Evolution of Atomic Theory

Mrs. Baldessari Chemistry

Scientists that changed our view of the atom

?Greatest Chemistry Discoveries?? – YouTube Aristotle

What: All matter is a combo of fire, air, earth or water When: 350 B.C. Where: Greece Democritus • 460BC-370BC •Born in Abdera, Thrace.

• Greek philosopher who developed the atomic theory of the universe.

•There was no Atomic Theory before Democritus - he developed the first theory Democritus

• Did not conduct any experiments

• All matter is made of (atomos = indivisible) atoms

• Atoms are indivisible and indestructible

• ATOMIC STRUCTURE Democritus’ Atomic Theory

 Atoms differ in shape, size, weight, sequence, and position.  Atoms spin and collide, creating larger aggregations of matter.  Democritus performed no experiments and had almost no evidence for his theory of the atomic model. John Dalton

 John Dalton was born on September 6, 1766 in Cumbria, England.  He died on July 27, 1844  Dalton was a chemist, meterologist, and physicist.  Attended college in Manchester.  He provided us with the first concrete ideas about the atom. John Dalton – solid sphere model Four points to his theory in 1805 1. All matter is composed of atoms that are indestructible and indivisible 2. Atoms of one element are all the same; atoms of a different element are different (Au atoms are different from Ag atoms) 3. Atoms cannot be changed into different atoms by chemical of physical changes (Pb atoms cannot be turned into Au atoms) 4. All atoms are like Billiard Balls – a solid, sphere 5. Atoms react in whole numbers Guess What? Atoms are not the smallest particles

Joseph John Thomson was born in Cheetham Hill, a suburb of Manchester on December 18, 1856.

He is given credit for the discovery of the electron (particles with a negative charge). JJ Thomson

• Conducted experiment with a Cathode Ray Tube • Model = plum pudding • When: 1897 • Where: England Cathode-ray Tubes JJ Thomson – His Experiment 1897

 watch JJ Thomson’s cathode ray tube experiment

Quick Facts: Born: December 18,1856 Cheetham Hill Manchester, UK Died: August 30, 1940 Cambridge UK Nationality: British Fields: Physics Institutions: Cambridge University Known for: Plum pudding model, discovery of electron, discovery of isotopes Awards: Nobel Prize for Physics 1906 JJ Thomson – His Experiment Cathode Ray Tube Thomson’s Plum pudding model

 The atomic model before his work was thought to of had resembled a billiard ball a small, solid sphere. Thomson’s work suggested that it wasn’t one particle but a “jigsaw puzzle” of smaller pieces.  Thomson discovered the electron by using a cathode-ray tube, He interpreted that the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms“. All gases contained these particles and the charge: mass ratio was the same for all gases and metals used.  In 1904 he suggested a model (By which some called the “Plum pudding model”) of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. They called it the Plum pudding model because the pudding was the atom and the bits of plum were the electrons positioned there.

 Plum Pudding Model JJ Thomson

3 Points to his theory 1. * All gases had the same mass: charge ratio 2. * All atoms contain small, charged particles called electrons 3. * The atom now looked like “Plum Pudding” Goldstein

 Eugene Goldstein was born in 1850 in Poland  Died December 25, 1930 in Berlin Germany  Goldstein's with cathode-ray and worked with anode rays. He observed particles moving toward the cathode (negative terminal) to suggested the presence of the proton -- a positively charged particle. Goldstein – His Experiment 1886- 1887 Cathode Ray Tube

Eugene Goldstein discovered positive particles by using a tube filled with hydrogen gas (this tube was similar to Thomson's tube). This resulted in The positive particle had a charge equal and opposite to the electron. The positive particle was named the proton. Goldstein

Major Point to his theory

He is given credit for the discovery of a positively charged particle = Proton

The atom was still thought to resemble only Only now there are also positive charges in the Plum Pudding Robert A. Millikan

 Born March 22, 1868 Morrison Illinois, USA  Died December 19, 1953 (Age 85)  Attended Oberlin College 1891 and Columbia University in 1895  He discovered from his measurements and Thomson’s charge: mass ratio the charge of an electron and its mass.  Millikan received the 1932 Nobel Prize in Physics for this discovery. Millikan’s 1910 Oil Drop Experiment

