Atoms and their StructureAtoms and their Structure

History of the AtomHistory of the Atom Original idea Ancient Greece (400 B.C..)Original idea Ancient Greece (400 B.C..) Democritus* and Leucippus: Greek Democritus* and Leucippus: Greek

philosophers philosophers

Democritus ReasonedDemocritus Reasoned Looked at beachLooked at beach Once had rocksOnce had rocks Break down rocks?Break down rocks? Eventually sandEventually sand

Smallest possible piece?Smallest possible piece?Atomos - indivisibleAtomos - indivisible

Another GreekAnother Greek Aristotle - Famous philosopherAristotle - Famous philosopher All substances are made of 4 “humors”All substances are made of 4 “humors” Fire – Hot, vaporizes earth Fire – Hot, vaporizes earth Air – light, vapor (old term – “ether”)Air – light, vapor (old term – “ether”) Earth - cool, heavy, rocks, treesEarth - cool, heavy, rocks, trees Water – wet, liquid but also ice, vaporWater – wet, liquid but also ice, vapor Blend these in different proportions to Blend these in different proportions to

get all substances get all substances

Who Was Right?Who Was Right? Greek society was slave basedGreek society was slave based Beneath the famous to work with handsBeneath the famous to work with hands Did not experimentDid not experiment Greeks settled disagreements by Greeks settled disagreements by

argumentargument Aristotle was more famousAristotle was more famous He wonHe won His ideas carried through middle ages.His ideas carried through middle ages.

AlchemistsAlchemistsHad two dreams:Had two dreams:

1: live forever (Philosopher’s Stone)1: live forever (Philosopher’s Stone) 2: turn “base”metals into gold2: turn “base”metals into gold Lots of trial and error experimentsLots of trial and error experiments Accomplishments?Accomplishments? Metallurgy – Metal from ores; AlloysMetallurgy – Metal from ores; Alloys natural pharmacy and medicinenatural pharmacy and medicine

Who’s Next?Who’s Next? Late 1700’s - John Dalton- EnglandLate 1700’s - John Dalton- England Teacher- summarized results of his Teacher- summarized results of his

experiments and those of other’sexperiments and those of other’s In Dalton’s Atomic TheoryIn Dalton’s Atomic Theory Combined ideas of elements with that of Combined ideas of elements with that of

atomsatoms

Dalton’s Atomic TheoryDalton’s Atomic Theory All All mattermatter is made of tiny is made of tiny indivisibleindivisible

particles called atoms.particles called atoms. Atoms of the same element are identical, Atoms of the same element are identical,

those of different atoms are different.those of different atoms are different. Atoms of different elements combine in Atoms of different elements combine in

whole number ratios to form compounds.whole number ratios to form compounds. Chemical reactions involve the Chemical reactions involve the

rearrangement of atoms. No new atoms rearrangement of atoms. No new atoms are created or destroyed. (Familiar idea?)are created or destroyed. (Familiar idea?)

Law of Definite ProportionsLaw of Definite Proportions Each compound has a specific ratio of Each compound has a specific ratio of

elements.elements. It is a ratio by mass. It is a ratio by mass. Water is always 8 grams of oxygen for Water is always 8 grams of oxygen for

each gram of hydrogen.each gram of hydrogen.

Law of Multiple ProportionsLaw of Multiple Proportions if two elements form more that one if two elements form more that one

compound, the ratio of the second compound, the ratio of the second element that combines with 1 gram of element that combines with 1 gram of the first element in each is a simple the first element in each is a simple whole number.whole number.

What?!What?! Water is 8 grams of oxygen per gram of Water is 8 grams of oxygen per gram of

hydrogen.hydrogen. Hydrogen Peroxide is 16 grams of Hydrogen Peroxide is 16 grams of

oxygen per gram of hydrogen.oxygen per gram of hydrogen. 16 to 8 is a 2 to 1 ratio16 to 8 is a 2 to 1 ratio True because you have to add a whole True because you have to add a whole

atom, you can’t add a piece of an atom. atom, you can’t add a piece of an atom.

