Unit 2 2.1 – History of the Atomic Model. Atomic Structure .

Unit 22.1 – History of the

Atomic Model

Atomic Structure

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Democritus400 B.C.

A. He was a Greek philosopher of science.

B. First to use the term “atom” to describe the basic particle of nature.

1. “atom” means “indivisible”2. Atom – the smallest particle of an

element that still retains the chemical properties of that element.

John Dalton1808

• He was an English schoolteacher.

• He was the first to propose an “Atomic theory” that states the 5 following statements:

John Dalton1808

1. All matter is composed of extremely small particles called “atoms”.

2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

a. This has since been modified based on Isotopes and ions.

3. Atoms cannot be subdivided, created, or destroyed.a. This has since been modified based upon current studies

in quantum physics. b. Such examples include muons and quarks.

John Dalton1808

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated, or rearranged.

J.J. Thomson (1897)with the Cathode Ray Tube


A. He was an English Physicist.B. He worked with glass gas-filled tubes

referred to as Cathode-Ray tubes.1. The glass tubes were filled with a gaseous

element under low pressure.2. He then passed an electrical current using a

battery and wires.


a. The electrical current caused the gas within the tube to glow with a beam (“ray”) within the tube to intensely glow within the tube.i. Magnets could make the “ray” move/deflect in various

directions.ii. The ray is being deflected by the negative charge of the

magnet.iii. Negative charge repels/deflects like negative charges.iv. The ray is made of negatively charged particles, Thompson

called electrons (since they were associated with the electrical current.)• The symbol for an electron is: e-


b. The electrical current came into the chamber (by a wire) at the cathode end. (The end where electricity enters the tube.)

c. The electrical current left the tube on the anode end. (The end where the electricity goes back into the wire.)

d. Hence the term Cathode Ray tubes.

Discovery of the Electron

C. Further investigations using different elements in Cathode-Ray tubes confirmed that every elements atoms possess electrons.

Actual Cathode Ray in action

Thomson’s Atomic Model

D. He proposes the “Plum Pudding” model of atoms. 1. It states that negatively charged electrons are

evenly placed inside a positively charged mass.2. Thomson believed that the electrons were like

plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Robert A. Millken1909

A. He was an American Physicist.

B. He was the first to measure the charge and mass of an electron.

Millikan’s Oil Drop ExperimentHelps to calculate the mass & charge of an

electron

How it worked inside


1. Electron charge = 1.602 x 10-19 Coulombs.a. This is an extremely small quantity of energy.

2. Electron mass = 9.11 x 10-31 kga. Electrons are 1/1837th the mass of a single

proton or neutron.b. This is a very, very, very small amount and size.


C. Milliken’s experiments allow for 2 inferences (conclusions based upon evidence and reasoning) to be made:1. Because atoms, in the natural state, are

electrically neutral, they must also contain an equal amount of positively charged particles.

2. Because electrons have so little mass, atoms must contain other particles with much greater mass (protons & neutrons).

Ernest Rutherford (1911)on New Zealand Money

Hans Geiger (1911)

Ernest Marsden (1911)

Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment

A. Geiger and Marsden were students of the New Zealand Physicist.

B. They performed the Gold Foil Experiment.C. They used high-energy alpha particle

radiation( 2 protons & 2 neutrons ejected from a decomposing, radioactive element) to bombard a piece of gold foil that is surrounded by a fluorescent screen.


1. As alpha particles struck the fluorescent screen, they would produce a small detectable burst of light.

2. As the experiment was running, they detected light burst mainly behind the gold foil, but also occasionally all around the ring.


a. These bursts of light around the ring were because of the positively charged alpha particles been deflected by positively charged particles in the atoms of the foil.

b. The particles became known as protons.i. Just as with the electrons, positive charges

repel/deflect like positive charges.


a. As most of the bursts of light occurred behind the gold foil, they concluded that the majority of space in an atom is “empty space”. The alpha particles travelled through and never hit anything.

b. Rutherford proposes the idea of the neutrally charged neutron particle in 1920.


Alpha () particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Rutherford’s Findings

The nucleus is small The nucleus is dense The nucleus is positively charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”“Like howitzer shells bouncing off of tissue paper!”

Conclusions:Conclusions:

Atomic ParticlesAtomic Particles

Niels Bohr 1913

A. He was also a student of Rutherford’s.

B. He proposed the Bohr model of an atom.1. The electrons move in

a circular pattern around the positively charged center. a. Much like the planets

revolve around the sun.

Bohr Model of an Atom

Erwin Schrödinger1926

• He was an Austrian Physicist.• He helped develop the

Quantum Theory of Atoms– This Theory tries to describe,

by mathematics, the wave-like properties of electrons & other very small particles.

Schrödinger

• This re-enforces that electrons travel in orbitals.• Orbital is defined as a 3- Dimensional region

around a nucleus that indicates the probable location of a single electron within an orbital

Werner Heisenberg1927

• He was a German Physicist.• He calculated that electrons

and photons have about the same amounts of energy.– Photons are used to help detect

the presence of electrons.– When they collide, the electron is

deflected in a random direction.

Heisenberg’s Uncertainty Principle• This states that it is impossible to determine

exactly both position and velocity simultaneously of an electron in an orbit.

Dmitri Ivanenko (1930)

Victor Ambartsumian (1930)

Ivanenko & Ambartsumian1930

A. These gentlemen were Russian Physicists.B. They proposed a model of the nucleus of an

atom being composed of positively charged protons and neutral charged particles (neutrons).

Ivanenko & Ambartssumian Model of a Nucleus

James Chadwick1932

A. He was an English Physicist.

B. He also studied under Rutherford.

C. He proved that the nucleus is definitely composed of protons and neutrons through his experiments with alpha particle radiation.

His Experiment using alpha particle radiation

Unit 2 2.1 – History of the Atomic Model. Atomic Structure .

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