Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings Phones must be put away...

Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings

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Chapter 2: Chapter 2: Chemical Context of LifeChemical Context of Life Atoms and MoleculesAtoms and Molecules

Chapter 2: Chapter 2: Chemical Context of LifeChemical Context of Life Atoms and MoleculesAtoms and Molecules

Elements and CompoundsElements and Compounds

Organisms are composed of matter, whichOrganisms are composed of matter, whichis anything that takes up space and hasis anything that takes up space and hasmassmass

Matter is made up of elements, Matter is made up of elements, substancessubstancesthat cannot be broken down to other that cannot be broken down to other substances by chemical reactionssubstances by chemical reactions

92 unique elements exist in nature92 unique elements exist in nature• More have been created in the labMore have been created in the lab

Organisms are composed of matter, whichOrganisms are composed of matter, whichis anything that takes up space and hasis anything that takes up space and hasmassmass

Matter is made up of elements, Matter is made up of elements, substancessubstancesthat cannot be broken down to other that cannot be broken down to other substances by chemical reactionssubstances by chemical reactions

92 unique elements exist in nature92 unique elements exist in nature• More have been created in the labMore have been created in the lab


Compounds

Sodium Chloride Sodium Chloride

+

• A compound

• Is a substance consisting of two or more elements combined in a fixed ratio

• Has characteristics different from those of its elements

Essential Elements of LifeEssential Elements of LifeEssential Elements of LifeEssential Elements of LifeEssential elementsEssential elements

• Include carbon, hydrogen, oxygen, Include carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur.nitrogen, phosphorus and sulfur.

• Make up 96% of living matterMake up 96% of living matter

Essential elementsEssential elements• Include carbon, hydrogen, oxygen, Include carbon, hydrogen, oxygen,

nitrogen, phosphorus and sulfur.nitrogen, phosphorus and sulfur.• Make up 96% of living matterMake up 96% of living matter


Other Elements

A few other elements make up the remaining 4% of living matter

Each elementEach elementConsists of a certain kind of atom that is Consists of a certain kind of atom that is

different from those of other elementsdifferent from those of other elements

An An atomatomIs the smallest unit of matter that still Is the smallest unit of matter that still

retains the properties of an elementretains the properties of an element

Each elementEach elementConsists of a certain kind of atom that is Consists of a certain kind of atom that is

different from those of other elementsdifferent from those of other elements

An An atomatomIs the smallest unit of matter that still Is the smallest unit of matter that still

retains the properties of an elementretains the properties of an element

ElementsElements

Subatomic ParticlesSubatomic ParticlesSubatomic ParticlesSubatomic Particles• Atoms of each element are composed Atoms of each element are composed

of even smaller parts called subatomic of even smaller parts called subatomic particlesparticles

• Relevant subatomic particles includeRelevant subatomic particles include• Neutrons, which have no electrical chargeNeutrons, which have no electrical charge• Protons, which are positively chargedProtons, which are positively charged• ElectronsElectrons, which are negatively charged, which are negatively charged

• Atoms of each element are composed Atoms of each element are composed of even smaller parts called subatomic of even smaller parts called subatomic particlesparticles

• Relevant subatomic particles includeRelevant subatomic particles include• Neutrons, which have no electrical chargeNeutrons, which have no electrical charge• Protons, which are positively chargedProtons, which are positively charged• ElectronsElectrons, which are negatively charged, which are negatively charged

Subatomic ParticlesSubatomic ParticlesSubatomic ParticlesSubatomic ParticlesProtons and neutronsProtons and neutrons

Are found in the atomic nucleusAre found in the atomic nucleus

ElectronsElectronsSurround the nucleus in a “cloud”Surround the nucleus in a “cloud”

Protons and neutronsProtons and neutronsAre found in the atomic nucleusAre found in the atomic nucleus

ElectronsElectronsSurround the nucleus in a “cloud”Surround the nucleus in a “cloud”


Model of the Atom

Nucleus

(a) (b) In this even more simplifiedmodel, the electrons areshown as two small bluespheres on a circle around thenucleus.

Cloud of negativecharge (2 electrons)

Electrons

This model represents theelectrons as a cloud ofnegative charge, as if we hadtaken many snapshots of the 2electrons over time, with eachdot representing an electron‘sposition at one point in time.

