unit 3 cmpnd and bonding

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UNIT 3 COMPOUND AND BONDING

3.1 Octet rule and Ions3.2 Ionic compounds3.3 Naming and writing Ionic Formulas3.4 Polyatomic Ions3.5 Covalent Compounds3.6 Electronegativity and Bond Polarity3.7 Attractive Forces in Compounds

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TOPIC LEARNING OUTCOME

At the end of the topic, students should be able:� Explain the octet rule and why element form octet � Explain the formation of ion (cation and anion)� Describe the ionic compound and covalent

compound� Express the naming and writing ionic formula� State the meaning of polyatomic ion� Interpret the electronegativity and bond polarity

trend � Distinguish the attractive force for ionic compound

and covalent compound 2

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UNIT 3.1 OCTET RULE AND IONAn octet�means 8 valence electrons.� is associated with the stability of the noble

gases.Exception:Helium (He) is stable with 2 valence electrons.

valence electronsHe 2 2Ne 2, 8 8Ar 2, 8, 8 8Kr 2, 8, 18, 8 8 3

Electron level arrangement

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IONIC AND COVALENT BONDS

Atoms form octets� to become more

stable. � by losing, gaining, or

sharing valence electrons.

� by forming ionic bonds or covalent bonds.

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METALS FORM POSITIVE IONSMetals form positive ions� by a loss of their valence

electrons.�with the electron configuration of

the nearest noble gas.� that have fewer electrons than

protons. Group 1A metals → ion 1+

Group 2A metals → ion 2+

Group 3A metals → ion 3+

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Ion – atom or a group of atoms that has a net positive or negative charge

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FORMATION OF A SODIUM ION, NA+

Sodium achieves an octet by losing its one valence electron.

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2, 8, 1 2, 8

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CHARGE OF SODIUM ION, NA+

With the loss of its valence electron,the sodium ion has a +1 charge.

Sodium atom Sodium ion11p+ 11p+

11e- 10e-

0 1+

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2, 8

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FORMATION OF MG2+

Magnesium achieves an octet by losing its two valence electrons.

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2, 8, 2 2, 8

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LEARNING CHECK

A. The number of valence electrons in aluminum is1) 1e-. 2) 2e-. 3) 3e-.

B. The change in electrons for octet requires a1) loss of 3e-. 2) gain of 3e-. 3) a gain of 5e-.

C. The ionic charge of aluminum is1) 3-. 2) 5-. 3) 3+.

D. The symbol for the aluminum ion is1) Al3+. 2) Al3-. 3) Al+.

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FORMATION OF NEGATIVE IONS

In ionic compounds, nonmetals�achieve an octet arrangement.�gain electrons.�form negatively charged ions with 3-,

2-, or 1- charges.

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FORMATION OF A CHLORIDE, CL-

Chlorine achieves an octet by adding an electron to its valence electrons.

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2, 8, 7 2, 8, 8

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CHARGE OF A CHLORIDE ION, CL-

By gaining one electron, the chloride ion has a -1 charge.

Chlorine atom Chloride ion17p+ 17p+

17e- 18e-

0 1–

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+ 1 e-

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SOME IONIC CHARGES

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I-

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IONIC COMPOUNDS

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IONIC COMPOUNDSIonic compounds

� consist of positive and negative ions.

� have attractions called ionic bonds between positively and negatively charged ions.

� have high melting and boiling points.

� are solid at room temperature.

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SALT IS AN IONIC COMPOUNDSodium chloride or “table salt” is an example of an ionic compound.

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IONIC FORMULASAn ionic formula

� consists of positively and negatively charged ions.

� is neutral.

� has charge balance.total positive charge = total negative charge

The symbol of the metal is written first followed by the symbol of the nonmetal, e.g. NaCl.

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CHARGE BALANCE FOR NACL, “SALT”

In NaCl,

� a Na atom loses its valence electron. � a Cl atom gains an electron.� the symbol of the metal is written first followed by

the symbol of the nonmetal.

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Formation of NaCl

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CHARGE BALANCE IN MGCL2In MgCl2,

� a Mg atom loses two valence electrons.

� two Cl atoms each gain one electron.

� subscripts indicate the number of ions needed to give charge balance.

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CHARGE BALANCE IN NA2S

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In Na2S.

• two Na atoms lose one valence electron each.

• one S atom gains two electrons.

• subscripts show the number of ions needed to give charge balance.

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FORMULA FROM IONIC CHARGESWrite the ionic formula of the compound with Ba2+ and Cl−.

�Write the symbols of the ions.Ba2+ Cl−

� Balance the charges. Ba2+ Cl− two Cl- needed

Cl−�Write the ionic formula using a subscript 2 for two

chloride ions that give charge balance. BaCl2 22

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LETS TRY IT OUT!Select the correct formula for each of the following ionic compounds.

