Investigating Atoms and Atomic Investigating Atoms and Atomic TheoryTheory

Students should be able to:Students should be able to:Describe the particle theory of matter. PS.2aDescribe the particle theory of matter. PS.2aUse the Bohr model to differentiate among the Use the Bohr model to differentiate among the

three basic particles in the atom (proton, three basic particles in the atom (proton, neutron, and electron) and their charges, neutron, and electron) and their charges, relative masses, and locations. PS.3relative masses, and locations. PS.3

Compare the Bohr atomic model to the electron Compare the Bohr atomic model to the electron cloud model with respect to their ability to cloud model with respect to their ability to represent accurately the structure of the represent accurately the structure of the atom.PS.3atom.PS.3

Atomos: Not to Be CutAtomos: Not to Be Cut

The History of Atomic TheoryThe History of Atomic Theory

Atomic ModelsAtomic Models This model of the This model of the

atom may look atom may look familiar to you. This is familiar to you. This is the Bohr model. In the Bohr model. In this model, the this model, the nucleus is orbited by nucleus is orbited by electrons, which are electrons, which are in different energy in different energy levels. levels. A model uses familiar ideas to A model uses familiar ideas to

explain unfamiliar facts explain unfamiliar facts observed in nature.observed in nature.

A model can be changed as A model can be changed as new information is collected.new information is collected.

The atomic model The atomic model has changed has changed throughout the throughout the centuries, starting centuries, starting in 400 BC, when it in 400 BC, when it looked like looked like a a billiard ball billiard ball →→

Who are these men?Who are these men?

In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

DemocritusDemocritus

This is the Greek This is the Greek philosopher Democritus philosopher Democritus who began the search for who began the search for a description of matter a description of matter more than more than 24002400 years ago. years ago. He asked: Could matter He asked: Could matter

be divided into smaller be divided into smaller and smaller pieces and smaller pieces forever, or was there a forever, or was there a limitlimit to the number of to the number of times a piece of matter times a piece of matter could be could be divideddivided? ?

400 BC

AtomosAtomos

His theory: Matter could not His theory: Matter could not be divided into smaller and be divided into smaller and smaller pieces forever, smaller pieces forever, eventually the smallest eventually the smallest possible piece would be possible piece would be obtained.obtained.

This piece would be This piece would be indivisible.indivisible.

He named the smallest He named the smallest piece of matter “atomos,” piece of matter “atomos,” meaning “not to be cut.”meaning “not to be cut.”

AtomosAtomos

To Democritus, atoms To Democritus, atoms were were smallsmall, hard , hard particles that were all particles that were all made of the same made of the same material but were material but were differentdifferent shapes and shapes and sizes.sizes.

Atoms were Atoms were infiniteinfinite in in number, always number, always moving and capable of moving and capable of joining together.joining together.

This theory was This theory was ignored and forgotten ignored and forgotten for more than for more than 2000 2000 years!years!

Why?Why?The eminent The eminent

philosophers of philosophers of the time, the time, AristotleAristotle and and Plato, had a Plato, had a more more respected, respected, (and ultimately (and ultimately wrongwrong) theory.) theory.

Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

The Early History of ChemistryThe Early History of ChemistryThe Early History of ChemistryThe Early History of Chemistry

Before 16th CenturyBefore 16th Century AlchemyAlchemy: Attempts (scientific or : Attempts (scientific or otherwise) to change cheap metals into goldotherwise) to change cheap metals into gold

17th Century17th Century Robert BoyleRobert Boyle: First “chemist” to : First “chemist” to perform quantitative experimentsperform quantitative experiments

18th Century18th Century George StahlGeorge Stahl: : PhlogistonPhlogiston flows out of a flows out of a burning material.burning material. Joseph PriestleyJoseph Priestley: Discovers oxygen : Discovers oxygen gas, “dephlogisticated air.”gas, “dephlogisticated air.”