Parts of AtomsParts of Atoms J. J. Thomson - English physicist. 1897J. J. Thomson - English physicist. 1897 Worked with a piece of equipment Worked with a piece of equipment

called a Crookes tube.called a Crookes tube. It is a vacuum tube - all the air has been It is a vacuum tube - all the air has been

pumped out and a small amount of pumped out and a small amount of another gas has been let in.another gas has been let in.

Thomson’s Experiment Thomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-cathode anode

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative moving pieces were negative

+

-

But Look Going the Other But Look Going the Other Direction!Direction!

Faint particles were seen Faint particles were seen

Deflected oppositely to the Deflected oppositely to the electronselectrons

Called them “protons”Called them “protons”

Thomson’s ModelThomson’s Model Found the electron (-)Found the electron (-) Later found the Later found the

proton (+)proton (+) Said the atom was Said the atom was

like plum puddinglike plum pudding (Let’s say a banana-(Let’s say a banana-

nut muffin)nut muffin) A bunch of positive A bunch of positive

“stuff”, with the “stuff”, with the electrons able to be electrons able to be removed removed

Other piecesOther pieces Proton (Thomson) - positively charged Proton (Thomson) - positively charged

pieces 1840 times heavier than the pieces 1840 times heavier than the electronelectron

Where are the pieces?Where are the pieces?

Rutherford’s ExperimentRutherford’s Experiment Ernest Rutherford English physicist. (1910)Ernest Rutherford English physicist. (1910) Believed in the plum pudding model of the Believed in the plum pudding model of the

atomatom Wanted to see how big they are Wanted to see how big they are Used radioactivityUsed radioactivity Alpha particles - positively charged pieces Alpha particles - positively charged pieces

given off by polonium given off by polonium Shot them at gold foil which can be made a Shot them at gold foil which can be made a

few atoms thick few atoms thick

Rutherford’s experimentRutherford’s experiment When the alpha particles hit a florescent When the alpha particles hit a florescent

screen, it glows.screen, it glows. Here’s what it looked like.Here’s what it looked like.

Lead block

Uranium

Gold Foil

Florescent Screen

He ExpectedHe Expected The alpha particles to pass through The alpha particles to pass through

without changing direction very muchwithout changing direction very much Because:Because: The positive charges were spread out The positive charges were spread out

evenly. Alone they were not enough to evenly. Alone they were not enough to stop the heavy alpha particlesstop the heavy alpha particles

What he expected

Because

Because, he thought the mass was evenly distributed in the atom

Because, he thought the mass was evenly distributed in the atom

What he got

How he explained it

+

Atom is mostly emptyAtom is mostly empty Small dense,Small dense,

positive piecepositive piece at centerat center

Alpha particles Alpha particles are deflected byare deflected by

it if they get close it if they get close enough enough

+

Introducing the Rutherford-Bohr Introducing the Rutherford-Bohr ModelModel

Working under Rutherford, Neils Bohr decided the electrons were scattered around the nucleus in shells or energy levelsSimilar to the planets orbiting around the sun: Solar System or Planetary Model

Also in the NucleusAlso in the Nucleus Neutron - 1930’s(James Chadwick)Neutron - 1930’s(James Chadwick) - no charge but the same mass as a - no charge but the same mass as a

proton. Very faint stream, unaffected by proton. Very faint stream, unaffected by plates, therefore no chargeplates, therefore no charge

Note the late date for this discovery!Note the late date for this discovery!

Modern ViewModern View The atom is mostly The atom is mostly

empty spaceempty space Two regionsTwo regions Nucleus- protons Nucleus- protons

and neutronsand neutrons Electron cloud- Electron cloud-

region where you region where you might find an might find an electronelectron

Density and the AtomDensity and the Atom Since most of the particles went Since most of the particles went

through, it was mostly empty.through, it was mostly empty. Because the particles occasionally Because the particles occasionally

deflected a great deal, the positive deflected a great deal, the positive pieces were heavy.pieces were heavy.