Watch Millikan's oil drop experiment Millikan’s Experiment

 Millikan discovered the mass of an electron by introducing charged oil droplets into an electrically charged field. The charge of the electron was found to be 1.602E-19 coulombs. Using Thomson's mass ration, Millikan found the mass of one electron to be 9.11E-28 grams.  In his Oil Drop experiment he took a atomizer from a perfume bottle and sprayed drops of oil or water into the sample chamber, Then some droplets fell threw a pinhole and ended up between two plates. One of the plates were positive and the other was negative. The middle chamber was ionized by x-rays. Particles that did not capture any electrons fell to the bottom plate due to gravity. The others that picked up an electron would stick to the upper plate or fall noticeably slower. Millikan

2 Points to his theory 1. All electrons have the same mass About 1/2000 atomic mass unit 2. All electrons have the same charge -1.6 x 10 -19 Coulomb

No new model - The atom was still thought to resemble Plum Pudding Rutherford

 Ernest Rutherford was born August 30, 1871 in Nelson, New Zealand  Died: October 19, 1937 in Cambridge, England  Proved nucleus is dense, positively charged core of atom, in 1911  Won Nobel Prize in Chemistry, 1908 Rutherford’s 1910 Gold Foil Experiment

watch Rutherfords gold foil experiment Results of foil experiment if Plum Pudding model had been correct What Actually Happened Rutherford’s observations and conclusions

3 Points to his theory 1. * Most alpha particle went through = Atom is mostly empty space (area of electrons) 2. * Some alpha particles bounced straight back = atom has a solid mass (nucleus) 3. * Some alpha particles reflected = nucleus is positively charged

JJ Thomson’s Plum pudding model thrown out New Model = Nuclear Model (stationary planetary model) Gold Foil Experiment

Observations and Conclusions: Most of the alpha particle went straight through with no deflection – the atom is mostly empty space

A few alpha particles bounced straight back – The atom contains a small concentrated area that contains most of the mass (nucleus)

Some alpha particles were deflected at slight angles – the nucleus contained a positive charge which repelled the alpha particle as it came close

New model: Nuclear Model Schrödinger

 Erwin Schrödinger born August 12 in Vienna, Austria in 1887.  Died: January 4, 1961 in Vienna  Fields: Physics  Won Nobel Prize in Physics in 1933 Schrödinger video of Schrodinger's cat

Schrödinger hypothesized that the cat can exist as both alive and dead until it is observed. Once it is observed, the characteristics of the cat have changed. He also believed this to be true of the motion of the electrons. Schrödinger

Schrödinger used wave mechanics to define the probability regions (Orbitals) of finding an electron of a given energy. Once we measure the electron, some of its characteristics must be changed.

New Model: Quantum Mechanical Model Schrödinger

Major points 1. Electrons do not follow fixed paths 2. They move randomly in areas of probability (orbitals) 3. There are specific energies associated with each orbital

New Model = Quantum Mechanical Model James Chadwick

 Chadwick was born in Cheshire, England, on 20th October, 1891.  Died: July 24, 1974 in Cambridge, England  Discovered the neutron  Chadwick’s discovery allowed the creation of elements heavier than uranium in the laboratory.  Won Nobel Prize in Physics in 1935 Chadwick – His 1932 Experiment

Experiment: 1. Bombarded a Be atom with large alpha particles 2. This caused an unknown “radiation” to be emitted. (many thought it was a gamma ray – which has no charge and no mass) 3. The unknown radiation was allow to bombard paraffin wax. 4. This dislodged protons into a detector. Chadwick

2 Points to his theory 1. Discovered the Neutron – Same mass as a proton, but without a charge 2. Mass could be converted into energy

Model was still thought to resemble the Quantum Mechanical Model Importance of Chadwick’s 1932 Discovery

Conclusions: 1. The unknown “radiation” did not carry a charge and was not deflected by magnetic fields 2. It could not be gamma rays because the particle contained enough mass to dislodge a proton. 3. New particle – Neutron – no charge; mass ~ the same as a Proton.

No new model: Quantum Mechanical Model