Valence Electrons, Bonds and Valence Electrons, Bonds and Chemical ReactionsChemical Reactions

05 October 200605 October 2006

Principles of Valence Electrons and BondsPrinciples of Valence Electrons and BondsIonic BondsIonic BondsMetallic BondsMetallic BondsCovalent BondsCovalent BondsIntermolecular ForcesIntermolecular ForcesCommon Chemical ReactionsCommon Chemical Reactions

Atoms in ProximityAtoms in Proximity

Observation: when two atoms are brought together, Observation: when two atoms are brought together, electrons reelectrons re--arrange to the lowest energy state (i.e., arrange to the lowest energy state (i.e., valence electrons are most stable)valence electrons are most stable)Consequence: distribution of electrons among atoms are Consequence: distribution of electrons among atoms are rere--arranged into arranged into bondsbonds

Give away electronsGive away electronsAccept electronsAccept electronsShare electronsShare electrons

Atoms in ProximityAtoms in Proximity

Ionic BondingIonic Bonding

Atoms give away electrons, whereas other atoms Atoms give away electrons, whereas other atoms receive electronsreceive electronsExample of lithium (Li) chloride (Cl) Example of lithium (Li) chloride (Cl)

3366Li + Li + 1717

35.535.5Cl = Cl = LiClLiCl

Ionic BondingIonic Bonding

Lithium (Li)Lithium (Li)

Li gives up 1 electron and is left with 2 electrons (Li gives up 1 electron and is left with 2 electrons (--) and 3 protons (+); net positive (+) ) and 3 protons (+); net positive (+) chargecharge

Chlorine (Cl)Chlorine (Cl)

Cl has 1 unpaired electron in valence shell, so Cl tends to acceCl has 1 unpaired electron in valence shell, so Cl tends to accept an electron and is left pt an electron and is left with 18 electrons (with 18 electrons (--) and 17 protons; net negative () and 17 protons; net negative (--) charge) charge

Ionic BondingIonic Bonding

Some atoms tend to give away electrons, whereas other atoms Some atoms tend to give away electrons, whereas other atoms tend to receive electronstend to receive electronsExample of lithium chlorideExample of lithium chlorideLi + Cl = Li + Cl = LiClLiCl

Bonding Bonding viavia electrical attractionelectrical attraction between Li+ and Clbetween Li+ and Cl--Li+ + Cl Li+ + Cl -- = = LiLi++ClCl--

Consequence: ionic bonds are underpinned by charged Consequence: ionic bonds are underpinned by charged ionsions and and tend to form crystals of very specific and repeating geometry tend to form crystals of very specific and repeating geometry (very rigid)(very rigid)Example: Example: NaClNaCl is based on ionic bonds and is saltis based on ionic bonds and is salt

Ionic Bonding: SaltIonic Bonding: Salt

Metallic BondsMetallic Bonds

Elements that do not give or take electrons (ionic Elements that do not give or take electrons (ionic bonds) but share electronsbonds) but share electronsValence electrons tend to move freely between both Valence electrons tend to move freely between both atoms (contrast with ionic bonds)atoms (contrast with ionic bonds)Significance of sharing electrons: compounds tend to Significance of sharing electrons: compounds tend to show two featuresshow two features

Malleability (easily worked or pounded)Malleability (easily worked or pounded)Conductive of electricity (good conductors)Conductive of electricity (good conductors)

ExamplesExamplesGold jewelryGold jewelryCopper wireCopper wire

Covalent BondsCovalent Bonds

Extremes of behavior in bondingExtremes of behavior in bondingAccept or give away electrons (ionic bonds)Accept or give away electrons (ionic bonds)No tendency to share (noble gases)No tendency to share (noble gases)

Intermediate between these two extremesIntermediate between these two extremesDo not form ionic bondsDo not form ionic bondsDo not form metallic bondsDo not form metallic bondsYet share 1, 2, 3 and 4 electrons in unique Yet share 1, 2, 3 and 4 electrons in unique arrangement called arrangement called covalentcovalent bondsbonds

Key: Key: orbits of valence electrons are sharedorbits of valence electrons are shared so that so that electrons are shared (and move) between valence shells of electrons are shared (and move) between valence shells of adjacent atomsadjacent atoms

Covalent BondsCovalent Bonds

Example of hydrogen fluoride (HF)Example of hydrogen fluoride (HF)1111H and H and 99

1919FF

Note: Valence shell for both atoms are fullNote: Valence shell for both atoms are fullSingle bond sharedSingle bond sharedDouble bondDouble bond

Covalent Bonds: CarbonCovalent Bonds: Carbon

661212C is a special case (important in biology & chemistry)C is a special case (important in biology & chemistry)

Valence electrons for C are 4 (1 in each orbit) and intermediateValence electrons for C are 4 (1 in each orbit) and intermediatebetween giving and acceptingbetween giving and acceptingC C -- C single covalent bond (1 orbit)C single covalent bond (1 orbit)CCC C -- C two covalent bonds involving 2 orbitsC two covalent bonds involving 2 orbitsUnique behavior of C CUnique behavior of C C