• Simplified models of an atom

Atomic Number and Atomic MassAtomic Number and Atomic MassAtomic Number and Atomic MassAtomic Number and Atomic MassAtoms of the various elementsAtoms of the various elements

• Differ in their number of subatomic Differ in their number of subatomic particlesparticles

The The atomic number atomic number of an elementof an element• Is the number of protonsIs the number of protons• Is unique to each elementIs unique to each element

The The mass numbermass number of an elementof an element• Is the sum of protons plus neutrons in Is the sum of protons plus neutrons in

the nucleus of an atomthe nucleus of an atom• Is an approximation of the atomic mass Is an approximation of the atomic mass

of an atomof an atom

Atoms of the various elementsAtoms of the various elements• Differ in their number of subatomic Differ in their number of subatomic

particlesparticles

The The atomic number atomic number of an elementof an element• Is the number of protonsIs the number of protons• Is unique to each elementIs unique to each element

The The mass numbermass number of an elementof an element• Is the sum of protons plus neutrons in Is the sum of protons plus neutrons in

the nucleus of an atomthe nucleus of an atom• Is an approximation of the atomic mass Is an approximation of the atomic mass

of an atomof an atom

IsotopesIsotopesIsotopesIsotopes• Atoms of a given element may occur Atoms of a given element may occur

in different forms called Isotopesin different forms called Isotopes• Isotopes of a given elementIsotopes of a given element

• Differ in the number of neutrons Differ in the number of neutrons in the in the atomic nucleusatomic nucleus

• Have the same number of protonsHave the same number of protons

• Radioactive isotopesRadioactive isotopes• Spontaneously give off particles and Spontaneously give off particles and

energyenergy

• Atoms of a given element may occur Atoms of a given element may occur in different forms called Isotopesin different forms called Isotopes

• Isotopes of a given elementIsotopes of a given element• Differ in the number of neutrons Differ in the number of neutrons in the in the

atomic nucleusatomic nucleus• Have the same number of protonsHave the same number of protons

• Radioactive isotopesRadioactive isotopes• Spontaneously give off particles and Spontaneously give off particles and

energyenergy


Radioactive IsotopesRadioactive Isotopes

Can be used in biology and medicineCan be used in biology and medicine

Cancerous throat tissue

Energy Levels of ElectronsEnergy Levels of ElectronsEnergy Levels of ElectronsEnergy Levels of Electrons

An atom’s electronsAn atom’s electrons• Vary in the amount of energy they possessVary in the amount of energy they possess

EnergyEnergy• Is defined as the capacity to cause changeIs defined as the capacity to cause change

Potential energyPotential energy• Is the energy that matter possesses because of Is the energy that matter possesses because of

its location or structureits location or structure

An atom’s electronsAn atom’s electrons• Vary in the amount of energy they possessVary in the amount of energy they possess

EnergyEnergy• Is defined as the capacity to cause changeIs defined as the capacity to cause change

Potential energyPotential energy• Is the energy that matter possesses because of Is the energy that matter possesses because of

its location or structureits location or structure


Energy Levels

The electrons of an atom

• Differ in the amounts of potential energy they possess

A ball bouncing down a flightof stairs provides an analogyfor energy levels of electrons,because the ball can only reston each step, not betweensteps.

(a)


Electron Shells

Energy levels

• Are represented by electron shellsThird energy level (shell)

Second energy level (shell)

First energy level (shell)

Energyabsorbed

Energylost

An electron can move from one level to another only if the energyit gains or loses is exactly equal to the difference in energy betweenthe two levels. Arrows indicate some of the step-wise changes inpotential energy that are possible.

(b)

Atomic nucleus

Electron Configuration and Electron Configuration and Chemical PropertiesChemical Properties

Electron Configuration and Electron Configuration and Chemical PropertiesChemical Properties

Valence electronsValence electrons• Are those in the outermost, or valence shellAre those in the outermost, or valence shell• Determines the chemical behavior of an atomDetermines the chemical behavior of an atom

The The valence numbervalence number of an atom is the number of unpaired of an atom is the number of unpaired electrons in its valence shell electrons in its valence shell

This valence number determines the number of bonds the atom can This valence number determines the number of bonds the atom can form.form.

Valence electronsValence electrons• Are those in the outermost, or valence shellAre those in the outermost, or valence shell• Determines the chemical behavior of an atomDetermines the chemical behavior of an atom

The The valence numbervalence number of an atom is the number of unpaired of an atom is the number of unpaired electrons in its valence shell electrons in its valence shell

This valence number determines the number of bonds the atom can This valence number determines the number of bonds the atom can form.form.


