Chapter 4 Atoms and their structure History of the atom n Not the history of atom, but the idea of the atom. n Original idea Ancient Greece (400 B.C.)

Chapter 4Chapter 4

Atoms and their structureAtoms and their structure

History of the atomHistory of the atom Not the history of atom, but the idea of the Not the history of atom, but the idea of the

atom.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.

History of AtomHistory of Atom Looked at beachLooked at beach Made of sandMade of sand Cut sand - smaller sandCut sand - smaller sand

Smallest possible Smallest possible piece?piece?Atomos - not to be cutAtomos - not to be cut

Another GreekAnother Greek Aristotle - Famous philosopherAristotle - Famous philosopher All substances are made of 4 elementsAll substances are made of 4 elements Fire - HotFire - Hot Air - lightAir - light Earth - cool, heavyEarth - cool, heavy Water - wetWater - wet Blend these in different proportions to Blend these in different proportions to

get all substances get all substances

Who Was Right?Who Was Right? Did not experiment.Did not experiment. Greeks settled disagreements by Greeks settled disagreements by

argument.argument. Aristotle was a better debater - He won.Aristotle was a better debater - He won. His ideas carried through middle ages.His ideas carried through middle ages. Alchemists tried to change lead to gold.Alchemists tried to change lead to gold.

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

experiments and those of others.experiments and those of others. Elements substances that can’t be Elements substances that can’t be

broken downbroken down In Dalton’s Atomic TheoryIn Dalton’s Atomic Theory Combined idea of elements with that of Combined idea of elements with that of

atoms.atoms.

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.are created or destroyed.

Law of Definite Proportions (#3)Law of Definite Proportions (#3) 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.

Parts of AtomsParts of Atoms J. J. Thomson - English physicist. 1897J. J. Thomson - English physicist. 1897 Made a piece of equipment called a Made a piece of equipment called a

cathode ray tube.cathode ray tube. It is a vacuum tube - all the air has been It is a vacuum tube - all the air has been

pumped out.pumped out. A limited amount of other gases are put A limited amount of other gases are put

inin

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Metal Disks

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

+-

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

+

- By adding an electric field By adding an electric field

Thomson’s ExperimentThomson’s Experiment Used many different metals and gasesUsed many different metals and gases Beam was always the sameBeam was always the same By the amount it bent he could find the By the amount it bent he could find the

ratio of charge to massratio of charge to mass Was the same with every materialWas the same with every material Same type of piece in every kind of Same type of piece in every kind of

atomatom

Thomsom’s ModelThomsom’s Model Found the electron.Found the electron. Couldn’t find Couldn’t find

positive (for a while).positive (for a while). Said the atom was Said the atom was

like plum pudding.like plum pudding. A bunch of positive A bunch of positive

stuff, with the stuff, with the electrons able to be electrons able to be removed.removed.

Millikan’s ExperimentMillikan’s Experiment

Atomizer

Microscope

-

+

Oil

Metal Plates

Millikan’s ExperimentMillikan’s Experiment

Oil

Atomizer

Microscope

-

+

Oil droplets

Millikan’s ExperimentMillikan’s Experiment

X-rays

X-rays give some drops a charge by knocking offelectrons

-

Millikan’s ExperimentMillikan’s Experiment

+

Millikan’s ExperimentMillikan’s Experiment

They put an electric charge on the plates

++

--

Millikan’s ExperimentMillikan’s Experiment

Some drops would hover

++

--

Millikan’s ExperimentMillikan’s Experiment

+

+ + + + + + +

- - - - - - -

Some drops would hover

Millikan’s ExperimentMillikan’s Experiment

From the mass of the drop and the charge on the plates, he calculated the charge on an electron

++

--

Rutherford’s ExperimentRutherford’s Experiment Ernest Rutherford English physicist. Ernest Rutherford English physicist.

(1910)(1910) Believed the plum pudding model of the Believed the plum pudding model of the

atom was correct.atom was correct. Wanted to see how big they are.Wanted to see how big they are. Used radioactivity.Used radioactivity. Alpha particles - positively charged Alpha particles - positively charged

pieces given off by uranium.pieces given off by uranium. Shot them at gold foil which can be made Shot them at gold foil which can be made

a few atoms thick.a 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 (pg 72)Here’s what it looked like (pg 72)

Lead block

Uranium

Gold Foil

Flourescent Screen

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

without changing direction very much.without 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 alpha particles.stop the 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 empty.Atom is mostly empty. Small dense,Small dense,

positive piecepositive piece at at center.center.

Alpha particles Alpha particles are deflected byare deflected by

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

+

Modern ViewModern View The atom is mostly The atom is mostly

empty space.empty space. Two regions.Two regions. NucleusNucleus- protons - protons

and neutrons.and neutrons. Electron cloud-Electron cloud-

region where you region where you might find an might find an electron.electron.

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 pieces turned so much, Because the pieces turned so much,

the positive pieces were heavy.the positive pieces were heavy. Small volume, big mass, big density.Small volume, big mass, big density. This small dense positive area is the This small dense positive area is the

nucleus.nucleus.

Other piecesOther pieces Proton - positively charged pieces 1840 Proton - positively charged pieces 1840

times heavier than the electron.times heavier than the electron. Neutron - no charge but the same mass Neutron - no charge but the same mass

as a proton.as a proton. Where are the pieces?Where are the pieces?

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 regions.There are two regions. The nucleus.The nucleus. With protons and neutrons.With protons and neutrons. Positive charge.Positive charge. Almost all the mass.Almost all the mass. Electron cloud- most of the volume of Electron cloud- most of the volume of

an atom.an atom. The region where the electron can be The region where the electron can be

found.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 radius.Hydrogen atom, 32 pm radius. Nucleus tiny compared to atom.Nucleus 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 is near 10Radius of the nucleus is near 10-15-15m.m. Density near 10Density near 101414 g/cm g/cm33..

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. the same as the number of electrons in the same as the number of electrons in

the neutral atom.the neutral atom. Mass Number = Mass Number = the number of protons the number of protons

+ neutrons.+ neutrons. All the things with mass.All the things with mass. NOT on the periodic tableNOT on the periodic table

IsotopesIsotopes Dalton was wrong.Dalton was wrong. Atoms of the same element can have Atoms of the same element can have

different numbers of neutrons.different numbers of neutrons. different mass numbers.different mass numbers. called called isotopes.isotopes.

SymbolsSymbols Contain the symbol of the element, the Contain the symbol of the element, the

mass number and the atomic number.mass number and the atomic number.

SymbolsSymbols Contain the symbol of the element, the Contain the symbol of the element, the

mass number and the atomic number.mass number and the atomic number.

X Massnumber

Atomicnumber

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

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

SymbolsSymbols 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

– NameName

Na2411

SymbolsSymbols 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– NameName

Br80 35

SymbolsSymbols 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– NameName

SymbolsSymbols 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– NameName

SymbolsSymbols 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– NameName

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 small.would be too small.

Measuring Atomic MassMeasuring Atomic Mass Unit is the Unit is the Atomic Mass Unit Atomic Mass Unit (amu)(amu) One twelfth the mass of a carbon-12 One twelfth the mass of a carbon-12

atom. atom. 6 p6 p++ and 6 n and 6 n00

Each isotope has its own atomic massEach isotope has its own atomic mass we get the average using percent we get the average using 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 +

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.

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

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

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

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

average. average. are the decimal numbers on the periodic are the decimal numbers on the periodic

table.table.