Chapter: Chemical Bonds - Hanover Area School District...The Ionic Bond •An ionic bond is the force of attraction between the opposite charges of the ions in an ionic compound. Types

Chapter: Chemical BondsTable of Contents

Section 3: Writing Formulas andNaming Compounds

Section 1: Stability in Bonding

Section 2: Types of Bonds

Compounds

• Some of the matter around you is in theform of uncombined elements such ascopper, sulfur, and oxygen.

• Like many other sets of elements, thesethree elements unite chemically to form acompound when the conditions are right.

Stability in Bonding1

Compounds• The green

coating on theStatue ofLiberty andsome oldpennies is aresult of thischemicalchange.


New Properties• The compound formed when

elements combine often hasproperties that aren’tanything like those of theindividual elements.

• Sodium chloride, forexample, is a compoundmade from the elementssodium and chlorine.


Formulas

• A chemical formula tells what elements acompound contains and the exact numberof atoms of each element in a unit of thatcompound.

• The compound that you are probably mostfamiliar with is H2O, more commonlyknown as water.


Formulas

• This formula contains the symbols Hfor the element hydrogen and O for theelement oxygen.

• Notice the subscript number 2 writtenafter the H for hydrogen.


Formulas

• A subscriptwritten after asymbol tells howmany atoms ofthat element arein a unit of thecompound.


Formulas• If a symbol has

no subscript,the unitcontains onlyone atom of thatelement. A unitof H2O containstwo hydrogenatoms and oneoxygen atom.


Atomic Stability• The electric forces between oppositely

charged electrons and protons hold atomsand molecules together, and thus are theforces that cause compounds to form.

Stability in Bonding

• Atoms of noble gases are unusually stable.

• Compounds of these atoms rarely formbecause they are almost always less stablethan the original atoms.

1

The Unique Noble Gases• To understand the

stability of the noblegases, it is helpful tolook at electron dotdiagrams.


• Electron dotdiagrams show onlythe electrons in theouter energy levelof an atom.

1

Chemical Stability• The noble gases are

stable because theyeach have acomplete outerenergy level.


• Notice that eightdots surround Kr,Ne, Xe, Ar, andRn, and two dotssurround He.

1

Energy Levels and Other Elements• Hydrogen contains one electron in its lone

energy level.


• A dot diagram for hydrogenhas a single dot next to itssymbol. This means thathydrogen’s outer energylevel is not full.

• It is more stable when itis part of a compound.

1

Energy Levels and Other Elements• In contrast, helium’s outer energy level

contains two electrons.


• Helium already has a full outer energylevel by itself and is chemically stable.

• Helium rarely formscompounds but, by itself,the element is a commonlyused gas.

1

Energy Levels and Other Elements• When you look at the elements in Groups

13 through 17, you see that each of themfalls short of having a stable energy level.


Energy Levels and Other ElementsStability in Bonding

• Each group contains too few electrons for astable level of eight electrons.

1

Outer Levels —Getting Their Fill

• How does hydrogen, or any other element,trying to become stable, gain or lose itsouter electrons?


• They do this by combining with otheratoms that also have partially completeouter energy levels.

• As a result, each achieves stability.

1

Outer Levels —Getting Their Fill • This illustration shows electron dot

diagrams for sodium and chlorine.


• When theycombine,sodium losesone electronand chlorinegains oneelectron.

1

Outer Levels —Getting Their Fill


• You can see from the electron dot diagramthat chlorine now has a stable outer energylevel.

• Sodium had onlyone electron inits outer energylevel, which itlost to combinewith chlorine insodium chloride.

1

Stability Is Reached


• Look to the next, outermost energy level ofsodium.

• This is now thenew outerenergy level,and it is stablewith eightelectrons.

1

Stability Is Reached

• When atoms gain, lose, or share electrons, anattraction forms between the atoms, pullingthem together to form a compound.


• This attraction is called a chemical bond. Achemical bond is the force that holds atomstogether in a compound.

1

Section Check

What shows the elements a compoundcontains and how many atoms of eachelement are found in the compound?

1Question 1

Section Check

A chemical formula is a form of chemicalshorthand that tells what elements and howmany atoms of each are in one molecule of acompound.

1Answer

Section Check

The number of __________ in each group’souter energy level increases across the periodictable.

A. electronsB. neutronsC. protonsD. protons and neutrons

1Question 2

Section Check

The answer is A. Protons and neutrons arelocated in the nucleus of the atom.

1Answer

Section Check

What is the force that holds atoms together incompounds?

