Download Models of the Atom

Models of the Atom
Model
Greeks – 450 BC
Democritus and Leucippus
Dalton – 1803
John Dalton’s Atomic Theory
Thomson – 1897
Positively charged
matter
Electron
Rutherford – 1911
Bohr – 1913
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Characteristics
The word "atom" comes from the Greek "a-tomos" and
signifies indivisible.
solid (no void inside)
surrounded by an empty
All matter consists of atoms, tiny indivisible particles
of an element that cannot be created or destroyed.
• Atoms of one element CANNOT be converted into
atoms of another element.
• Compounds form from the chemical combination of a
specific ratio of atoms of different elements
Scientist’s noticed that:
• Cathode rays traveled in straight lines unless they
were placed in a magnetic or electrical field
• When placed in a magnetic field, the cathode rays bent
• When placed in an electrical field, the cathode rays
bent towards the positive plate
• The cathode ray was identical regardless of the metal
used as the cathode
J.J. Thompson realized that this was evidence that the
atom was divisible and he proposed his model of the atom
“the raisin bun in a plum pudding” with electrons
embedded in a positively charged sphere.
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• Tiny positively charged alpha particles from radioactive
radium were aimed like projectiles at a thin gold foil
• 1 in 20,000 alpha particles was deflected back by more
than 90
• Something small in the centre of the atom was repelling
the alpha particles
• Rutherford concluded that the atom was mostly empty
space occupied by electrons but in the centre, a small
“nucleus” contained all of the positive charge and
essentially all the mass of the atom
• Rutherford explained the nature of the atom but not
the mass. In 1931 James Chadwick discovered the
neutron which explained the mass
• Bohr hypothesized that hydrogen only had certain
allowable energy levels on which electrons could be
found
Modern – post 1932
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This is the model that is accepted today According to
the Principles of Quantum Mechanics electrons are
distributed around the nucleus in "probability
regions". These probability regions are called
"atomic orbitals". According to Quantum Mechanics,
these orbitals are mathematically defined and are
described by a uniquely different math function for
each electron.
Animations
1. Go to
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther1
4.swf
Watch the animation.
2. Go to
http://www.wwnorton.com/COLLEGE/chemistry/gilbert/tutorials/ch3.htm
Watch Section 3.2 Rutherford Experiment (p. 106-109) Click view tutorial.