Transcript Democritus 460 BC

The History of the Atom…
Democritus
460 BC - 370 BC
• went against, Aristotle, who believed that matter was composed
of four qualities: earth, fire, air and water
• all matter is composed of small, indivisible particles
• coined the term ‘atomos’
John Dalton
1766-1844
• developed a model on the atom based on already established
laws:
• the law of conservation of matter (Lavoisier)
• the law of definite proportions (Proust)
• the law of multiple proportions (Dalton)
Dalton’s Atomic Theory
1803
1) Each element is made up of indivisible particles of uniform density atoms. (now know atoms are divisible)
2) All atoms of a given element have the same mass, atoms of different
elements have different masses. (now know about isotopes)
3) Atoms combine in fixed, small, whole number ratios to form
compounds. (explains law of definite and law of multiple
proportions)
4)
Chemical reactions involve the rearrangement of atoms to form new
substances. (atoms are neither created nor destroyed) (explains law
of conservation of mass)
Dalton’s model of the atom:
Law of Conservation of Matter
Matter is neither created nor destroyed in a chemical reaction
Law of Definite Proportions
Elements combine in definite, fixed ratios to form compounds
Example - water is H2O every time.
Law of Multiple Proportions
- The same elements can combine in different fixed ratios to form
different compounds. Example H2O versus H2O2.
- When 2 or more elements combine to form different compounds, the
ratio of the second element to a fixed mass of the first is always a
small, whole number ratio (math exs from w/s)
Crookes and the Cathode Ray Tube (1870’s)
Particles or light?? Test with a magnet:
J. J. Thomson
1856 - 1940
• What were these particles???
• Thomson’ hypothesis: since the beam came from the cathode (a
negative electrode), and given how the ray is deflected by a magnet,
the particles have a negative charge.
J. J. Thomson’ experiments with the CRT
1856 - 1940
• Tried to separate the charge from the particles by bending the rays with
a magnet. He could not separate the two.
• Studied the deflection of the cathode ray by an electric field: cathode
ray was deflected by the negatively charged plate.
J. J. Thomson’ experiments with the CRT
1856 - 1940
• Calculated the charge to mass ratio of these particles by measuring the
degree of deflection with different strength magnetic and electric
fields.
• no matter which gas he used in the tube, the calculated charge to mass
ratio of the particles was the same.
AP disk chap 2 screen 6 description
More J. J. Thomson
• But atoms are electrically neutral - must also have a positive component.
• Reverse the electrodes on the cathode ray tube: deflection by positive
plate!!
Voila! A positive component. Thomson used the hydrogen ion, which is just a
single proton, so he missed that atoms have positively charges particles - he
thought the whole atom was a ball of positive charge.
Thomson’s Plum Pudding Model (1896ish)
• Thomson concluded that the cathode ray was made up of negatively
charged subatomic particles, later called electrons (e-)
• No matter what metal the electrodes were made of, or what gas filled
the tube, the same charge to mass ratio was calculated. All atoms have
them, and they are identical in all atoms
• Atoms also have a positive component - he reasoned a ball of positive
charge, protons were found later.
ball o’ positive charge
electrons
Millikan’s Oil Drop Experiment 1909
• Determined the charge of an electron. Given Thomson’s charge
to mass ratio calculation, the mass of an electron could then
also be determined.
• Oil drops were given a negative charge by attaching electrons to
them. By balancing the force of gravity with the force from an
electric field on these droplets, Millikan could determine the
magnitude of the charge on each oil drop, and found that the
charge was always some multiple of 1.6 x 10-19 C.
• Therefore, the charge of a single electron was 1.6 x 10-19 C.
•
AP disk chapter 2 screen 7 description for animation
Ernest Rutherford
1871 - 1937
• Was trying to prove the Plum Pudding model with his famous
Gold Foil Experiment in 1909
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Ernest Rutherford - The Hypothesis
1871 - 1937
• expected most of the (+)ly charged alpha particles to pass straight through
the gold atoms in the foil.
Rutherford’s Observations
• Most of the alpha particles passed straight through, as expected.
• Some were deflected at large, large angles - some came right back at him!!
This was unexpected…. AP disk chapter 2 screen 9 description
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Rutherford’s Thoughts
What was going on???
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QuickTime™ and a
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Rutherford’s Conclusions and Model
• Majority of the atom was empty space, with all the positive charge and
99% of the atom’s mass concentrated in a small volume in the center. The
nucleus!! (small and dense)
• Electrons orbited around the nucleus. Much smaller than protons.
Two problems:
1. The mass of atom did not equal the mass of the positive charge alone.
2. Why didn’t the electrons fall into the nucleus? What keeps them in
motion?
Rutherford’s First Problem
• Rutherford’s model suggested atoms composed of two subatomic particles
with equal but opposite charges: electrons and protons.
• Problem - mass of atom not equal to mass of protons alone - must be another
subatomic particle to make up the difference.
• 1932 - Chadwick discovers the neutron - a subatomic particle present in all
atoms that has about the same mass as a proton but no electrical charge!!
Rutherford’s Second Problem
Why didn’t the electrons fall into the nucleus? What keeps them in
motion?
…..answered by Bohr…..next chapter…...stay tuned