Periodic Table Periodic Table (Campbell & Reece, p.32)(Campbell & Reece, p.32)

The periodic table of the elementsThe periodic table of the elements

• Shows the electron distribution for all the Shows the electron distribution for all the elementselements

Secondshell

Helium

2He

Firstshell

Thirdshell

Hydrogen

1H

2He

4.00Atomic mass

Atomic number

Element symbol

Electron-shelldiagram

Lithium

3LiBeryllium

4BeBoron

3BCarbon

6CNitrogen

7NOxygen

8OFluorine

9FNeon

10Ne

Sodium

11NaMagnesium

12MgAluminum

13AlSilicon

14SiPhosphorus

15PSulfur

16SChlorine

17ClArgon

18Ar


Which drawing depicts an atom with a valence of 2?

a. A

b. B

c. C

d. D

e. E

OrbitalsOrbitalsOrbitalsOrbitals

An orbitalAn orbital• Is the three-dimensional space where an Is the three-dimensional space where an

electron is found 90% of the timeelectron is found 90% of the time

An orbitalAn orbital• Is the three-dimensional space where an Is the three-dimensional space where an

electron is found 90% of the timeelectron is found 90% of the time


OrbitalsOrbitals

Each electron shellEach electron shell

• Consists of a specific number of orbitalsConsists of a specific number of orbitals

Electron orbitals.Each orbital holds

up to two electrons.

1s orbital 2s orbital Three 2p orbitals 1s, 2s, and 2p orbitals

(a) First shell (maximum 2 electrons)

(b) Second shell (maximum 8 electrons)

(c) Neon, with two filled shells (10 electrons)

Electron-shell diagrams.Each shell is shown withits maximum number of

electrons, grouped in pairs.

x

Z

Y

Filling pattern of electron orbitals:Filling pattern of electron orbitals:Electrons fill lower shells first before outer shellsElectrons fill lower shells first before outer shells

• First: two electrons in 1s orbitalFirst: two electrons in 1s orbital• Then: two electrons 2s orbitalThen: two electrons 2s orbital• Then: one electron in each of 2p orbitals Then: one electron in each of 2p orbitals

before adding more to any 2p if necessarybefore adding more to any 2p if necessaryCarbon breaks these rules: spCarbon breaks these rules: sp3 3 hybridizationhybridization

• Allows 4 unpaired electrons = 4 bondsAllows 4 unpaired electrons = 4 bonds

Filling pattern of electron orbitals:Filling pattern of electron orbitals:Electrons fill lower shells first before outer shellsElectrons fill lower shells first before outer shells

• First: two electrons in 1s orbitalFirst: two electrons in 1s orbital• Then: two electrons 2s orbitalThen: two electrons 2s orbital• Then: one electron in each of 2p orbitals Then: one electron in each of 2p orbitals

before adding more to any 2p if necessarybefore adding more to any 2p if necessaryCarbon breaks these rules: spCarbon breaks these rules: sp3 3 hybridizationhybridization

• Allows 4 unpaired electrons = 4 bondsAllows 4 unpaired electrons = 4 bonds

OrbitalsOrbitals

http://www.wisc-online.com/objects/index_tj.asp?objid=GCH904http://www.wisc-online.com/objects/index_tj.asp?objid=GCH904

Structure of an Atom

http://www.wisc-online.com/objects/index_tj.asp?objid=GCH904




BondsBondsBondsBonds• The formation and function of The formation and function of

molecules depend on chemical molecules depend on chemical bonding between atoms.bonding between atoms.

• The formation and function of The formation and function of molecules depend on chemical molecules depend on chemical bonding between atoms.bonding between atoms.

BondsBondsBondsBonds


• A covalent bondA covalent bond• Is the Is the sharingsharing of a pair of valence of a pair of valence

electronselectrons


• A covalent bondA covalent bond• Is the Is the sharingsharing of a pair of valence of a pair of valence

electronselectrons


Covalent Bonds

Formation of a covalent bondHydrogen atoms (2 H)

Hydrogenmolecule (H2)

+ +

+ +

+ +

In each hydrogenatom, the single electronis held in its orbital byits attraction to theproton in the nucleus.

1

When two hydrogenatoms approach eachother, the electron ofeach atom is alsoattracted to the protonin the other nucleus.

2

The two electronsbecome shared in a covalent bond,forming an H2

molecule.

