Atomic Model Timeline

Main

Democritus

400 BC

•First person to propose the "ultimate particle"
•believed that matter could only be divided a limited number of times, eventually there would be a "smallest piece"
•Called it "atomos", meaning “not to be cut.”
•Thought that atoms were small particles that were made out of the same material but were all different sizes and shapes
•believed that they were in constant motion

Aristotle

350 BC

•based on the four elements
•thought atoms didn't exist
•He didn't think particles could be in constant motion in a void
•His theory was used for almost 2000 years, until after the scientific revolution

Antoine Lavoisier

1780

• His work was mainly devoted to the study of combustion, through experiments combining and separating substances through combustion, he proved that there was a substance in the air which he called “oxygen”
• He noticed that it never behaved like it was a mixture of different substances, concluded that it was an element
• He believed that this would be true for all chemical changes (he was right). This is now called the law of conservation of mass.
• Although he did not do any work relating to atoms, his work laid the groundwork for thinking about elements.

John Dalton

1803

• Recognized the existence of atoms and elements
• Assigned a relative atomic weight of one to hydrogen, from that developed a relative atomic weight scale from assumed atomic ratios
• Law of multiple proportions
• Proposal of the atomic theory in 1803

Joseph Gay-Lussac

1808

• Law of combining volumes in 1808 – at the same temperature and pressure, two volumes of hydrogen gas react with one volume of oxygen gas to produce two volumes of water.
• Found that if you take a specific volume of gas and raise the pressure, the temperature increases.
• Gay-Lussac never worked with the atomic theory, but his combinations of gases were a great advance.

Amedeo Avogadro

1811

•Hypothesis in 1811: at the same temperature and pressure, equal volumes of gases contain the same number of molecules or atoms
•When combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2 and H2O
•radical statement, not widely accepted until fifty years later
•Avogadro also astutely reasoned that simple gases were not formed of solitary atoms but were instead compound molecules of two or more atoms. (Avogadro did not actually use the word atom; at the time the words atom and molecule were used almost interchangeably. He talked about three kinds of “molecules,” including an “elementary molecule”—what we would call an atom.)

Henri Becquerel

1896

•worked with properties of the atom, such as magnetism and radioactivity
-found that some atoms have natural radioactivity
•His discovery of radioactivity allowed later scientists to perfect the atomic model

JJ Thomson

1897

•Discovered electron
•"plum pudding" model
---the atom was positive and there were negative forces wandering around it
-discovered atoms of the same element but different atomic weight

Marie and Pierre Curie

1898

•explored the mysteries of radioactivity
•discovered the elements radium and polonium after investigating uranium ore Marie found that it wasn't just the uranium that was radioactive, but also other new elements.
*Fun Fact: In their honor the units for radio activity was named after them "curie"

Planck

1900

•Quantum Theory
•energy is given off in little packets
•energy in wave form is restricted to specific quantities
•led to understanding of atom levels and advancement in the atomic model

Albert Einstein

1905

•postulated that light was made up of different particles that, in addition to wavelike behavior, demonstrate certain properties unique to particles. He also brought forth the theory of relativity

Hans Geiger

1906

•Developed an electrical device to "click" when hit with alpha particles.

Robert Millikan

1908

•very interested in J.J. Thomson's finding of the electron. J.J. Thomson predicted that the electron was 1000 time smaller than the atom
•involved in the Quantum Theory after he was inspired by Max Planck
"Oil Drop Experiment":
-Determined unit the unit charge of an electron
               e = 1.60 x 10-19 coulombs
-Also determined their mass:
m = 9.11 x 10^ -28 gram
•inspired other scientists to explore parts of the atom

Rutherford

1909

(Thought Thomson was wrong)
•thought of an atom having a positive nucleus with negative electrons surrounding it
•Atom was mostly empty
- Came to this conclusion through the "Gold Foil Experiment"
-Had particles pass through gold foil with detective screen surrounding them
-The particles that didn't pass made him come to the conclusion that most of the atom is empty space
-Flaws: thought electrons contained charges that are accelerating

Niels Bohr

1913

•decided to make a new model based off of Rutherford's model, but changed the orbit of the electron
•created energy levels in the atom, where only a certain amount of electrons could fit on one energy level of the atom
•used Planck's ideas in order to create quantum mechanics

Henry Mosely

1913

•worked with Niels Bohr in order to create the real atomic number
•used X-rays to find the frquencies of elements on the periodic table
•used these frequencies to find that the number of protons in the nucleus correlated with the atomic number
"The atomic number of an element is equal to the number of protons in the nucleus". -Moseley

Ernest Marsden

1926

•contribution to Rutherford atomic model while expirementing with alpha particles
-Used Rutherford's gold foil experiment
-Discovered proton
-Found connection between tHe atomic number and the nuclear charge

James Chadwick

1932

•discovered the neutron in 1932
•discovery of the neutron led directly to the discovery of fission because he found out that Sice neutrons have no electrical charge they could spilt the nucleus without anything to get in their way.
and ultimately to the atomic bomb