The force that holds atoms together incompounds is a chemical bond.

Answer

1Question 3

Gain or Loss of Electrons

• Atoms lose or gain to meet a standard—astable energy level.

2Types of Bonds

• An atom that has lost or gained electrons iscalled an ion. An ion is a charged particlebecause it now has either more or fewerelectrons than protons.

Gain or Loss of Electrons

• The positive and negative charges are notbalanced.

Types of Bonds

• It is the electric forces between oppositelycharged particles, such as ions, that holdcompounds together.

2

Gain or Loss of Electrons • Some of the most common compounds are

made by the loss and gain of just one electron.

Types of Bonds

• Some examples aresodium chloride,commonly known as tablesalt; sodium fluoride, ananticavity ingredient insome toothpastes; andpotassium iodide, aningredient in iodized salt.

2

A Bond Forms

• A neutral atom of potassium has oneelectron in its outer level. This is not astable outer energy level.

Types of Bonds

• When potassium forms a compound withiodine, potassium loses one electron fromits fourth level, and the third level becomesa complete outer level.

2

A Bond Forms

• The potassium atom has become an ion.

Types of Bonds

• When a potassium atom loses an electron,the atom becomes positively chargedbecause there is one electron less in theatom than there are protons in the nucleus.

• The 1+ charge is shown as a superscriptwritten after the element’s symbol, K+ , toindicate its charge.

2

A Bond Forms

• The iodine atom in this reaction undergoeschange, as well.

Types of Bonds

• An iodine atom has seven electrons in itsouter energy level.

• During the reaction with potassium, theiodide atom gains an electron, leaving itsouter energy level with eight electrons.

2

A Bond Forms

• This atom is no longer neutral because itgained an extra negative particle.

Types of Bonds

2

• It now has a charge of and is called aniodide ion, written as .

A Bond Forms • Notice that the resulting compound has a

neutral charge because the positive andnegative charges of the ions cancel eachother.

Types of Bonds

2

The Ionic Bond • An ionic bond is the force of attraction

between the opposite charges of the ions inan ionic compound.

Types of Bonds

• In an ionic bond, a transfer of electronstakes place.

• If an element loses electrons, one or moreelements must gain an equal number ofelectrons to maintain the neutral charge ofthe compound.

2

The Ionic Bond • The formation of magnesium chloride,

MgCl2, is another example of ionicbonding.

Types of Bonds

• When magnesium reacts with chlorine, amagnesium atom loses two electrons andbecomes a positively charged ion, Mg2+.

2

The Ionic Bond • At the same time, two chlorine atoms gain

one electron each and become negativelycharged chloride ions, Cl −.

Types of Bonds

2

Zero Net Charge

• The result of this bond is a neutralcompound.

Types of Bonds

• The compound as a whole is neutralbecause the sum of the charges on the ionsis zero.

2

Zero Net Charge • When atoms form an ionic compound,

their electrons are shifted to the otheratoms, but the overall number of protonsand electrons of the combined atomsremains equal and unchanged. Therefore,the compound is neutral.

Types of Bonds

• Ionic bonds usually are formed by bondingbetween metals and nonmetals.

2

Sharing Electrons • Some atoms of nonmetals are unlikely to

lose or gain electrons.

Types of Bonds

• For example, the elements in Group 4 ofthe periodic table have four electrons intheir outer levels.

• They would have to either gain or lose fourelectrons in order to have a stable outerlevel.

2

Sharing Electrons • The loss of this many electrons takes a

great deal of energy.

Types of Bonds

• Therefore, these atomsbecome morechemically stable bysharing electrons, ratherthan by losing orgaining electrons. Click image to view movie.

2

Sharing Electrons • The attraction that

forms between atomswhen they shareelectrons is known asa covalent bond.

Types of Bonds

• A neutral particlethat forms as a resultof electron sharing iscalled a molecule.

2

Single Covalent Bonds • A single covalent bond is made up of two

shared electrons.

Types of Bonds

• A water molecule contains two singlebonds. In each bond, a hydrogen atomcontributes one electron to the bond andthe oxygen atom contributes the other.

• The result of this type of bonding is astable outer energy level for each atom inthe molecule.

2

Multiple Bonds

• A covalent bond also can contain morethan one pair of electrons.

Types of Bonds

• An example of this is the bond in nitrogen(N2).

2

Multiple Bonds • A nitrogen atom has five electrons in its

outer energy level and needs to gain threeelectrons to become stable.

