+ unit 3 the history of the atom and atomic structures democritus & dalton thomson rutherford...

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Unit 3 The History of the ATOMand Atomic Structures

+Democritus 460-370 B.C.Democritus 460-370 B.C.

Greek Philosopher

He called nature’s basic particle an atomos, based on the Greek word “indivisible.”

Had no evidence, so people didn’t take him seriously.

He considers education to be the noblest of pursuits, but cautioned that learning without sense leads to error. (THERE MUST BE A POINT!!!)

Aristotle said empty space did not exist & rejected Democritus; said matter was EARTH, WATER, AIR & FIRE

+John Dalton 1803Dalton’s Atomic Theory

1. All elements are composed of tiny indivisible particles called atoms.

ElementA2. Atoms of the same

element are identical. The atoms of any one element are different from those of any other elements.

+ Dalton’s Atomic Theory

3. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds.

+ Dalton’s Atomic Theory

4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

+ J.J. Thomson 1897 (with Crookes & Millikan)

discovery of the electron (e-)

disproved Dalton’s theory; atoms DID have smaller parts after all

Used a cathode ray tube to prove that there were charged particles (stream of (-) particles) within the atom

+Thomson Model

Plum pudding model aka chocolate chip cookie model

Millikan found out that electrons were negatively charged and had a mass of 1/1840 of a hydrogen atom

+Video Links to Experiments

JJ Thomson’s Cathode Ray Tube

How does it work?

Rutherford’s Gold Foil Experiment What did he do?

+Ernest Rutherford 1911

Used alpha particles to shoot at gold foil. Got unexpected results. Something was deflecting particles back to the source.

Buried next to Sir Issac Newton and Lord Kelvin in Westminster Abbey.

+

NucleusEmpty Space

Electrons

+Rutherford said, “Atoms have a nucleus!”

Dense & positively charged

Contains most of the mass of the atom

Contains the protons (positive charge; heavy)

Electrons are around the outside of the nucleus flying around the empty space

+Video Links to Experiments

JJ Thomson’s Cathode Ray Tube

How does it work?

Rutherford’s Gold Foil Experiment What did he do?

+James Chadwick

Discovered the neutron in 1932

Has a neutral charge

Found in the nucleus

About ½ of the mass of the nucleus

Wanted to know where the extra mass was coming from in an atom; shot particles @ beryllium

Eventually led to experiments to bombard atoms with neutrons –> uranium bombs!

+Niels Bohr - 1913

Created the Bohr model

Electrons travel in definite orbits/ energy levels around the nucleus

aka “Planetary Model” – orbit the nucleus like planets around the sun

Electrons have fixed amounts of energy “quanta”Low energy closer to nucleusHigh energy further from nucleus

+ Bohr Model

+Werner Heisenberg, 1927

It’s impossible to know both the location and the motion of an electron at the same time

Heisenberg Uncertainty Principle

+ Quantum Mechanical Model(aka wave mechanical model)

Erwin Schrodinger – 1926

Electrons DO NOT orbit the nucleus

Electrons DO have quanta of energy that determine placement in an atom

+Electron Cloud

Area where there is a high probability that the electron will reside there 90% of the time

+Schrodinger’s Cat

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Subatomic Particles

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Nucleus - protons - neutrons

Electron Cloud - electrons

++ oo- -

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p+ n0

e-Found in the nucleus

Mass = 1 (amu)

Relative charge = +1

Found in the nucleus

Same mass as a proton

Mass = 1 amu

Relative charge = 0

Found in the electron cloud around the nucleus

Mass 1836 times smaller than a proton

Therefore, mass doesn’t really count! (0.0005 amu)

Relative charge = -1

+ Atomic Number Atomic Number Z = p+

the number of protons in the nucleus

ALL atoms of the PROTONS SAME element have the IDENTIFY SAME atomic number and THE

ELEMENT! SAME protons

In a neutral atom, p+ = e-

Magnesium = Z = 12 12 protons

18.99

+Mass Number (nucleus!)

Mass number = p+ + n0

(electrons don’t have much mass!!!)

Z = 82 LEAD, Pb Mass 207

Protons? Electrons? Neutrons? 82 82 82 125

#n0 = MN - # p+

= 207 – 82= 125 neutrons

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An element has an atomic number of 6 and a mass number of 14. What is the element? How many neutrons?

Z = 6 CARBON

MN = 14 #n = MN – #p+

= 14 – 6 = 8 neutrons

+ 17

2

18.99

Average atomic mass (not the mass #)

Period number (row) the number of energy levels outside of the nucleus

Group number (column) the second digit tells us the number of valence (outer) electrons in a Bohr atom

+Bohr model of the atom

Yep! Somebody lied when they told you 8! We’ll go into details in Unit 4. Hold your horses…

(protons & neutrons)

+Identify this atom: (assuming it is neutral p+ = e-)

NUCLEAR NOTATION

16O8

HYPHEN NOTATION

Oxygen - 16

+If we drew a Bohr model of sodium: How many protons?

How many neutrons?

How many energy levels?

How many electrons?

23Na

+Scanning Electron Microscopes http://www.youtube.com/watch?v=egPQZw0QkVw