History of Atom Timeline

Rubric: https://drive.google.com/file/d/0B5kixEO3VLnwcFYxRVFEdl9qcUE/view

~400 BC (5th Century BC)​ Democritus ​ ​ • Atomos –indivisible

• First to identify possibility of atom

1803, 1805, 1808​ ​ John Dalton​ ​ • Father of modern atomic theory

• Five parts to theory
• Includes elements are composed of atoms
• Also includes element’s atoms are
identical in mass

1896 ​ ​ ​ ​ Henri Becquerel​ • Chemical spontaneously decompose

1897​ ​ ​ ​ JJ Thomson​ ​ • Discovered electron

​ ​ ​ ​ ​ ​ ​ • Described the nature of cathode rays
​ ​ ​ ​ ​ ​ ​ • Plum Pudding model of atom

1911​ ​ ​ ​ Ernest Rutherford​ • Gold foil experiment

• Atom has small positive nucleus,
remainder is empty space
​ ​ ​ ​ ​ ​ ​ • Predicted existence of neutrons

1898​ ​ ​ ​ Marie Curie​ ​ • Studied uranium and thorium

• Coined term “radioactivity”

1909​ ​ ​ ​ Robert Millikan​ • Measured charge of the electron

• Oil drop experiment
• Completed with Thomson

1922​ ​ ​ ​ Niels Bohr​ ​ • Planetary model of atom

• Electrons travel in specified energy levels
• Spectrum lines produced when electrons
move

1923​ ​ ​ ​ Louis de Broglie​ • Electrons have properties of both waves

and particles
 • Group of waves named after scientist

1927​ ​ ​ ​ Werner Heisenberg​ • “uncertainty principle”

• Impossible to determine the position and
the momentum of a particle at the same
time
• “cloud” model aka quantum mechanical
model

1930​ ​ ​ ​ Erwin Schrodinger​ • Showed mathematically that waves can

be used to describe electrons in atoms
​ ​ ​ ​ ​ ​ ​ • Determined probability of electrons in
atoms

1932​ ​ ​ ​ James Chadwick​ • Helped with the development of the

nuclear model
• Discovered neutron

Democritus
http://www.universetoday.com/60058/democritus-atom/
https://the-history-of-the-atom.wikispaces.com/Democritus
http://www.chemteam.info/AtomicStructure/Greeks.html

John Dalton
http://www.biography.com/people/john-dalton-9265201#daltons-law-
http://www.famousscientists.org/john-dalton/

JJ Thomson
http://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/atomic-
and-nuclear-structure/thomson.aspx
http://www.biography.com/people/jj-thomson-40039#personal-life-and-later-years

Ernest Rutherford
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1908/rutherford-bio.html
http://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/atomic-
and-nuclear-structure/rutherford.aspx
http://www.famousscientists.org/ernest-rutherford/
James Chadwick
http://www.nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-facts.html
http://www.famousscientists.org/james-chadwick/
http://www.atomicheritage.org/profile/james-chadwick
http://www.pbs.org/wgbh/aso/databank/entries/dp32ne.html

Democritus
Democritus was an ancient Greek philosopher who lived from 460 BC to 370 BC. His mentor,
Leucippus, was actually the one who came up with the atomic theory first, then this theory was
adopted by Democritus. The atomic theory stated that “The universe is composed of two
elements: the atoms and the void in which they exist and move.” His hypothesis about the
atoms was created in 465 BC and it had 5 important points:
1.​ All matter is composed of atoms, which are indivisible
2.​ Atoms are indestructible
3.​ Between atoms are empty space (void)
4.​ Atoms are homogeneous, with the same structure throughout.
5.​ Atoms differ in size, shape, mass, position, and arrangement.
He also reasoned that the solidness of the material corresponded to the shape of the atoms
involved (ex. iron atoms are solid and strong with hooks that lock them into a solid; water atoms
are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air
atoms are light and whirling.)

John Dalton
John Dalton was a British chemist, born in Eaglesfield, England, on September 6, 1766. From
1801 to 1803, Dalton was researching about gases, the pressure related to it, and how they turn
into liquids. His study of gases led Dalton to wonder about what these invisible substances were
actually made of. He carried out countless chemical reactions, and in 1808 published what we
now call Dalton’s Law in his book A New System of Chemical Philosophy: “If two elements form
more than one compound between them, then the ratios of the masses of the second element
which combine with a fixed mass of the first element will be ratios of small whole numbers.”
Dalton’s Atomic Theory states that:
1.​ The elements are made of atoms, which are tiny particles, too small to see.
2.​ All atoms of a particular element are identical.
3.​ Atoms of different elements have different properties: their masses are different,
and their chemical reactions are different.
4.​ Atoms cannot be created, destroyed or split.
5.​ In a chemical reaction, atoms link to one another, or separate from one another.
 6.​ Atoms combine in simple whole-number ratios to form compounds.
Dalton’s system of atoms and molecules is almost identical to how we might represent them
today, and he also composed a table listing the atomic weights of all known elements.

JJ Thomson
J.J. Thomson was born on December 18, 1856, in Cheetham Hill, England. In 1894, Thomson
began studying cathode rays, which are glowing beams of light that follow an electrical
discharge in a high-vacuum tube. He discovered that the ratio of the electrical charge to the
mass of the particles was the same no matter what type of gas was used, by experimenting with
the cathode rays. Through this, he determined that all matter is made up of tiny particles that
are much smaller than atoms. He originally called these particles 'corpuscles’, which are now
called electrons. In 1906, he also proved the existence of isotopes in a stable element using
mass spectrometry.
Bibliography
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Coffey, Jerry. "Democritus Atom." Universe Today. N.p., 22 Mar. 2010. Web. 28 Oct. 2015.
<http://www.universetoday.com/60058/democritus-atom/>.

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<https://the-history-of-the-atom.wikispaces.com/Democritus>.

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<http://www.chemteam.info/AtomicStructure/Greeks.html>.

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<http://www.famousscientists.org/john-dalton/>.

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