Atomic Theory CSII Honors

Neubert 2011

The Ancient Greeks…• Democritus – first to

describe the atom around 400 BC

• Aristotle did not believe in atoms

Born: c. 460 BCBirthplace: Abdera, ThraceDied: c. 370 BCCause of death: unspecifiedOccupation: Philosopher, Mathematician Nationality: Ancient GreeceExecutive summary: Early atomic theorist

Born: 384 BCBirthplace: Stagira, Chalcidice, GreeceDied: 322 BCCause of death: unspecifiedOccupation: Philosopher, Scientist Nationality: Ancient GreeceExecutive summary: Politics

What is an atom?

Democritus• The basic particle that

makes up all matter.• Comes from Greek word

“atomos” meaning indivisible

Aristotle• believed all matter was

continuous.• His belief was accepted for

~2000 years

No evidence to support either theory until…

Antoine Lavoisier• Law of the conservation of mass

– Mass is neither created or destroyed during chemical or physical changes

– Or more simply, the mass of substances produced (products) by a chemical reaction is always equal to the mass of the reacting substances (reactants).

– E.g., 1.00g carbon + 5.34g sulphur = 6.34g carbon disulphide

– 2.00g carbon + 10.68g sulphur =12.68g carbon disulphide

Born: 26-Aug-1743Birthplace: Paris, France

Died: 8-May-1794Cause of death: ExecutionReligion: Roman Catholic

Occupation: Chemist Executive summary: Father of modern

chemistry

Joseph Louis Proust• Law of Definite Proportions

– Chemical compounds contain the same proportions by mass regardless of the source or amount

• EX: Sodium chloride (table salt) is always 39.34% Sodium and 60.66% Chloride by mass

Born September 26, 1754(1754-09-26)Angers, France Died July 5, 1826(1826-07-05) (aged 71)Occupation Chemist

John Dalton• Early proponent of Atomic Theory

1. All matter is composed of extremely small particles called… ________.

2. Atoms of an element are identical to each other and different from atoms of other elements.

3. Atoms cannot be created or destroyed.

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

5. Atoms are combined, separated or rearranged when chemical reactions occur.

Born: 6-Sep-1766Birthplace: Eaglesfield, Cumberland, EnglandDied: 27-Jul-1844Cause of death: StrokeReligion: QuakerOccupation: Chemist, Physicist Executive summary: Early proponent of Atomic Theory

Dalton’s TheoryImpact• Turns Democritus’ ideas

into a scientific theory which could be tested – he related atoms to mass.

• Model of atom is small, solid, and indivisible particle.

Flaws

We now know that…• Atoms are divisible into

event smaller particles• Atoms of the same element

can have different masses

Warm Up 9/1(Put in the warm up section of your

binder.)• Name the flaws to Dalton’s ideas of the atom.

– Finish the statement• Dalton figured out that Atoms cannot…

Cathode Ray Tube and the Electron…JJ Thomson

• Discovered the electron– Concludes that all cathode rays

are composed of identical negatively charged sub-atomic particles = electons.

William Crookes• He also developed the Crookes

tubes, investigating cathode rays.• Investigated the conduction of

electricity in low pressure gases– as the pressure was lowered, the

negative electrode (cathode) appeared to emit rays

Born: 18-Dec-1856Birthplace: Cheetham Hill, EnglandDied: 30-Aug-1940Cause of death: unspecified

Religion: Anglican/EpiscopalianOccupation: Physicist Executive summary: Discovered the electron

Born 17 June 1832London, England Died 4 April 1919 (age 86)London, EnglandFields Physical chemistry

Cathode-Ray Tube Experiments

• Electric current passed through gasses at low pressures– Cathode end glows– Ray passes through to anode end when current was

passed.

• Rays were also observed to deflect away from magnetic fields and negatively charged particles.

Plum-Pudding Model

• Thomson’s experiments show that (1) atoms are divisible and (2) they include negatively charged electrons.

• Thomson’s Plum-Pudding Model of the Atom has negative particles spread evenly throughout a solid, positively charged sphere.

Robert Milikan

• Measured the charge of the electron using an oil drop experiment

• Using this charge + Thompson’s charge to mass ratio = mass of electron

• Mass of one electron = 9.109x10-31

Born: 22-Mar-1868Birthplace: Morrison, IL

Died: 19-Dec-1953Cause of death: Heart Failure

Religion: UnitarianOccupation: Physicist

Nationality: United StatesExecutive summary:

Determined the charge of an electron

Ernest Rutherford

• Conducted the Gold-Foil Experiment

• Positively charged alpha particles were fired at a thin piece of gold foil.

• About 1 out of every 8,000 particles were actually deflected back toward the source.

Born: 30-Aug-1871Birthplace: Brightwater, New ZealandDied: 19-Oct-1937Cause of death: Complications of Surgery Occupation: Physicist Nationality: EnglandExecutive summary: Father of nuclear physics

Ernest Rutherford

• Concludes that there must be a very small and densely packed region of positive charge in the atom that would repel these alpha particles.

• Discovered nucleus

The Nuclear Model of the Atom

• Nucleus = very small and dense center of the atom, where protons and neutrons are found.

• Protons have a positive charge.

• Neutrons are electrically neutral.

• Negatively charged electrons orbit the nucleus like the planets orbit the sun.

• Most of the atom is empty space.

Subatomic Particles

Proton Neutron Electron

Charge + 0 —

Mass* 1 amu 1 amu ~ 0

Location Nucleus Nucleus Electron Cloud

*Mass of protons and neutrons = 1.67 x10-27 kg compared to the electron’s mass of 9.109 x10-31 kg

Quick Review!

Mililkan Oil Drop Activity• Follow the directions on the paper. • Each group will only get one or two envelopes

and as a group we will share out the answers. • Answer the questions on a separate piece of

paper. Be sure to staple this paper to the original worksheet. Answers should be your own, although you may work as a group to come up with your ideas.

• Remember: you can agree to disagree.