Atomic Theory

Two atoms are walking down the street when one atom says to the other, "I think I lost an electron."The other atom says "Are you sure?""Yeah I'm positive!"

Democritus430 B.C.E.

Click icon to add picture"by convention bitter, by convention sweet, but in reality atoms are void"

DemocritusScientific Method

Click icon to add pictureQuestionWhat is the world made of?

Claim: World is made of empty space and tiny particles.

Evidence: ?????

Conclusion: •Atoms are smallest Particle of matter•There are different types of atoms for all different types of matter

Aristotle384-322 BC

Click icon to add picture•Philosopher•Very influential

•Democritus theory was rejected!

Question: Could Democritus be right?

Claim: There is no such thing as empty space.

Experiment: ???Evidence: ???

Conclusion: Democritus is wrong.

Antoine Lavoisier1743-1794

Click icon to add picture1782

Law of Conservation of Matter

Question: ??

Claim: ???

Experiment: Compared mass of reactants before and after reaction

Evidence: Mass of reactants and products were the same.A mass and B mass Before =A mass

+ B mass After

Conclusion: Matter is neither created or destroyed, only changed! Law of Conservation of matter

Joseph Proust

Click icon to add picture1799

Law of Definite Proportion

Question: Does the composition of elements in compounds change? (H2O)

Claim: ???

Experiment: ????

Evidence: H2O composition is always 11.2% H and 88.8% O by Mass

Conclusion: The proportion of elements in a compound never changes, The law of Definite Proportions

John Dalton1766-1844

Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.

John DaltonScientific Method

Click icon to add picture1803Atomic Theory

•Supported Law of Conservation of Matter•Supported Law of Definite Proportions

Question: How are the results of Proust and Lavoisier’s Experiments connected?

Claim: If matter were composed of indivisible atoms then a chemical reaction would only rearrange the atoms and none of them would disappear or be formed. Also if each element consisted of atoms of a specific type and mass then a compond would always be made of a certain combination of atoms that never varied.Experiment: ????Evidence:???

Conclusions: Atomic Theory1. All matter is made of atoms2. Atoms are indivisible and can not be

divided into smaller particles3. All atoms of an element are exactly

alike, and unique not like any other element

Dalton’s Atomic Theory of 1803 was almost correct!

Later discoveries proved 2 errors; 1. atoms are made of subatomic particles 2. atoms of the same element are nearly, but not exactly identical

Dmitri Mendeleev1869

Arranged elements into 7 groups with similar properties.  He discovered that the properties of elements  "were periodic functions of their atomic weights".  This became known as the Periodic Law.

Many Scientists

Cathode Ray

Emits rays from both ends•Positive charge (Anode)•Negative Charge (Cathode)

Believed that an atom was a tiny, solid, unbreakable ball.

•Vacuum tube, all gasses pumped out•Metal piece, called electrodes sticking out of each end• Become charged when attached to strong

electrical current•Rays travel in the tube from the negative electrode (cathode) to the positively charged electrode (anode)

J.J. Thomsom1897

Used a cathode ray tube to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 108 coulombs/gram. 

Discovered subatomic particles called Electrons

•Experiment: Used cathode Ray•Rays bent toward positively charged plate– must be negative•Conclusion: cathode rays are made of invisible, negatively charged particles called electrons which came from the atoms of the cathode!

Marie Curie1898

Studied uranium and thorium and called their spontaneous decay process "radioactivity".

She and her husband Pierre also discovered the radioactive elements polonium and radium.

Frederick Soddy1900

Click icon to add pictureObserved spontaneous disintegration of radioactive elements into variants he called "isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released during decay.

Nagaoka1903

Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.

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HGJ Moseley1914

Using x-ray tubes, determined the charges on the nuclei of most atoms.

He wrote: “ The atomic number of an element is equal to the number of protons in the nucleus".

This work was used to reorganize the periodic table based upon atomic number instead of atomic mass.

Ernest Rutherford1911

Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil.

He established that the nucleus was ‘very dense, very small and positively charged.’

He also assumed that the electrons were located outside the nucleus. 

Francis W. Aston1918

Discovered the existence of isotopes through the use of a mass spectrograph. 

Niels Bohr1922

Developed an explanation of  atomic structure that underlies regularities  of the periodic table of elements.

His atomic model had atoms built up of successive orbital shells of electrons.

Erwin Schrodinger1930

Click icon to add pictureViewed electrons as continuous clouds.

James Chadwick1932

Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. 

Enrico Fermi1941

Click icon to add pictureConducted the first controlled chain reaction releasing energy from the atoms nucleus.