The Early History of ChemistryThe Early History of Chemistry

1818thth Century, continued: Century, continued:Antoine Lavoisier (1743-1794) Antoine Lavoisier (1743-1794) The The

Father of Modern Chemistry, suggested Father of Modern Chemistry, suggested the law of conservation of mass: Mass is the law of conservation of mass: Mass is neither created nor destroyed in a neither created nor destroyed in a chemical reaction.chemical reaction.

Dalton’s ModelDalton’s Model

In the early 1800s, In the early 1800s, the English the English Chemist John Chemist John Dalton Dalton performed a performed a number of number of experiments that experiments that eventually led to eventually led to the acceptance of the acceptance of the idea of atoms.the idea of atoms.

Other Fundamental Chemical LawsOther Fundamental Chemical LawsOther Fundamental Chemical LawsOther Fundamental Chemical Laws

A given compound always contains exactly A given compound always contains exactly the same proportion of elements by mass.the same proportion of elements by mass.

Carbon tetrachloride is always 1 atom Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine.carbon per 4 atoms chlorine.

Law of Definite Proportion (Joseph Proust)

Other Fundamental Chemical LawsOther Fundamental Chemical LawsOther Fundamental Chemical LawsOther Fundamental Chemical Laws

When two elements form a series of When two elements form a series of compounds, the ratios of the masses of compounds, the ratios of the masses of the second element that combine with 1 the second element that combine with 1 gram of the first element can always be gram of the first element can always be reduced to small whole numbers.reduced to small whole numbers.

The ratio of the masses of oxygen in The ratio of the masses of oxygen in HH22O and HO and H22OO22 will be a small whole will be a small whole

number (“2”).number (“2”). http://wps.prenhall.com/wps/media/objects/4974/5093961/emedia/ch02/LawOfMultipleProport

ions.html

Law of Multiple Proportions (John Dalton)

Dalton’s TheoryDalton’s Theory

He deduced that all He deduced that all elementselements are composed of are composed of atoms. Atoms are atoms. Atoms are indivisible and indivisible and indestructible particles.indestructible particles.

Atoms of the Atoms of the samesame element element are exactly alike.are exactly alike.

Atoms of Atoms of differentdifferent elements elements are are differentdifferent..

CompoundsCompounds are formed by are formed by the joining of atoms of two the joining of atoms of two or more elements.or more elements.

..

This theory This theory became one became one of the of the foundations foundations of modern of modern chemistry.chemistry.

Thomson’s Plum Pudding Thomson’s Plum Pudding ModelModel

In In 18971897, the , the English scientist English scientist J.J. Thomson J.J. Thomson provided the first provided the first hint that an atom hint that an atom is made of even is made of even smallersmaller particles. particles.

Thomson ModelThomson Model He proposed a He proposed a

model of the atom model of the atom that is sometimes that is sometimes called the “called the “PlumPlum PuddingPudding” model. ” model.

Atoms were made Atoms were made from a positively from a positively chargedcharged substancesubstance with negatively with negatively charged electrons charged electrons scatteredscattered about, like about, like raisins in a pudding.raisins in a pudding.

Thomson ModelThomson Model

Thomson studied Thomson studied the the passage passage of an of an electric current electric current through a gas.through a gas.

As the current As the current passed through passed through the gas, it gave off the gas, it gave off rays of rays of negatively negatively charged particles.charged particles.

Thomson ModelThomson Model

This surprised This surprised Thomson, Thomson, because the because the atoms of the gas atoms of the gas were uncharged. were uncharged. Where had the Where had the negative charges negative charges come from?come from?

Where did they come from?

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!

Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.

But he could never find them.

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

In 1908, the English In 1908, the English physicist Ernest physicist Ernest Rutherford was hard Rutherford was hard at work on an at work on an experiment that experiment that seemed to have little seemed to have little to do with unraveling to do with unraveling the mysteries of the the mysteries of the atomic structure.atomic structure.

