History of the Atomic Model 2.1 Unit 2 History of the Atomic Model 2.1

Atomic Structure www.lab-initio.com

Democritus 400 B.C.

Democritus 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 Dalton 1808

Dalton A. He was an English schoolteacher. B. He was the first to propse an “Atomic theory” that contains the 5 following statements: 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. This has since been modified based on isotopes and ions

Dalton 3. Atoms cannot be subdivided, created, or destroyed A. This has since been modified based upon current studies in quantum physics. Such examples include muons and quarks. 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.

Dalton’s view of atoms of elements Can you see B. 1-4 in this picture?

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

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Actual Cathode Ray in action

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 through the gas using a battery and wires. a.The electrical current caused the gas within the tube to intensely glow with a beam (“ray”).

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/deflectslike negative charges. iv.The ray is made of a negative charge that Thompson called electrons (since they were associated with the electrical current.)

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

Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Robert A. Millken 1909

He was an American Physicist. He was the first to measure the charge and mass of an electron. The symbol for an electron is: e- Electron charge = 1.602 x 10-19 Coulombs. This is an extremely small quantity of energy. Electron mass = 9.11 x 10-31 kg This might be a good time to actually write the number out to reinforce minutiae.

Electrons are 1/1837th the mass of a single proton or neutron. This is a very, very, very small amount and size. Milliken’s experiments allow for 2 inferences (conclusions based upon evidence and reasoning) to be made:

Because atoms, in the natural state, are electrically neutral, they must also contain an equal amount of positively charged particles. Because electrons have so little mass, atoms must contain other particles with much greater mass (protons & neutrons).  

Millikan’s Oil Drop Experiment Helps to calculate the mass & charge of an electron

How it worked inside https://www.youtube.com/watch?v=XMfYHag7Liw

Ernest Rutherford (1911) on New Zealand Money

Hans Geiger (1911)

Ernest Marsden (1911)

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

As the experiment was running, they detected light burst mainly behind the gold foil, but also occasionally all around the ring. 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. The particles became known as protons.

Just as with the electrons, positive charges repel/deflect like positive charges. 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” that the alpha particles travelled through and never hit anything. Rutherford proposes the idea of the neutrally charged neutron particle in 1920.

Rutherford’s Gold Foil Experiment 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 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!” Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged

Atomic Particles Particle Charge Mass # Location Electron -1 Electron cloud Proton +1 1 Nucleus Neutron

Niels Bohr (1913) His comment on the structure of atoms Niels Bohr (1913) His comment on the structure of atoms. (Which are mostly empty space.)

Bohr He was also a student of Rutherford’s. He proposed the Bohr model of an atom. The electrons move in a circular pattern around the positively charged center. (Much like the planets revolve around the sun.)

Bohr Model of an Atom

Dmitri Ivanenko (1930)

Victor Ambartsumian (1930)

Dmitri Ivanenko & Victor Ambartsumian (1930) These gentlemen were Russian Physicists. They proposed a model of the nucleus of an atom that is composed of positively charged protons and neutral charged particles (neutrons).

Ivanenko & Ambartssumian Model of a Nucleus

James Chadwick 1932

He was an English Physicist. 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