particles, nuclei, and isotopes the nuts and bolts of chemistry

Particles, Nuclei, and Isotopes

The Nuts and Bolts of Chemistry

Atomic History: An Overview

• Early Greeks - Democritus vs Aristotle

• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom


• Early Greeks - Democritus vs. Aristotle


Early Greeks

• What is the nature of matter?-Infinitely divisible pieces of

“stuff”-Earth, Air, Fire ,and Water

• No experiments

Early Greeks

Democritus vs. Aristotle

• Democritus• New idea• “atomos” -

indivisible• Eventually,

can’t divide matter any more

• First “atomic theory”

• Aristotle• Didn’t buy it…• All things are

infinitely divisible

Guess who won?

(Aristotle)

For 2000 years, scientists thought all matter was infinitely

divisible…


• Early Greeks - Democritus vs Aristotle


Dalton’s Atomic Theory

• 1807• Idea of “atom”

– Solid spheres that were indestructible

– Unique to each element

– Combine evenly– Reactions are

rearrangements

John Dalton

Dalton’s Atomic Theory

• Atoms combine evenly in compounds– Small, whole number proportions– (e.g.) water - H20 (not H1O1/2 )

H

OH

H

OH

Thomson’s Discovery• Studied electricity not

atomic structure• His 1st observation was

that a magnet could deflect the current and realized that it was made of particles

• So to study current he pumped all the air out of a glass tube and applied voltage to two metal plates + (anode)/- (cathode)

J.J. Thomson 1897

Thomson’s Discovery (cont…)

• He then sent a ray of particles through the tube and saw that they came out of the cathode side and were attracted to the anode side and the tube glowed

• This told him that the particles were negative… he called his discovery a Cathode Ray Tube– Sends a “ray” of -

particles– Used in TVs and

Computers

J.J. Thomson 1897

Thomson’s Discovery

• The beam was attracted to the positive plate.• So, cathode rays are negative• He realized that these negative particle must be

ELECTRONS

Thomson’s Discovery

• Most books give Thomson credit for discovering the proton

• He and Millikan found the mass of an electron to be much smaller than an atom

• So, electrons are VERY VERY small• Protons must be large in

comparison– Plum Pudding Model

Plum Pudding Model

Preface: “Plum Pudding” atomic model

• atoms are solid• made of positively-charged

material• with negative “bits” scattered

throughout(like raisins in plum pudding)(or raisin bread)

Rutherford’s Discovery

Image courtesty ofhttp://www.nobel.se/chemistry/laureates/1908/rutherford-

bio.html

Ernest Rutherford

Gold Foil Experiment

To test the Plum Pudding Model (1907):

• Shot alpha rays at thin gold foil– (about 2000 atoms thick)

• EXPECTED to see the ray scatter as it hit all the solid positive atoms– Like spray from a nozzle

Gold Foil ExperimentHere is what they saw:

Gold Foil ExperimentIn Detail:

Click here to see an

animation.

http://micro.magnet.fsu.edu/electromag/java/rutherford/

Gold Foil ExperimentRESULTS

• Most of the particles were not deflected

• Some were minimally deflected• VERY few (1 in 20,000) bounced

back– “as if you had fired a 15-inch shell at

a piece of tissue paper and it came back and hit you.” -

Rutherford

Gold Foil ExperimentCONCLUSIONS

• Plum pudding model wrong• A “nucleus” exists

– It is tiny– It is densely-packed and positively-

charged (this is the only way a strong positive particle could be deflected)

• Empty spaces exist in atoms– LOTS of it!!!!

Gold Foil Experiment

How much empty space?• Use a billiard ball to represent a

nucleus• The electrons occupy a volume one

kilometer in ALL DIRECTIONS• Most of that space is EMPTY.

Chadwick’s Discovery (1932)

• PROBLEM• There was more

mass in nucleus than explained by protons alone

• Where did it come from?

• NEUTRONSChadwick

Bohr’s Atomic Model

• Nucleus has + charge

• Electrons have - charge

• Why don’t electrons simply “fall into” the nucleus?

Bohr

Bohr’s Atomic Model

• Bohr pictured atoms as little solar systems

• Nucleus in center• Electrons “orbiting” in

circles–We now know this isn’t correct

–But it helps to explain many

things about atoms

What Theory Do We Currently Support?

• Electron Cloud Model– Nucleus is composed of Protons and

Neutrons– Electrons spin very quickly around the

nucleus forming a “giant cloud” of negative energy

– These electrons can move further away or closer to the nucleus at anytime

– Their distance and space is infinite!

HOMEWORK

Notes 4.2

Protons, Neutrons, and Electrons

• Protons– Located in nucleus– Positively charged (+1)– Mass = “1” mass unit (or 1

gram/mole)• Number of protons = atomic

number (Z)

Three Main Subatomic Particles


• Neutrons– Located in nucleus– no charge (0)– Mass = “1” mass unit (or 1

gram/mole)



• Electrons– Located outside nucleus– negative charge (-1)– So small we assume mass = 0

• Actual mass of 0.000 5 g/mol



Atomic number (Z)• shown in lower

left• ALWAYS equals

the number of protons

• Equals the number of electrons in a neutral atom

C6

Isotope Notation


Mass Number (A)• shown in upper left• Is the sum:

neutrons + protons• In this example,

carbon has 6 protons and 7 neutrons

C6

Isotope Notation

13


We call this atom

“carbon-13” C6

Isotope Notation

13


• Atoms with the same number of protons can have different numbers of neutrons.

• We call such atoms isotopes


• Atoms with the same number of protons can have different numbers of neutrons.

• We call such atoms isotopes

C612

C613

C614

carbon -12 carbon -13 carbon -14


C612

C613

C614

6 protons6 electrons6 neutrons



All three kinds of carbon atoms have the same chemistry!


H1

1

Pb82

208

F9

19

1 proton1 electron0 neutrons

Neutrons = A - Z1 - 1 = 0


Neutrons = A - Z208 - 82 = 126


Neutrons = A - Z19 - 9 = 10

+2


?3

7

What element is this?

How do you know?


Li3

7

What element is this?

How do you know?

Lithium

the atomic number is 3

look on the periodic table


Pb?

207

?92

235

K19

?

# of protons (Z)

# of neutrons

# of electrons

mass number (A)

name

82

125

82

207

lead-207

92

143

92

235

uranium-235

19

21

19

40

potassium-40

Homework…Complete the

Atomic Structure Worksheets

((((( Back AND Front )))))

particles, nuclei, and isotopes the nuts and bolts of chemistry

Documents

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thomsons discovery conthe

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ray of particles

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