Rutherford’s experiment Involved firing Rutherford’s experiment Involved firing a stream of tiny a stream of tiny positively chargedpositively charged particles at a thin sheet of particles at a thin sheet of gold foilgold foil (2000 atoms thick)(2000 atoms thick)

MostMost of the positively of the positively charged “bullets” passed charged “bullets” passed right through the gold right through the gold atoms in the sheet of atoms in the sheet of gold gold foilfoil without changing without changing course at all.course at all.

SomeSome of the positively of the positively charged “bullets,” however, charged “bullets,” however, did bounce away from the did bounce away from the gold sheet as if they had hit gold sheet as if they had hit something something solidsolid. He knew . He knew that positive charges that positive charges repelrepel positive charges.positive charges.

http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

This could only mean that the gold atoms in the This could only mean that the gold atoms in the sheet were mostly sheet were mostly open spaceopen space. Atoms were . Atoms were notnot a pudding filled with a positively charged a pudding filled with a positively charged material.material.

Rutherford concluded that an atom had a Rutherford concluded that an atom had a small, small, dense, positively charged centerdense, positively charged center that that repelledrepelled his positively charged “bullets.”his positively charged “bullets.”

He called the center of the atom the “He called the center of the atom the “nucleusnucleus”” The nucleus is The nucleus is tiny tiny compared to the atom as a compared to the atom as a

whole. whole.

RutherfordRutherford

Rutherford reasoned Rutherford reasoned that all of an atom’s that all of an atom’s positively charged positively charged particles were particles were containedcontained in the in the nucleus. The nucleus. The negatively charged negatively charged particles were particles were scatteredscattered outside the outside the nucleus around the nucleus around the atom’s atom’s edgeedge..

Bohr ModelBohr ModelIn 1913, the Danish In 1913, the Danish

scientist Niels Bohr scientist Niels Bohr proposed an proposed an improvement. In his improvement. In his model, he placed model, he placed each electron in a each electron in a specificspecific energy energy level.level.

Bohr ModelBohr Model According to Bohr’s According to Bohr’s

atomic model, atomic model, electrons move in electrons move in definite definite orbitsorbits around around the nucleus, much the nucleus, much like planets circle the like planets circle the sun. These orbits, or sun. These orbits, or energy energy levelslevels, are , are located at certainlocated at certain distances from the distances from the nucleus.nucleus.

Wave Model

The Wave ModelThe Wave Model Today’s atomic Today’s atomic

model is based on model is based on the principles of the principles of wavewave mechanicsmechanics..

According to the According to the theory of wave theory of wave mechanics, electrons mechanics, electrons do not movedo not move about about an atom in a an atom in a definite definite path,path, like the planets like the planets around the sun.around the sun.

The Wave ModelThe Wave Model

In fact, it is In fact, it is impossibleimpossible to determine the exact to determine the exact location of an electron. The location of an electron. The probableprobable location of location of an electron is based on how much an electron is based on how much energyenergy the the electron has.electron has.

According to the modern atomic model, at atom According to the modern atomic model, at atom has a has a small positively charged nucleussmall positively charged nucleus surrounded by a large region in which there are surrounded by a large region in which there are enough electrons to make an atom neutral.enough electrons to make an atom neutral.

Electron Cloud:Electron Cloud: A space in which A space in which

electrons are likely to be electrons are likely to be found.found.

Electrons Electrons whirlwhirl about the about the nucleus billions of times nucleus billions of times in one secondin one second

They are not moving They are not moving around in around in randomrandom patterns.patterns.

Location of electrons Location of electrons depends upon how much depends upon how much energyenergy the electron has. the electron has.

Electron Cloud:Electron Cloud:

Depending on their energy they are locked into a Depending on their energy they are locked into a certain area in the cloud.certain area in the cloud.

