The Modern Atomic TheoryHow it all began

Large Hadron Collider

Atom• An atom has three parts:• Proton = positive• Neutron = no charge• Electron = negative• The proton & neutron are

found in the center of the atom, a place called the nucleus.

• The electrons orbit the nucleus.

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Empedocles(later supported by Aristotle)

ATOMOS!

Democritus (with Leucippus)

ATOMS!

John Dalton

Dalton Atom

Indivisible hard sphere

Dalton’s Model

• Dalton proposed a modern atomic modelbased on experimentation not on pure reason.– All matter is made of atoms.– Atoms of an element are identical.– Each element has different atoms.– Atoms of different elements combine in constant

ratios to form compounds.– Atoms are rearranged in reactions.

Indivisible? Not convinced!

Experiment!

Sir Joseph John Thomson

Cathode rays are negatively charged!

• J.J. Thomson• --discovered sub-atomic particles common to

all elements in cathode ray tube experiments• --named negatively charged particles with very

little mass electrons

Electron

Positively Charged Sphere

Thomson Atom (1899)

Plum Pudding Model of the Atom

Plum Pudding Model of the Atom

Raisin Bread Model of the Atom

Electrons!

Sir Joseph John Thomson

Only electrons? Let’s do an

experiment!

Ernest Rutherford

Rutherford

• --designed alpha-particle scattering experiment using radioactive source and gold foil

• --observed that most particles passed through foil undeflected, some slightly deflected and a very few reflected

• --concluded that Thomson's model was incorrect• MODEL: "nuclear" atom : positive, very small center

of atom is nucleus containing protons; electrons move around nucleus like planets around sun; atom is mostly empty space

Ernest RutherfordHans Geiger Ernst Marsden

Gold Foil Experiment

Positively Charged(2 protons and

2 neutrons)

Gold Foil Experiment or Geiger-Marsden Experiment

A New Model of the Atom

Expected based onPlum pudding model

Rutherford’s modelBased on results

ElectronElectron

Nucleus

Rutherford Atom (1911)

Charged Nucleus!

Ernest Rutherford

What about the electrons?

Let’s take a look again!

Niels Bohr

Bohr Atom (1913)

Electrons move in orbits

(energy levels)!

Niels Bohr

I DISCOVERED NEUTRONS!

Sir James Chadwick

Particles in orbits? Can we

see them? Really?

Erwin Schrödinger

WAVE BEHAVIOR FTW!

Werner Heisenberg

Wave? Particle? BOTH!

Electron cloud FTW!

Erwin Schrödinger

Mentors and Mentees

Evolution of Modern Atomic Models