History of the Atomic Model

The Battle of the two Philosophers… VS. Aristotle Democritus

DEVELOPS THE IDEA OF THE ATOM. 460 BC Democritus – Greek Model DEVELOPS THE IDEA OF THE ATOM. He pounds materials in his mortar and pestle until he reduces them to smaller and smaller particles which he ultimately calls… ATOMOS (greek for indivisible)

Democritus – Greek Model Atoms are solid & homogeneous all atoms made of the same material. Different types of atoms have different shapes and sizes. The different shapes and sizes of the atoms determine the different properties of the substances Atoms are infinite in number.

Aristotle – Greek Model Earth, Fire, Air and Water approach to the nature of matter. All substances made of these four elements Blend these in different proportions to get all substances Transmute Lead into GOLD

ARISTOTLE WINS!!! Democritus’ idea of “atoms” was ignored and forgotten for more than 2000 years!

Dalton combines the idea of elements with that of atoms! HISTORY OF THE ATOM John Dalton - Billiard Ball Model (England) 1808 All matter is made up of tiny spheres called … ATOMS Dalton combines the idea of elements with that of atoms!

Dalton’s Atomic Theory Billiard Ball Model All elements are composed of atoms. indivisible and indestructible Atoms of the same element are alike Atoms of different elements are different Chemical reactions involve the rearrangement of atoms.

Here comes the Electron…

HISTORY OF THE ATOM ELECTRON Joseph John Thomson Plum Pudding Model 1898 found that atoms could sometimes eject a far smaller negative particle which he called an ELECTRON

J. J. Thomson (1898) – Plum Pudding Model

J. J. Thomson (1898) Plum Pudding Model Thomson studied the passage of an electric current through a gas using cathode-ray tubes. As the current passed through the gas, it gave off rays of negatively charged particles.

Thomson’s Experiment Voltage source - + Metal Disks

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end These rays were a stream of negatively charged particles

Thomson’s Experiment + - Voltage source By adding an electric field he found that the moving pieces were negative

http://www.chem.uiuc.edu/clcwebsite/video/Cath.wmv Watch a real one in action!

Thomson was surprised If atoms of the gas were uncharged. Where had the negative charges come from? Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. The atom was divisible!

Thomson (1898) Plum Pudding Model Thomson called the negatively charged “corpuscles,” today known as electrons. He was unable to discover the positive charge that allows an atom to be neutral.

J.J. Thomson (1898) Plum Pudding Model His model is called the “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. Plum Pudding

Atoms are now considered divisible. Thank you J. J. !! Atoms are now considered divisible.

Here comes the Proton…

Ernest Rutherford (1909) Nuclear Model Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) Was a Student of J.J. Thomson.

Here’s what it looked like Rutherford’s experiment Here’s what it looked like

Flourescent Screen Lead block Uranium Gold Foil

He Expected The alpha particles to pass through without changing direction very much. Because… The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles.

What he expected…

What he expected…

Because…

Because he thought the positive charge was evenly distributed in the atom

Because, he thought the positive charge was evenly distributed in the atom

What he got… The alpha particles were deflected in all different directions

How he explained it… + Atom is mostly empty Small dense, positive piece at center Alpha particles are deflected by it if they get close enough.

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Density and the Atom Since most of the particles went through, the atom was mostly empty Because the alpha particles deflected so much, the positive pieces in the core of the atom had to be very heavy Positive core had a small volume, big mass, big density This small dense positive area is the nucleus

Ernest Rutherford (1909) Nuclear Model Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge. He called the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole.

Rutherford’s Gold Foil Experiment

Niels Bohr (1913) Planetary Model In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.

Niels Bohr (1913) Planetary Model Electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus. Each orbit contains a set number of electrons.

Erwin Schrodinger (1926) Wave Model The exact location of an electron cannot be stated. Electrons are in regions called electron clouds. Atom mostly empty space

Erwin Schrodinger (1926) Wave Model

Wave model No definite path for electron. Can only predict the odds of the location of the electron.

James Chadwick (1931) Realized that the atomic mass of most elements was double the number of protons. Bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists.

Democritus & John Dalton Progression of the Atomic Model + - The structure of an atom, according to: Democritus & John Dalton James Chadwick J.J. Thomson Neils Bohr Ernest Rutherford Erwin Schrodinger

Murray Gell-Mann & George Zweig (1964) Quantum Theory – Standard Model Gell-Mann found that all the elements of an atom are held together by quarks. To find this, he blasted high speed electrons into a hydrogen atom. Zweig proposed the existence of quarks. George Zweig

Quarks – Quantum Theory- Standard Model Protons are made from 2 up quarks and 1 down quark Neutrons are made from 1 up quark and 2 down quarks.

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