Electrons with the Electrons with the lowestlowest energy are found in energy are found in the energy level the energy level closestclosest to the nucleus to the nucleus

Electrons with the Electrons with the highesthighest energy are found energy are found in the in the outermostoutermost energy levels, farther from energy levels, farther from the nucleus.the nucleus.

IndivisibleIndivisible ElectronElectron NucleusNucleus OrbitOrbit Electron Electron CloudCloud

DemocritusDemocritus XX

DaltonDalton XX

ThomsonThomson XX

RutherfordRutherford XX XX

BohrBohr XX XX XX

WaveWave XX XX XX

The Modern View of Atomic The Modern View of Atomic StructureStructure

The Modern View of Atomic The Modern View of Atomic StructureStructure

electronselectrons

protonsprotons: found in the nucleus, they have : found in the nucleus, they have a positive charge equal in magnitude to a positive charge equal in magnitude to the electron’s negative charge.the electron’s negative charge.

neutronsneutrons: found in the nucleus, virtually : found in the nucleus, virtually same mass as a proton but no charge.same mass as a proton but no charge.

The The atomatom contains: contains:

The Mass and Change of the The Mass and Change of the Electron, Proton, and NeutronElectron, Proton, and NeutronThe Mass and Change of the The Mass and Change of the Electron, Proton, and NeutronElectron, Proton, and Neutron

Particle Mass (kg) Charge

Electron 9.11 10 31 1

Proton 1.67 10 27 1+

Neutron 1.67 10 27 0

Particles and ChargeParticles and Charge

SymbolSymbol MassMass ChargeCharge LocationLocationProton (pProton (p++) 1amu +1 nucleus) 1amu +1 nucleus

Neutron (nNeutron (noo) 1amu 0 nucleus) 1amu 0 nucleus

Electron (eElectron (e--) 1/1840 -1 electron) 1/1840 -1 electron

cloudcloud

The Chemists’ Shorthand: The Chemists’ Shorthand: Atomic SymbolsAtomic Symbols

The Chemists’ Shorthand: The Chemists’ Shorthand: Atomic SymbolsAtomic Symbols

K Element Symbol39

19

Mass number

Atomic number

p+ + no = mass number

Atomic number = #p+ or #e- in a neutral atom

Mass number – atomic number = #no

Atomic MassesAtomic Masses

Elements occur in nature as mixtures Elements occur in nature as mixtures of isotopesof isotopes

Atomic mass is the weighted average Atomic mass is the weighted average of all isotopes for an element.of all isotopes for an element.

Carbon =Carbon = 98.89% 98.89% 1212CC

1.11% 1.11% 1313CC

<0.01% <0.01% 1414CC

Carbon atomic mass = 12.01 amuCarbon atomic mass = 12.01 amu

MASS NUMBER AND AVERAGE MASS NUMBER AND AVERAGE ATOMIC MASSATOMIC MASS

Atomic masses are based on CARBON. Atomic masses are based on CARBON. The The atomic mass unitatomic mass unit is 1/12 of the is 1/12 of the mass of one carbon atom. mass of one carbon atom.

How do we calculate average atomic How do we calculate average atomic mass?mass?

Multiply the % times the mass for each Multiply the % times the mass for each isotope, then add them together.isotope, then add them together.

Average atomic massAverage atomic mass

Calculate the average mass of isotopes of Calculate the average mass of isotopes of neptunium with:neptunium with:

50.0% at 238.05 amu50.0% at 238.05 amu29.4% at 235.1 amu29.4% at 235.1 amu20.6% at 237.98 amu20.6% at 237.98 amu

(.500 x 238.05) + (.294 x 235.1) + (.206 x (.500 x 238.05) + (.294 x 235.1) + (.206 x 237.98) = 237.17amu237.98) = 237.17amu

Another problem:Another problem:

Calculate the average atomic mass of calcium Calculate the average atomic mass of calcium with these isotopes:with these isotopes:

