ATOMIC TIMELINE - Dublin Schools

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ATOMIC TIMELINE
1. DEMOCRITUS
2. DALTON
3. JJ THOMSON
4. RUTHERFORD
5. BOHR
6. CHADWICK
1. DEMOCRITUS
460-370 BC
• Greek Philosopher (not a scientist)
• Developed the theory of the atom
• Theorized that the world is made up of hard, indivisible uncuttable particles of matter moving through
empty space.
• Did not perform any scientific experiments
• Believed that matter could not be divided into smaller and smaller pieces forever, eventually the
smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece
of matter “atomos,” meaning “not to be cut.”
DALTON
1766-1844
• proposed the Atomic theory of matter based on his experimental observations
• His Discoveries:
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All matter is made of atoms. Atoms are indivisible and indestructible.
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All atoms of a given element are identical in mass and properties.
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Compounds are formed by a combination of two or more different kinds of atoms.
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A chemical reaction is a rearrangement of atoms.
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Atoms can be neither created nor destroyed.
JJ THOMSON
1856–1940
• In 1897 he discovered the electron!
• Yet scientists knew atoms had no charge (neutral) so how could this be?!
• Thomson figured atoms must consist of a positive charge as well (to cancel out the negatively charged
electrons)
• He designed the “Plum Pudding Model” or “Blueberry Muffin Model”. Positively charged atom with
negatively charged electrons scattered within.
RUTHERFORD
1871–1937
• JJ Thomson’s student
• In 1911 he found evidence that countered (went against) Thomson’s model of the atom.
• He used the Gold Foil Experiment to figure out that.
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He aimed a beam of positively charged particles at a thin sheet of gold foil.
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IF Thomson’s model is CORRECT then the charged particles would pass right through the foil in a straight line.
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The gold foil would not have enough positive charge in any specific area to repel the positively charged particles that
were being shot.
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Most of the particles acted how they predicted ^^^^
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BUT a few particles were STRONGLY DEFLECTED. Since like charges repel each other ( + and +) he inferred that an atom’s
positive charge must be clustered in a tiny region in the center.
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He called this the NUCLEUS! (the deflected particles were repelled by a gold atoms nucleus)
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Other scientists figured out that electrons have almost no mass…..so they realized that all of the atoms mass must be in
the positively charged nucleus.
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Rutherford named these positively charged particles in the nucleus PROTONS!
BOHR
1885–1962
• https://www.youtube.com/watch?feature=player_embedded&v=wCCz20JOXXk
• In 1913 Bohr Refined Rutherford’s atomic model by proposing the position of electrons.
• He determined that electrons:
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Orbit around the nucleus
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Can move only in fixed orbits of specific energies
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Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit
further from the nucleus
CHADWICK
1891-1974
• In 1932 Chadwick discovered another particle in the nucleus…. THE NEUTRON!
• This completes the modern atomic model that we use today 
• It was hard to detect because it has no electrical charge (neutral) but it has nearly the same mass as a
proton.
• He was also now able to explain the existence of ISOTOPES!
• Isotopes of the same element have the same number of protons and electrons, but differ in the number
of neutrons found in the nucleus.

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