Types of Bonds

• It does this by sharing its three electronswith another nitrogen atom.

2

Multiple Bonds

• When each atom contributes threeelectrons to the bond, the bond containssix electrons, or three pairs of electrons.

Types of Bonds

• Each pair of electrons represents a bond.

• Therefore, three pairs of electronsrepresent three bonds, or a triple bond.

2

Unequal Sharing

• Electrons are not always shared equallybetween atoms in a covalent bond.

Types of Bonds

• These elements are close together in theupper right-hand corner of the periodictable. The strength of the attraction of eachatom to its electrons is related to the size ofthe atom, the charge of the nucleus, and thetotal number of electrons the atomcontains.

2

Unequal Sharing

• Part of the strength of attraction has to dowith how far away from the nucleus theelectron being shared is.

Types of Bonds

• The other part of the strength of attractionhas to do with the size of the positivecharge in the nucleus.

2

Unequal Sharing

Types of Bonds

• Chlorine atoms have astronger attraction forelectrons thanhydrogen atoms do.

• One example of thisunequal sharing isfound in a molecule ofhydrogen chloride, HCl.

Click image to view movie.

2

Unequal Sharing

Types of Bonds

• As a result, the electrons shared in hydrogenchloride will spend more time near the chlorineatom than near the hydrogen atom.

2

Tug-of-War

Types of Bonds

• Each atom in the molecule attracts theelectrons that they share. However,sometimes the atoms aren’t the same size.

• You might think of the bond as the rope ina tug-of-war, and the shared electrons asthe knot in the center of the rope.

2

Polar or Nonpolar?

Types of Bonds

• A polar molecule is onethat has a slightly positiveend and a slightly negativeend although the overallmolecule is neutral. Wateris an example of a polarmolecule.

• The charge is balanced but not equallydistributed. This type of molecule is calledpolar.

2

Polar or Nonpolar?

Types of Bonds

• Such a molecule does not have oppositelycharged ends.

• A nonpolar molecule is one in whichelectrons are shared equally in bonds.

• This is true of molecules made from twoidentical atoms or molecules that aresymmetric, such as CCl4.

2

Section Check

When ionic bonds form, the resultingcompounds are __________.

A. electrically neutralB. electrically unstableC. negatively chargedD. positively charged

2Question 1

Section Check

The answer is A. In an ionic bond, a transfer ofelectrons takes place and the overall neutralcharge is maintained.

2Answer

Section Check

The attraction that forms between atoms whenthey share electrons is __________.

A. a binary compoundB. a covalent bondC. an ionic bondD. the oxidation number

2Question 2

Section Check

The answer is B. A single covalent bond ismade up of two shared electrons.

2Answer

Section Check

In what type of molecule are electrons sharedequally?

A. diatomicB. nonpolarC. polarD. water

2Question 3

Section Check

The answer is B. In a nonpolar molecule,electrons are shared equally and the moleculedoes not have oppositely charged ends.

2Answer

Binary Ionic Compounds 3Writing Formulas and Naming Compounds

• A binary compound is one that iscomposed of two elements.

• The first formulas of compounds you willwrite are for binary ionic compounds.

• Before you can write a formula, you musthave all the needed information at yourfingertips.

Binary Ionic Compounds

Writing Formulas and Naming Compounds

• The relationship between an element’sposition on the periodic table and the numberof electrons it gains or loses is called theoxidation number of an element.

• You need to know which elements areinvolved and what number of electronsthey lose, gain, or share in order to becomestable.

3

Binary Ionic Compounds


• For ionic compounds the oxidation numberis the same as the charge on the ion.

• An oxidation number tells you how manyelectrons an atom has gained, or shared tobecome stable.

• For example, a sodium ion has a charge of1+ and an oxidation number of 1+.

3

Oxidation Numbers


• The number at the top of each column isthe most common oxidation number ofelements in that group.

3

Oxidation Numbers


• The elements in this tablecan have more than oneoxidation number.

• When naming thesecompounds, the oxidationnumber is expressed in thename with a romannumeral. For example, theoxidation number of ironin iron (III) oxide is 3+.

3

Compounds Are Neutral


• When writing formulas it is important toremember that although the individual ionsin a compound carry charges, thecompound itself is neutral.

• A formula must have the right number ofpositive ions and the right number ofnegative ions so the charges balance.

3

Compounds Are Neutral


• What if you have a compound like calciumfluoride?

• In this case you need to have two fluorideions for every calcium ion in order for thecharges to cancel and the compound to beneutral with the formula CaF2.