28.4% at 40.06 amu28.4% at 40.06 amu

34.1% at 41.02 amu34.1% at 41.02 amu

22.8% at 40.89 amu22.8% at 40.89 amu

14.7% at 39.98 amu14.7% at 39.98 amu

(.284x40.06)+(.341x41.02)+(.228x40.89)+(.147x39.98)(.284x40.06)+(.341x41.02)+(.228x40.89)+(.147x39.98)

40.5640.56

Atomic MassAtomic Mass

Atomic mass is the Atomic mass is the weighted average of weighted average of all of the known all of the known isotopes of an isotopes of an element, so will element, so will always be shown as a always be shown as a decimal number.decimal number.

Chemical BondsChemical BondsChemical BondsChemical Bonds

The forces that hold atoms together in The forces that hold atoms together in compounds. compounds.

Covalent bondsCovalent bonds result from atoms result from atoms sharing sharing electronselectrons..

MoleculeMolecule: a collection of covalently-bonded : a collection of covalently-bonded atoms.atoms.

Includes all covalent compounds and Includes all covalent compounds and diatomic molecules.diatomic molecules.

The Chemists’ Shorthand:The Chemists’ Shorthand:FormulasFormulas

The Chemists’ Shorthand:The Chemists’ Shorthand:FormulasFormulas

Chemical Formula:Chemical Formula:

SymbolsSymbols = types of atoms = types of atoms

SubscriptsSubscripts = relative numbers of atoms = relative numbers of atoms

COCO22

Structural Formula:Structural Formula: Individual bonds are shown Individual bonds are shown by linesby lines..

OO==CC==OO

Chemical BondsChemical BondsChemical BondsChemical Bonds

Ionic BondingIonic Bonding: Force of attraction : Force of attraction between oppositely charged ions. between oppositely charged ions. Smallest particle is formula unit.Smallest particle is formula unit.

CationCation: A positive ion: A positive ion

MgMg2+2+, NH, NH44++

AnionAnion: A negative ion: A negative ion

ClCl, SO, SO4422

Periodic TablePeriodic TablePeriodic TablePeriodic TableElements classified by:Elements classified by:

propertiesproperties

atomic numberatomic number

Groups Groups (vertical column)(vertical column)

1A = alkali metals1A = alkali metals

2A = alkaline earth metals2A = alkaline earth metals

7A = halogens7A = halogens

8A = noble gases8A = noble gases

PeriodsPeriods (horizontal row) (horizontal row)

Stupendous SevenStupendous Seven

Periodic TablePeriodic Table

Antoine Lavoisier , 1790’s Antoine Lavoisier , 1790’s made first list of known made first list of known elements, 23 total. By 1870, there were 70elements, 23 total. By 1870, there were 70!!

John Newlands, 1864—Law of Octaves: When element John Newlands, 1864—Law of Octaves: When element were placed in order of increasing atomic mass, every 8were placed in order of increasing atomic mass, every 8thth element repeated properties.element repeated properties.

Lothar Meyer, 1869—Periodic table based on physical Lothar Meyer, 1869—Periodic table based on physical characteristics only and increasing atomic mass.characteristics only and increasing atomic mass.

Dmitri Mendeleev, 1869—Periodic Dmitri Mendeleev, 1869—Periodic table based on physical and chemical characteristics and increasing atomic mass. Predicted new elements.

Henry Moseley, 1913—Modern periodic law based on Henry Moseley, 1913—Modern periodic law based on subatomic particles: subatomic particles: There is a periodic repetition of There is a periodic repetition of chemical and physical properties of the elements when chemical and physical properties of the elements when they are arranged by increasing atomic number (protons).they are arranged by increasing atomic number (protons).

Periodic TrendsPeriodic Trends

Periodic TrendsPeriodic Trends

GroupsGroups Periods Periods

Periodic TrendsPeriodic Trends

Periodic TrendsPeriodic Trends

Periodic TrendsPeriodic Trends