A. Na+ and S2-

1) NaS 2) Na2S 3) NaS2

B. Al3+ and Cl-1) AlCl3 2) AlCl 3) Al3Cl

C. Mg2+ and N3-

1) MgN 2) Mg2N3 3) Mg3N223

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CHARGES OF REPRESENTATIVE ELEMENTS

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NAMING AND WRITING IONIC FORMULAS

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NAMING IONIC COMPOUNDS WITH TWO ELEMENTS

To name a compound that contains two elements,

�identify the cationand anion.�name the cation

first followed by the name of the anion.

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NAMES OF SOME COMMON IONS

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EXAMPLES OF IONIC COMPOUNDS WITH TWO ELEMENTS

Formula Ions Namecation anion

NaCl Na+ Cl- sodium chlorideK2S K+ S2- potassium sulfideMgO Mg2+ O2- magnesium oxideCaI2 Ca2+ I- calcium iodideAl2O3 Al3+ O2- aluminum oxide

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LEARNING CHECK

Write the formulas and names for compounds of the following ions:

Br− S2− N3−

Na+

Al3+

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SOLUTION

Br− S2− N3−

Na+

Al3+

NaBrsodium bromide

Na2Ssodium sulfide

Na3Nsodium nitride

AlBr3

aluminum bromide

Al2S3

aluminum sulfide

AlNaluminum nitride

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NAMING VARIABLE CHARGE METALS

Transition metals with two different ions use a Roman numeral after the name of the metal to indicate ionic charge.

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GUIDE TO WRITING FORMULAS FROM THE NAME

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WRITING FORMULASWrite a formula for potassium sulfide.

1. Identify the cation and anion.potassium = K+

sulfide = S2−

2. Balance the charges.K+ S2−

K+

2(1+) + 1(2-) = 0

3. 2 K+ and 1 S2− = K2S33

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POLYATOMIC IONS

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POLYATOMIC IONS

A polyatomic ion (also known as molecular ion)� a group of atom � has an overall ionic charge.

Some examples of polyatomic ions areNH4

+ ammonium OH− hydroxide

NO3− nitrate NO2

− nitrite

CO32− carbonate PO4

3− phosphateHCO3

− hydrogen carbonate(bicarbonate)

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NAMES AND FORMULAS OF COMMON POLYATOMIC IONS

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NAMES AND FORMULAS OF COMMON POLYATOMIC IONS

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3−

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32−

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3−

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43−

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3−

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42−

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NAMING COMPOUNDS WITH POLYATOMIC IONS� The positive ion is named first followed by the

name of the polyatomic ion.NaNO3 sodium nitrate

K2SO4 potassium sulfate

Fe(HCO3)3 iron(III) bicarbonate

or iron(III) hydrogen carbonate

(NH4)3PO3 ammonium phosphite

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WRITING FORMULAS WITH POLYATOMIC IONS

The formula of an ionic compound � containing a polyatomic ion must have a charge

balance that equals zero (0).Na+ and NO3

−à NaNO3

�with two or more polyatomic ions has the polyatomic ions in parentheses.Mg2+ and 2NO3

− à Mg(NO3)2

subscript 2 for charge balance45

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COVALENT COMPOUNDS

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COVALENT BONDS

Covalent bonds form

� when atoms share electrons to complete octets.

� between two nonmetal atoms.

� between nonmetal atoms from Groups 4A(14), 5A(15), 6A(16), and 7A(17).

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HYDROGEN MOLECULEA hydrogen molecule� is stable with two electrons (like helium).� has a shared pair of electrons.

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FORMING OCTETS IN MOLECULES

In a fluorine, F2,, molecule, each F atom� shares one electron.� attains an octet.

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NAMING COVALENT COMPOUNDS

To name covalent compounds

� STEP 1: Name the first nonmetal as an element.

� STEP 2: Name the second nonmetal with an ideending.

� STEP 3: Use prefixes to indicate the number of atoms (subscript) of each element.

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Table4.12

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NAMING COVALENT COMPOUNDS

What is the name of SO3?

1. The first nonmetal is S sulfur. 2. The second nonmetal is O named oxide.3. The subscript 3 of O is shown as the prefix tri.

SO3 à sulfur trioxide

The subscript 1 (for S) or mono is understood.

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FORMULAS AND NAMES OF SOME COVALENT COMPOUNDS

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LEARNING CHECK

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Select the correct name for each compound.A. SiCl4 1) silicon chloride

2) tetrasilicon chloride3) silicon tetrachloride

B. P2O5 1) phosphorus oxide2) phosphorus pentoxide3) diphosphorus pentoxide

C. Cl2O7 1) dichlorine heptoxide2) dichlorine oxide3) chlorine heptoxide

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NAME THE FOLLOWING COMPOUNDS

A. Ca3(PO4)2

ionic Ca2+ PO43− calcium phosphate

B. FeBr3

ionic Fe3+ Br − iron(III) bromide

C. SCl2

covalent 1S 2 Cl sulfur dichloride

D. Cl2O

covalent 2 Cl 1 O dichlorine monoxide54

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WRITE THE FORMULAS FOR THE FOLLOWING:

A. calcium nitrateCa2+, NO3

− Ca(NO3)2

B. boron trifluoride

1 B, 3 F BF3

C. aluminum carbonate

Al3+, CO32− Al2(CO3) 3

D. dinitrogen tetroxide

2 N, 4 O N2O455

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ELECTRONEGATIVITY AND BOND POLARITY

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ELECTRONEGATIVITY

The electronegativity value

� indicates the attraction of an atom for shared electrons.