CC--CC--CCCC

Behavior of Valence Electrons: Five OptionsBehavior of Valence Electrons: Five Options

No action (e.g., inert gases)No action (e.g., inert gases)Give away one or more electrons in valence Give away one or more electrons in valence state (positive ion leading to ionic bond)state (positive ion leading to ionic bond)Accept one or more electrons to valence state Accept one or more electrons to valence state (negative ion leading to ionic bond)(negative ion leading to ionic bond)Share an electron with many other atoms Share an electron with many other atoms without respect to an orbit (metallic bond)without respect to an orbit (metallic bond)Share one or more electrons plus their orbits Share one or more electrons plus their orbits with another atom (covalent bond)with another atom (covalent bond)

Next Week’s Lab: Next Week’s Lab: Evaporation and Chemical StructureEvaporation and Chemical Structure

Vaporization and chemical properties of Vaporization and chemical properties of moleculesmolecules

Liquid to gas state changeLiquid to gas state changeState change has energy cost: endothermic State change has energy cost: endothermic (temperature decrease)(temperature decrease)

Temperature change is a function of chemical Temperature change is a function of chemical structure of moleculestructure of molecule

Bonding and polarityBonding and polarity

Intermolecular Forces: Polarization & Intermolecular Forces: Polarization & Hydrogen BondingHydrogen Bonding

Example of water (HExample of water (H22O)O)++HH HH++

OO--

When one molecule’s distribution of atoms results in one side ofWhen one molecule’s distribution of atoms results in one side ofthe molecule having either a + or the molecule having either a + or –– chargechargeResulting distribution of charges causes adjoining HResulting distribution of charges causes adjoining H22O molecule O molecule to align with + and to align with + and –– charges to be most stablecharges to be most stableCalled “polarity” of molecule (e.g., magnet)Called “polarity” of molecule (e.g., magnet)

Intermolecular Forces: Van der Waal Intermolecular Forces: Van der Waal ForcesForces

In polarity, specific and rigid + and In polarity, specific and rigid + and –– fields on each fields on each molecule that does not change over timemolecule that does not change over timeWhen molecules converge, inevitable that electrons When molecules converge, inevitable that electrons shift and reshift and re--distribute (e.g., planar compound)distribute (e.g., planar compound)In reIn re--distribution, small net attraction between distribution, small net attraction between molecules arise and two molecules for weak bondmolecules arise and two molecules for weak bond

Graphite pencil leadGraphite pencil leadStack of paperStack of paper

Valence Electrons and Chemical Valence Electrons and Chemical BondingBonding

Review valence electronsReview valence electronsPrinciples of “Bonds Away”Principles of “Bonds Away”Ionic BondsIonic BondsMetallic BondsMetallic BondsCovalent BondsCovalent BondsIntermolecular ForcesIntermolecular ForcesCommon Chemical Reactions (pH)Common Chemical Reactions (pH)

Acid Acid –– Base Reaction: MeasurementBase Reaction: Measurement

pH scalepH scaleAny increase in H+ results in more acid solution from Any increase in H+ results in more acid solution from 7 to 07 to 0Any increase in OHAny increase in OH-- results in more basic solution results in more basic solution from 7 to 14from 7 to 14ExamplesExamples

Rainwater of 5.6 means what?Rainwater of 5.6 means what?Cell pH value of 6Cell pH value of 6--8 means what?8 means what?Importance to biological systems and bufferingImportance to biological systems and buffering

Take Home MessageTake Home Message

When atoms combine to produce molecules and compounds, When atoms combine to produce molecules and compounds, expect the chemical properties of the molecules/compounds to expect the chemical properties of the molecules/compounds to be far different than that of the constituent atoms (hierarchy be far different than that of the constituent atoms (hierarchy theory)theory)Atoms bind together by reAtoms bind together by re--arranging and sharing their electronsarranging and sharing their electrons

Ionic bondsIonic bondsMetallic bondsMetallic bondsCovalent BondsCovalent BondsIntermolecular forces (e.g., hydrogen bond)Intermolecular forces (e.g., hydrogen bond)

Chemical interactions make and break bonds between atoms and Chemical interactions make and break bonds between atoms and in so doing effect a change in energy (potential and kinetic)in so doing effect a change in energy (potential and kinetic)Weak chemical bonds (e.g., covalent bonds) play a very importantWeak chemical bonds (e.g., covalent bonds) play a very importantrole in the chemistry of liferole in the chemistry of life

Chapter Deletions (No. 9)Chapter Deletions (No. 9)

Pp 202 (A Closer look)Pp 202 (A Closer look)Pp 204 (A Closer Look)Pp 204 (A Closer Look)Pp 203 (Composition of Compounds) Pp 203 (Composition of Compounds) –– 211 211 (Ion Exchange Reactions)(Ion Exchange Reactions)