules.The covalent bonds of covalent compounds are responsible formany of the properties of the compounds. Because valenceelectrons are shared in covalent compounds, rather thantransferred between atoms as they are in ionic compounds,covalent compounds have very different properties than ioniccompounds. Many covalent compounds, especially those containingcarbon and hydrogen, burn easily. In contrast, many ioniccompounds do not burn.compound describe the type and number of atoms in amolecule of the compound.When naming molecular compounds, the general rule is thatthe most metallic element appears first in the name. If bothelements are in the same group, the more metallic element iscloser to the bottom of the group. The names of the elementsin the compound reflect the actual number of atoms in amolecule. You put a prefix indicating the number of atomsbefore the element name. If there is only one atom in the firstelement, the mono is left off. The name of the second elementis changed to end in the suffix –ide. For example, N2O4 hastwo nitrogen atoms and four oxygen atoms. Its name wouldbe dinitrogen tetraoxide. Many covalent compounds do not dissolve in water, whereasmost ionic compounds dissolve well in water. Unlike ionic compounds, covalent compounds do not havefreely moving electrons, so they cannot conductelectricity. The individual molecules of covalent compounds are moreeasily separated than the ions in a crystal, so mostcovalent compounds have relatively low boiling points. Thisexplains why many of them are liquids or gases at roomtemperature.Like ionic compounds, molecular compounds have namesthat identify specific compounds, and formulas that matchthose names. The name and formula of a molecularWriting a formula for a molecular compound is easy. Writethe symbols for the element in the order the elements appear12
in the name. The prefixes indicate the number of atoms ofeach element appear in molecule. The prefixes appears assubscripts in the formulas. If there is no prefix, the number ofatoms is one.Section Review:1.How is a covalent bond different from an ionic bond?2. What keeps the atoms together in a molecule?3. What do you call it when an atom shared one pair ofelectrons?4. Two pairs of electrons?5. Three pairs of electrons?6. Which elements have the greatest attraction for electrons?7. Which atoms become more negative in a polar covalentbond? More positive?8. How do you determine if a molecule is polar or nonpolar?9. What does the name and formula for molecular compounds describe?10. What is the general rule for which element appears firstin a molecular compound?11. How is the second element name changed?12. What does the prefix hexa- mean?13. How do you write a formula for molecular compounds?13
S ECTION 11.4Metallic BondsO BJECTIVE :1. Describe the structure and strength of bond inmetals.The thick, rigid trunk of the oak tree might crack and break ina strong wind. The slim, flexible trunk of the willow tree mightbend without breaking. In one way, metals are like willowtrees. They can bend without breaking. That’s because metalsform special bonds called metallic bonds.The properties of metal are related to bonds within the metal.There is a way for metal atoms to lose and gain electrons. In ametal, valence electrons are free to move among the atoms,thus becoming a cation with a pool of shared electrons. Ametallic bond is the attraction between a metal cation andthe shared electrons that surrounds it. Cations in a metalform a lattice that is held in place by strong metallic bonds.Although the electrons are moving among the atoms, the totalnumber of electrons does not change. The valence electronsof metals move freely in this way because metals haverelatively low electronegativity, or attraction to electrons. Thepositive metal ions form a lattice-like structure held togetherby all the metallic bonds.The more valence electrons an atom can contribute to theshared pool, the stronger the metallic bond.Vocabulary:metallic bondductilemalleablealloyThe valence electrons surrounding metal ions are constantlymoving. This makes metals good conductors of electricity. Thelattice-like structure of metal ions is strong but quite flexible.This allows metals to bend without breaking. Metals are bothductile (can be shaped into wires) and malleable (can beshaped into thin sheets).14
Remember that a flow of charge particles is an electriccurrent. A metal has a built-in supply of charged particlesthat can flow from one location to another, the pool ofelectrons.The lattice in metals is flexible compared to the rigid lattice ofionic compounds. The metal ions shift their position andshape of the metal changes, but the metal does not shatter.The ions are still held together by the metallic bonds.other elements in it. The molten solution is then allowed tocool and harden. Alloys generally have more useful propertiesthan pure metals. Alloying one metal with other metal(s) ornon metal(s) often enhances its properties. For instance, steelis stronger than iron, its primary element. Unlike puremetals, most alloys do not have a single melting point.Instead, they have a melting range in which the material is amixture of solid and liquid phases.Metals such as iron are useful for many purposes because oftheir unique properties. For example, they can conductelectricity and bend without breaking. However, pure metalsmay be less useful than mixtures of metals with otherelements. For example, adding a little carbon to iron makes itmuch stronger. This mixture is called steel. Steel is so strongthat it can hold up huge bridges, like the one pictured above.Steel is also used to make skyscrapers, cargo ships, cars, andtrains. Steel is an example of an alloy.An alloy is a mixture of a metal with one or more otherelements. The other elements may be metals, nonmetals, orboth. An alloy is formed by melting a metal and dissolving the15
Section Review:1.How do metals achieve a stable electron configuration?2. What holds metal ions together in a metal lattice?3. What two important properties of metals can be explained by their structure?4. What are some useful ways alloys may differ from puremetals?16
of bonds in a compound determines many of its properties. Three types of bonds are ionic, covalent, and metallic bonds. An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals. Table salt contains ionic bonds. A covalent bond is the force of
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