3

A moleculeA molecule• Consists of two or more atoms held Consists of two or more atoms held together by covalent bondstogether by covalent bonds

A single bondA single bond• Is the sharing of one pair of valence Is the sharing of one pair of valence electronselectrons

A A double bonddouble bond• Is the sharing of two pairs of valence Is the sharing of two pairs of valence electronselectrons

A moleculeA molecule• Consists of two or more atoms held Consists of two or more atoms held together by covalent bondstogether by covalent bonds

A single bondA single bond• Is the sharing of one pair of valence Is the sharing of one pair of valence electronselectrons

A A double bonddouble bond• Is the sharing of two pairs of valence Is the sharing of two pairs of valence electronselectrons

Covalent BondsCovalent Bonds


Covalent Bonds

(a)

(b)

Name(molecularformula)

Electron-shell

diagram

Structuralformula

Space-fillingmodel

Hydrogen (H2). Two hydrogen atoms can form a single bond.

Oxygen (O2). Two oxygen atoms share two pairs of electrons to form a double bond.

H H

O O

• Single and double covalent bonds


Covalent Bonds

Name(molecularformula)

Electron-shell

diagram

Structuralformula

Space-fillingmodel

(c)

Methane (CH4). Four hydrogen atoms can satisfy the valence ofone carbonatom, formingmethane.

Water (H2O). Two hydrogenatoms and one oxygen atom arejoined by covalent bonds to produce a molecule of water.

(d)

HO

H

H H

H

H

C

• Covalent bonding in molecules

ElectronegativityElectronegativityElectronegativityElectronegativityElectronegativityElectronegativity

• Is the attraction of a particular kind of Is the attraction of a particular kind of atom for the electrons in a covalent bondatom for the electrons in a covalent bond

The more electronegative an atomThe more electronegative an atom• The more strongly it pulls shared The more strongly it pulls shared

electrons toward itselfelectrons toward itself

In a In a nonpolar covalentnonpolar covalent bond bond• The atoms have similar electronegativities The atoms have similar electronegativities • Share the electron equallyShare the electron equally

ElectronegativityElectronegativity• Is the attraction of a particular kind of Is the attraction of a particular kind of

atom for the electrons in a covalent bondatom for the electrons in a covalent bond

The more electronegative an atomThe more electronegative an atom• The more strongly it pulls shared The more strongly it pulls shared

electrons toward itselfelectrons toward itself

In a In a nonpolar covalentnonpolar covalent bond bond• The atoms have similar electronegativities The atoms have similar electronegativities • Share the electron equallyShare the electron equally

Thinking QuestionThinking QuestionThinking QuestionThinking QuestionElectronegativity values increase as Electronegativity values increase as the atom becomes smaller and as you the atom becomes smaller and as you go from left to right on the Periodic go from left to right on the Periodic Table. Why do you think this is so?Table. Why do you think this is so?

Electronegativity values increase as Electronegativity values increase as the atom becomes smaller and as you the atom becomes smaller and as you go from left to right on the Periodic go from left to right on the Periodic Table. Why do you think this is so?Table. Why do you think this is so?


Polar Covalent BondsPolar Covalent Bonds

This results in a partial negative charge on theoxygen and apartial positivecharge onthe hydrogens.

H2O

–

O

H H+ +

Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen.

In a polar covalent bond

• The atoms have differing electronegativities

• Share the electrons unequally


Water ModelsWater Models

This results in a partial negative charge on theoxygen and apartial positivecharge onthe hydrogens.

H2O

–

O

H H+ +

Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen.

•Make 10 or more paper cut-outs of water atomsMake 10 or more paper cut-outs of water atoms

•Arrange them to demonstrate water crystal latticeArrange them to demonstrate water crystal lattice

Covalent Bonds ReviewedCovalent Bonds ReviewedCovalent Bonds ReviewedCovalent Bonds Reviewed

Ionic BondsIonic BondsIonic BondsIonic Bonds

In some cases, atoms strip electrons In some cases, atoms strip electrons away from their bonding partnersaway from their bonding partners

Electron transfer between two atoms Electron transfer between two atoms creates ionscreates ions

IonsIons• Are atoms with more or fewer electrons Are atoms with more or fewer electrons

than usual (charged atoms)than usual (charged atoms)

• Such as NaSuch as Na++, Cl, Cl--, K, K++, PO, PO443-3-

In some cases, atoms strip electrons In some cases, atoms strip electrons away from their bonding partnersaway from their bonding partners

Electron transfer between two atoms Electron transfer between two atoms creates ionscreates ions

IonsIons• Are atoms with more or fewer electrons Are atoms with more or fewer electrons

than usual (charged atoms)than usual (charged atoms)