� increases from left to right going across a period on the periodic table.

� is high for the nonmetals with fluorine as the highest.

� is low for the metals. 57

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SOME ELECTRONEGATIVITY VALUES FOR GROUP A ELEMENTS

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Low values

High values

`Electronegativity increases

`E

lectronegativity decreases

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NONPOLAR COVALENT BONDSA nonpolar covalent bond

� occurs between nonmetals.� is an equal or almost equal sharing of electrons.� has almost no electronegativity difference (0.0 to 0.4).

Examples: Electronegativity

Atoms Difference Type of BondN-N 3.0 - 3.0 = 0.0 Nonpolar covalentCl-Br 3.0 - 2.8 = 0.2 Nonpolar covalentH-Si 2.1 - 1.8 = 0.3 Nonpolar covalent

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POLAR COVALENT BONDSA polar covalent bond

� occurs between nonmetal atoms.� is an unequal sharing of electrons.� has a moderate electronegativity difference (0.5 to 1.7).

Examples: Electronegativity

Atoms Difference Type of BondO-Cl 3.5 - 3.0 = 0.5 Polar covalentCl-C 3.0 - 2.5 = 0.5 Polar covalentO-S 3.5 - 2.5 = 1.0 Polar covalent

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COMPARING NONPOLAR AND POLAR COVALENT BONDS

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IONIC BONDSAn ionic bond� occurs between metal and nonmetal ions.� is a result of electron transfer.� has a large electronegativity difference (1.8 or more).

Examples: Electronegativity

Atoms Difference Type of BondCl-K 3.0 – 0.8 = 2.2 IonicN-Na 3.0 – 0.9 = 2.1 IonicS-Cs 2.5 – 0.7 = 1.8 Ionic

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Use the electronegativity (see Figure 4.6) difference (Δ) to identify the type of bond between the following asnonpolar covalent (NP), polar covalent (P), or ionic (I).

A. K-NΔEN = 2.2 ionic (I)

B. N-OΔEN = 0.5 polar covalent (P)

C. Cl-ClΔEN = 0.0 nonpolar covalent (NP)

D. H-ClΔEN = 0.9 polar covalent (P)

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EN: K=0.8, N=3.0

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3.7 ATTRACTIVE FORCES IN COMPOUND

1) Ionic CompoundIn ionic compounds, ionic bonds § require large amounts of energy to break.§ hold positive and negative ions together.§ explain their high melting points.

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2) COVALENT COMPOUNDSIn covalent compounds, the attractive forces betweensolid and liquid molecules § are weaker than ionic bonds.§ require less energy to break.§ explain why their melting points are lower than ionic

compounds.These attractive forces include§ dipole-dipole attractions,§ dispersion forces, and§ hydrogen bonding.

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DIPOLE-DIPOLE ATTRACTIONS

In covalent compounds, polar molecules exert attractive forces between molecules called dipole-dipole attractions.

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DIPOLE-DIPOLE ATTRACTIONS, HYDROGEN BONDS

In covalent compounds, some polar moleculesform strong dipole attractions called hydrogen bonds, which occur between the partially positive hydrogen atom of one molecule and a lone pair of electrons on a nitrogen, oxygen, or fluorine atom in another molecule.

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DISPERSION FORCESDispersion forces are§ weak attractions between nonpolar molecules.§ caused by temporary dipoles that develop when

electrons are not distributed equally.

Nonpolar molecules form attractions when they form temporary dipoles.

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COMPARISON OF BONDING AND ATTRACTIVE FORCES

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MELTING POINTS AND ATTRACTIVE FORCES

The stronger the attractive force between ions or molecules, the higher the melting points.Ionic compounds, have the strongest attractive force and, therefore the highest melting points. Covalent molecules have less attractive forces than ionic compounds and, therefore lower melting points.

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MELTING POINTS AND ATTRACTIVE FORCES

The attractive forces between covalent molecules vary in magnitude; the stronger the attractive force, the higher its melting point. § Hydrogen bonds are the strongest type of

dipole–dipole attractions, requiring the most energy to break, followed by dipole–dipole forces.§ Dispersion forces are the weakest, requiring

even less energy to break them, and therefore have lower melting points than hydrogen bonds and dipole–dipole forces.

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MELTING POINTS OF SELECTED SUBSTANCES

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LEARNING CHECK

Identify the main type of attractive forces for each of the following compounds: ionic bonds, dipole–dipole, hydrogen bonds or dispersion.1. NCl32. H2O3. Br2

4. KCl5. NH3

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SOLUTIONIdentify the main type of attractive forces for each of the following compounds: ionic bonds, dipole–dipole, hydrogen bonds or dispersion.1. NCl3 dipole–dipole2. H2O hydrogen bonds3. Br2 dispersion4. KCl ionic bonds5. NH3 hydrogen bonds