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!" Atomic Theory

Democritus 430 B.C.E. "by convention bitter, by convention sweet, but in reality atoms are void"

Democritus Scientific Method Question What 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

Democritus theory was rejected! Aristotle 384-322 BC 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.

A mass and B mass Before =A mass + B mass After Antoine Lavoisier 1743-1794 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 1782 Law of Conservation of Matter

Law of Definite Proportion Joseph Proust 1799 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 Dalton 1766-1844 Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.

John Dalton Scientific Method 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 Theory All matter is made of atoms Atoms are indivisible and can not be divided into smaller particles All atoms of an element are exactly alike, and unique not like any other element 1803 Atomic Theory Supported Law of Conservation of Matter Supported Law of Definite Proportions

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 Dalton’s Atomic Theory of 1803 was almost correct!

Dmitri Mendeleev 1869 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 Believed that an atom was a tiny, solid, unbreakable ball. Cathode Ray Emits rays from both ends Positive charge (Anode) Negative Charge (Cathode) 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)

Discovered subatomic particles called Electrons J.J. Thomsom 1897 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 Curie 1898 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 Soddy 1900 Observed 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.

Nagaoka 1903 Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.

HGJ Moseley 1914 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 Rutherford 1911 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. Aston 1918 Discovered the existence of isotopes through the use of a mass spectrograph. 

Niels Bohr 1922 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 Schrodinger 1930 Viewed electrons as continuous clouds.

James Chadwick 1932 Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. 

Enrico Fermi 1941 Conducted the first controlled chain reaction releasing energy from the atoms nucleus.