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Atomic Structure Timeline
• Scientific models- a theory, description, or explanation based on known facts and observable behaviors
• Good models– Based on observation– Can successfully make predictions– Is changeable as new info. becomes
available
Alchemy (next 2000 years)
• Mixture of science and mysticism. • Lab procedures were developed, but alchemists did not
perform controlled experiments like true scientists.
Democritus (500 B.C.)
• Proposed that matter was composed of tiny indivisible particles
• Not based on experimental data
• Greek: atomos “indivisible”
John Dalton (1803)
• British Schoolteacher– based his theory on others’
experimental data– Interest was in meteorology
• Billiard Ball Model– atom is a
uniform, solid sphere
John Dalton
Dalton’s Four Ideas
1. Elements are composed of small indivisible particles called atoms.
2. Atoms of the same element are identical. Atoms of different elements are different.
3. Atoms of different elements combine together in simple proportions to create a compound.
4. In a chemical reaction, atoms are rearranged, but not changed.
J. J. Thomson (1897)
• Cathode Ray Tube Experiments– beam of negative particles
• Discovered Electrons– negative particles within
the atom
• Plum-pudding Model
J. J. Thomson (1897)
Plum-pudding Model– positive sphere
(pudding) with negative electrons (plums) dispersed throughout
– Atoms are neutral-no charge
Ernest Rutherford (1910)
• Gold Foil Experiment
• Discovered the nucleus– dense, positive charge in
the center of the atom– Electrons scattered
around nucleus– Atoms are mostly empty
space
Ernest Rutherford (1910)
• Nuclear Model– dense, positive nucleus surrounded by
negative electrons
Niels Bohr (1913)
• Bright-Line Spectrum– tried to explain presence
of specific colors in hydrogen’s spectrum
• Energy Levels– electrons can only exist in
specific energy states
• Planetary Model
Niels Bohr (1913)
• Planetary Model
– electrons move in circular orbits within specific energy levels (orbits)
Bright-line spectrum
Erwin Schrödinger (1926)
• Quantum mechanics – electrons can only exist in
specified energy states
• Electron cloud model – orbital: region around the
nucleus where e- are likely to be found
Erwin Schrödinger (1926)
Electron Cloud Model (orbital)• dots represent probability of finding an e-
not actual electrons
Electron Cloud Model
• Electrons not in definite path, but in a probable location
• Location based on energy of electron• Atom is a small, + charged nucleus
surrounded by a region of electrons• # of protons and electrons are equal in a
neutral atom
Conclusion
• All updates were based on evidence• Proceeded rapidly from 1800 to the
present