• Such as NaSuch as Na++, Cl, Cl--, K, K++, PO, PO443-3-

Ionic BondsIonic BondsIonic BondsIonic Bonds

An anionAn anion• Atom that gains electronsAtom that gains electrons• Is negatively chargedIs negatively charged

• ClCl--, PO, PO443-3-

A A cationcation• Atom that lost electronsAtom that lost electrons• Is positively chargedIs positively charged• NaNa++, K, K++

An anionAn anion• Atom that gains electronsAtom that gains electrons• Is negatively chargedIs negatively charged

• ClCl--, PO, PO443-3-

A A cationcation• Atom that lost electronsAtom that lost electrons• Is positively chargedIs positively charged• NaNa++, K, K++


Ionic Bonds

Cl–

Chloride ion(an anion)

–

The lone valence electron of a sodiumatom is transferred to join the 7 valenceelectrons of a chlorine atom.

1 Each resulting ion has a completedvalence shell. An ionic bond can formbetween the oppositely charged ions.

2

Na NaCl Cl

+

NaSodium atom

(an unchargedatom)

ClChlorine atom(an uncharged

atom)

Na+

Sodium on(a cation)

Sodium chloride (NaCl)

An ionic bond

• Is an attraction between anions and cations


Ionic BondsIonic Bonds

Na+

Cl–

Ionic compounds

• Are often called salts, which may form crystals


Hydrogen Bonds

– +

+

Water(H2O)

Ammonia(NH3)

OH

H

+

–

N

HH H

A hydrogenbond results from the attraction between thepartial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.

+ +

A hydrogen bond

• Forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom

Molecular Shape and FunctionMolecular Shape and FunctionMolecular Shape and FunctionMolecular Shape and Function

The precise shape of a moleculeThe precise shape of a molecule• Is usually very important to its Is usually very important to its

function in the living cellfunction in the living cell• Is determined by the positions of its Is determined by the positions of its

atoms’ valence orbitalsatoms’ valence orbitals

The precise shape of a moleculeThe precise shape of a molecule• Is usually very important to its Is usually very important to its

function in the living cellfunction in the living cell• Is determined by the positions of its Is determined by the positions of its

atoms’ valence orbitalsatoms’ valence orbitals


Molecular Shape

Space-fillingmodel

Hybrid-orbital model(with ball-and-stick

model superimposed)UnbondedElectron pair

104.5°

O

HWater (H2O)

Methane (CH4)

H

H H

H

C

O

H

H

H

C

Ball-and-stickmodel

H H

H

H

Chemical ReactionsChemical ReactionsChemical ReactionsChemical Reactions

Chemical reactionsChemical reactions• Are the making and breaking of Are the making and breaking of

chemical bondschemical bonds• Lead to changes in the composition Lead to changes in the composition

of matterof matter

Chemical reactionsChemical reactions• Are the making and breaking of Are the making and breaking of

chemical bondschemical bonds• Lead to changes in the composition Lead to changes in the composition

of matterof matter


Chemical ReactionsChemical Reactions

Reactants Reaction Product

2 H2 O2 2H2O

+

+

• Chemical reactions

• Convert reactants to products

EquilibriumEquilibriumEquilibriumEquilibrium

Chemical equilibriumChemical equilibrium• Is reached when the forward and Is reached when the forward and

reverse reaction rates are equalreverse reaction rates are equal

Chemical equilibriumChemical equilibrium• Is reached when the forward and Is reached when the forward and

reverse reaction rates are equalreverse reaction rates are equal

Key Points of Chapter 2Key Points of Chapter 2Key Points of Chapter 2Key Points of Chapter 2

• Matter consists of chemical elements in Matter consists of chemical elements in pure form and in combinations called pure form and in combinations called compoundscompounds

• An element’s properties depend on the An element’s properties depend on the structure of its atomsstructure of its atoms

• The formation and function of The formation and function of molecules depends on chemical molecules depends on chemical bonding between atomsbonding between atoms

• Chemical reactions make and break Chemical reactions make and break chemical bondschemical bonds

• Matter consists of chemical elements in Matter consists of chemical elements in pure form and in combinations called pure form and in combinations called compoundscompounds

• An element’s properties depend on the An element’s properties depend on the structure of its atomsstructure of its atoms

• The formation and function of The formation and function of molecules depends on chemical molecules depends on chemical bonding between atomsbonding between atoms

• Chemical reactions make and break Chemical reactions make and break chemical bondschemical bonds

Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings Phones must be put away...

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