Small volume+big mass=big densitySmall volume+big mass=big density This small dense positive area is the This small dense positive area is the

nucleusnucleus

Subatomic particlesSubatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Structure of the AtomStructure of the Atom There are two regionsThere are two regions The nucleusThe nucleus

» With protons and neutrons With protons and neutrons

» Positive chargePositive charge

» Almost all the massAlmost all the mass

Electron cloud- Most of the volume of Electron cloud- Most of the volume of an atoman atom

» The region where the electron can be foundThe region where the electron can be found

Size of an atomSize of an atom Atoms are small.Atoms are small. Measured in picometers, 10Measured in picometers, 10-12-12 meters meters Hydrogen atom, 32 pm radiusHydrogen atom, 32 pm radius Nucleus tiny compared to atomNucleus tiny compared to atom IF the atom was the size of a stadium, the IF the atom was the size of a stadium, the

nucleus would be the size of a marble.nucleus would be the size of a marble. Radius of the nucleus near 10Radius of the nucleus near 10-15-15m.m. Density near 10Density near 101414 g/cm g/cm

Counting the PiecesCounting the Pieces *Atomic Number *Atomic Number = number of protons= number of protons # of protons determines kind of atom# of protons determines kind of atom Identifies the element*Identifies the element* Is equal to the number of electrons in Is equal to the number of electrons in

the neutral atom (Why?)the neutral atom (Why?)

How Heavy is the Atom?How Heavy is the Atom?

Mass Number = Mass Number = the number of the number of protons + neutronsprotons + neutrons

All the things with massAll the things with mass

Something’s Wrong!Something’s Wrong!

Atoms identical to hydrogen were Atoms identical to hydrogen were discovered to definitely have three discovered to definitely have three different Mass Numbers!different Mass Numbers!

masses of 1, 2, or 3 were identifiedmasses of 1, 2, or 3 were identified

IsotopesIsotopes Dalton was wrongDalton was wrong Atoms of the same element Atoms of the same element cancan have have

different numbers of neutronsdifferent numbers of neutrons different mass numbers*different mass numbers* called *called *isotopesisotopes

Nuclear NotationNuclear Notation Contain the symbol of the element, the Contain the symbol of the element, the

mass number and the atomic numbermass number and the atomic number

Nuclear NotationNuclear Notation Contains the symbol of the element, the Contains the symbol of the element, the

mass number and the atomic numbermass number and the atomic number

X Massnumber

Atomicnumber

But Why Different from P.T.?But Why Different from P.T.?

Nuclear notation allows for a quick Nuclear notation allows for a quick subtractionsubtraction

Gives the number of neutrons in this Gives the number of neutrons in this isotopesisotopes

Nuclear NotationNuclear Notation Find the Find the

– number of protonsnumber of protons

– number of neutronsnumber of neutrons

– number of electronsnumber of electrons

– Atomic numberAtomic number

– Mass NumberMass Number

F19 9

Find the Find the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Atomic numberAtomic number

–Mass NumberMass Number

Br80 35

if an element has an atomic if an element has an atomic number of 34 and a mass number number of 34 and a mass number of 78 what is the of 78 what is the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Complete symbolComplete symbol

if an element has 91 protons and if an element has 91 protons and 140 neutrons what is the 140 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of electronsnumber of electrons

–Complete symbolComplete symbol

if an element has 78 electrons and if an element has 78 electrons and 117 neutrons what is the 117 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of protonsnumber of protons

–Complete symbolComplete symbol

Naming IsotopesNaming Isotopes Put the mass number after the name of Put the mass number after the name of

the elementthe element carbon- 12carbon- 12 carbon -14carbon -14 uranium-235uranium-235

Why don’t we use the atomic number to Why don’t we use the atomic number to identify the isotope?identify the isotope?

Atomic MassAtomic Mass How heavy is an atom of oxygen?How heavy is an atom of oxygen? There are different kinds of oxygen atoms.There are different kinds of oxygen atoms. More concerned with More concerned with average average atomic mass.atomic mass. Based on abundance of each element in Based on abundance of each element in

nature.nature. Don’t use grams because the numbers Don’t use grams because the numbers

would be too smallwould be too small

Measuring Atomic MassMeasuring Atomic Mass Unit is the Unit is the Atomic Mass Unit Atomic Mass Unit (amu)(amu) One amu is equal to one twelfth the One amu is equal to one twelfth the

mass of a carbon-12 atom. mass of a carbon-12 atom. (Originally, the hydrogen atom was assigned (Originally, the hydrogen atom was assigned a mass of one amu. Why hydrogen?)a mass of one amu. Why hydrogen?)

