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Development of Atomic Theory

&Atomic

Structure

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Atomic Theory

Many models/theories hypothesis and experiments

Models and theories are based off each other and further developed

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Early Theories Democritus (460-370 B.C.)

Greek philosopher (scientist)

There are various basic elements from which all matter is made

Proposed – eventually end up with “uncuttable” particle

Named that particle ‘atom’ (atomos = “indivisible”)

Atom = small, hard particles – made of same material, different shapes

Some atoms are round, pointy, oily, have hooks, etc. to account for their properties

First concept of an atom

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Early Theories Aristotle (384-322 B.C.)

Greek philosopher

Disagreed w/ Democritus

NEVER end up with indivisible particle – they can be divided forever

More popular at the time than Democritus so ……….

Democritus' ideas rejected by leading philosophers/scientists for thousands of years

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John Dalton1766 – 1844

British chemist & teacher Late 1700’s – scientists learned

elements combined specific ratios (based on mass) to form compounds

Dalton used actual experiments

Introduced his ideas in 1803

All substances made of atoms

All the atoms of a given element are identical, but they differ from those of any other element

Atoms are neither created nor destroyed

A given compound always has the same relative numbers and kinds of atoms

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J.J. Thomson1856 – 1940

British Scientist

Discovered electron 1897 – Cathode Ray Tube (CRT) Experiment

Discovered small particle inside the atom

Called them corpuscles

Now called electrons

Found in all atoms

All were identical

All had negative charge

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J.J. Thomson1856 – 1940

British Scientist

Knew atoms had neutral charge

Must also be a + charge

Didn’t know location of + or - particles

Plum Pudding model 1904

Electrons in a soup of positive charges

He also discovered isotopes in 1913

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J.J. Thomson’sPlum Pudding Model

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Ernest Rutherford1871 – 1937

he was one of J.J. Thomson’s students In 1909 and 1910 he did his

gold foil experiment

Discovered the center of atom small, dense, with a + charge

Called that the nucleus

An atom’s mass is mostly in the nucleus

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Ernest Rutherford’s

Gold Foil Experiment

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Ernest Rutherford

Calculated that the diameter of nucleus is 100,000 times smaller than the diameter of the gold atom

Electrons in fixed orbit

Nucleus Theory

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Rutherford’s Model of the

Atom

Niels Bohr1885 – 1962

Danish Scientist Proposed Planetary Model in 1913

Nucleus surrounded by orbiting electrons at different energy levels (different distances from nucleus)

Electrons have definite orbits

No paths in between

Electrons can “jump” from level to level

Showed why different atoms interact and why they are reactive or not

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The Bohr Model of the Atom

Bohr later worked on the Manhattan Project

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Bohr Model of Nitrogen Atom

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Manhattan Project1942 - 1946

Developed the atomic bomb

Trinity test (July 16, 1945)

1st human-engineered atomic explosion

Little Boy 1st nuclear bomb used –

Hiroshima Fat Boy

2nd (last) nuclear bomb used - Nagasaki

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Modern Atomic Theory

Erwin Schrödinger (1887 – 1961)

Werner Heisenberg(1901 – 1976)

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Modern Atomic Theory

Schrödinger and Heisenberg Explained nature of electrons

Electron Cloud Model Electrons don’t travel in specific

paths but in a general area Can only predict probability of an

electrons location

400 BC 1803 1904 1910 1913 1926

Niels Bohr

J J Thomson

Democratus

John Dalton

Schroedinger / Heisenberg

Ernest Rutherford

Atomic Theory Atomic Theory Timeline

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Next Atomic Theory?

Which one of you will develop a better atomic theory and win the Nobel Prize?

Atomic Structure

composed of 3 subatomic particles

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Electron

Proton

Neutron

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The Atom

Made up of three subatomic particles

ProtonNeutronElectron

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Proton

Neutron

Electron

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The Nucleus Center of the atom

99.9% of mass 2 particles

Proton Positive charge 1.7 x 10-24 g 1 amu

Neutron Neutron Neutral charge 1 amu Slightly more mass than proton

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The Electron Electron

Negative charge ~0 amu (1/1836 amu) (takes 1836 electrons to = 1 amu)

Orbit around nucleus billions times/sec Location of electron depends on amount of

energy the electron has Properties of elements depend on the # of

electrons in various energy levels # of electrons in outer energy level determines an

elements ability to bond (combine) with other elements

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Subatomic Particles

All protons are identical to all other protons

All neutrons are identical to all other neutrons

All electrons are identical to all other neutrons

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Electron Cloud Region around the nucleus where

electrons are likely to be found

Located outside the nucleus Like bees around a beehive

The further away from the nucleus an electron is, the more energy it has

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Electron Cloud

1st ring - 2 electrons

2nd ring - 8 electrons

3rd ring - 8 electrons

Atomic Mass & Atomic Number

Atomic number = number of protons

Atomic Mass = average mass of isotopes of an element = the total number of protons & neutrons in the nucleus

29Atomic Mass

Atomic Number

Isotopes

an atom with the same number of protons but different number of neutrons Same atomic number, different

atomic mass

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Isotopes

Atomic number = total # of protons

Mass number = sum of protons and neutrons

Isotopes have different mass numbers but the same atomic numbers

Isotopes share most of same physical and chemical properties

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Radioactive Isotopes

Unique property of some isotopes

They are unstable Called radioactive decay

Nucleus changes composition Become different elements! Not random – follow specific path

of different elements32

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Drawing atoms – Carbon

(we will always use the Bohr model to draw atoms)

Steps:1. Find number of protons

Number of Protons = ?

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Atomic number

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Drawing an atom

Steps:2. Find number of electrons same as

number of protons3. Find number of neutrons

atomic mass – atomic number

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12-6=6

Final Drawing of a Carbon atom

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P=6N=6