3

• A calcium ion has a charge of 2+ and afluoride ion has a charge of .

Writing Formulas


• You can write formulas for ionic compoundsby using the following rules in this order.

1. Write the symbol of the element orpolyatomic ion (ions containing more thanone atom) that has the positive oxidationnumber or charge.

3

Writing Formulas


2. Write the symbol of the element orpolyatomic ion with the negative oxidationnumber.

3. The charge (without the sign) of one ionbecomes the subscript of the other ion.Reduce the subscripts to the smallest wholenumbers that retain the ratio of ions.

3

Writing Names


• You can name a binary ionic compoundfrom its formula by using these rules.

1. Write the name of the positive ion.

3

Writing Names


2. Check to see if thepositive ion is capableof forming more thanone oxidation number.If it is, determine theoxidation number of theion from the formula ofthe compound.

3

Writing Names


2. (continued) Write thecharge of the positive ionusing roman numerals inparentheses after theion’s name. If the ionhas only one possibleoxidation number,proceed to step 3.

3

Writing Names


3. Write the root nameof the negative ion.The root is the firstpart of the element’sname.

4. Add the ending -ideto the root. Thetable lists severalelements and their -ide counterparts.

3

Writing Names


• Subscripts do not become part of the namefor ionic compounds.

• However, subscripts can be used to helpdetermine the charges of these metals thathave more than one positive charge.

3

Compounds with Complex Ions


• Not all compoundsare binary.

• Baking soda has theformula NaHCO3.

• This is an example ofan ionic compoundthat is not binary.

3



• Some compounds,including bakingsoda, are composedof more than twoelements. Theycontain polyatomicions.

3



• A polyatomic ion is a positively ornegatively charged, covalently bondedgroup of atoms.

• So the polyatomic ions as a whole containstwo or more elements.

3

Writing Names


• The table lists severalpolyatomic ions.

• To name a compoundthat contains one ofthese ions, first writethe name of thepositive ion.

• Then write the nameof the negative ion.

3

Writing Formulas


• To write formulas for these compounds,follow the rules for binary compounds,with one addition.

• When more than one polyatomic ion isneeded, write parentheses around thepolyatomic ion before adding the subscript.

3

WritingFormulas


• Here is oneexample ofnaming acomplexcompound.

3

Compounds with Added Water


• Some ionic compounds have water moleculesas part of their structure. These compoundsare called hydrates.

• A hydrate is a compound that has waterchemically attached to its ions and writteninto its chemical formula.

3

Common Hydrates


• When a solution of cobalt chlorideevaporates, pink crystals that contain sixwater molecules for each unit of cobaltchloride are formed.

• The formula for this compound is CoCl2.

3

Common Hydrates


• Plaster of paris also forms a hydrate whenwater is added.

• It becomescalciumsulfatedihydrate,which isalso knownas gypsum.

3

Common Hydrates


• When writing a formula that contains ahydrate, the number is shown after a “⋅”.Following the number 2 is the formula forwater as shown.

3

Naming Binary CovalentCompounds


• Covalent compounds are those formedbetween elements that are nonmetals.

• Some pairs of nonmetals can form morethan one compound with each other.

• In the system you have learned so far, each ofthese compounds would be called nitrogenoxide. You would not know from that namewhat the composition of the compound is.

3

Using Prefixes


• Scientists use Greekprefixes to indicate howmany atoms of eachelement are in a binarycovalent compound.

• Notice that the last vowelof the prefix is droppedwhen the second elementbegins with a vowel as inpentoxide.

3

Using Prefixes


• Often the prefix mono- isomitted, although it is usedfor emphasis in some cases.

3

Using Prefixes


• These same prefixes are used whennaming the hydrates previously discussed.

• The main ionic compound is named theregular way, but the number of watermolecules in the hydrate is indicated bythe Greek prefix.

3

Section Check

What does the oxidation number of an elementtell you?

The oxidation number indicates how manyelectrons an atom has gained, lost, or sharedin order to become stable.

Answer

3Question 1

Section Check

A __________ is a compound composed oftwo elements.

A binary compound is composed of twoelements. Potassium iodide is a binary ioniccompound.

Answer

3Question 2

Section Check

What is a hydrate?

A hydrate is a compound that has waterchemically attached to its ions. Water is alsowritten into its chemical formula.

Answer

3Question 3

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Chapter: Chemical Bonds - Hanover Area School District...The Ionic Bond •An ionic bond is the force of attraction between the opposite charges of the ions in an ionic compound. Types