Each isotope has its own atomic mass Each isotope has its own atomic mass we need the average from percent we need the average from percent abundance.abundance.

Calculating averagesCalculating averages You have five rocks, four with a mass of 50 You have five rocks, four with a mass of 50

g, and one with a mass of 60 g. What is the g, and one with a mass of 60 g. What is the average mass of the rocks?average mass of the rocks?

Total mass = 4 x 50 + 1 x 60 = 260 gTotal mass = 4 x 50 + 1 x 60 = 260 g Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 5 5 Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 55 5 5

Calculating averagesCalculating averages Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 5 5 5 5 Average mass = .8 x 50 + .2 x 60Average mass = .8 x 50 + .2 x 60 80% of the rocks were 50 grams80% of the rocks were 50 grams 20% of the rocks were 60 grams20% of the rocks were 60 grams Average = % as decimal x mass + Average = % as decimal x mass +

% as decimal x mass + % as decimal x mass + % as decimal x mass + % as decimal x mass +

Averaging GradesAveraging Grades Test grade = 100%Test grade = 100% Quiz grade = 10%!Quiz grade = 10%! 100 + 10 = 110100 + 10 = 110 110/2 = 55%110/2 = 55%

Is this fair? Why?Is this fair? Why? WeightedWeighted Average Average

Let’s Pretend…Let’s Pretend…Suppose there are 2 types of lithium Suppose there are 2 types of lithium

atoms: lithium-6 and lithium -7atoms: lithium-6 and lithium -7

Let’s Pretend…Let’s Pretend…Suppose there are 2 types of lithium atoms: Suppose there are 2 types of lithium atoms:

lithium-6 and lithium –7lithium-6 and lithium –7

90% of the lithium in the world is Lithium-790% of the lithium in the world is Lithium-7

Let’s Pretend…Let’s Pretend…Suppose there are 2 types of lithium atoms: Suppose there are 2 types of lithium atoms:

lithium-6 and lithium –7lithium-6 and lithium –7

90% of the lithium in the world is Lithium-790% of the lithium in the world is Lithium-7

10% of the lithium in the world is Lithium-610% of the lithium in the world is Lithium-6

Let’s Pretend…Let’s Pretend…Suppose there are 2 types of lithium atoms: Suppose there are 2 types of lithium atoms:

lithium-6 and lithium –7lithium-6 and lithium –7

90% of the lithium in the world is Lithium-790% of the lithium in the world is Lithium-7

10% of the lithium in the world is Lithium-610% of the lithium in the world is Lithium-6

How do I find the average mass of lithium How do I find the average mass of lithium atoms?atoms?

Weighted Average!Weighted Average! % means out of 100—% means out of 100—

Then out of 100 Li atoms we haveThen out of 100 Li atoms we have

90(7) + 10(6)90(7) + 10(6) = 6.9 = 6.9

100100

weighted average of the masses of the weighted average of the masses of the isotopes.isotopes.

Why do we not divide by 2 now?Why do we not divide by 2 now?

Atomic MassAtomic Mass Calculate the atomic mass of copper if Calculate the atomic mass of copper if

copper has two isotopes. 69.1% has a mass copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of of 62.93 amu and the rest has a mass of 64.93 amu.64.93 amu.

(did you get about 63.55?)(did you get about 63.55?)

Atomic MassAtomic Mass Magnesium has three isotopes. 78.99% Magnesium has three isotopes. 78.99%

magnesium 24 with a mass of 23.9850 amu, magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the atomic mass of 25.9826 amu. What is the atomic mass of magnesium?magnesium?

If not told otherwise, the mass of the isotope is the If not told otherwise, the mass of the isotope is the mass number in amu mass number in amu

(did you get somewhere around 24.3?)(did you get somewhere around 24.3?)

Atomic MassAtomic Mass is not a whole number because it is an is not a whole number because it is an

average. average.

is the number with the decimal numbers is the number with the decimal numbers on the periodic table.on the periodic table.