Download Atomic Structure What is an atom?

Atomic Structure
What is an atom?
What does it look like?
How was it discovered?
We might as well attempt to introduce a new planet into the solar system, or to
annihilate one already in existence, as to create or destroy a particle of hydrogen.
John Dalton, 1808
All matter consists of tiny particles.
Atoms are indestructible and unchangeable.
Elements are characterized by the mass of their atoms.
When elements react, their atoms combine in simple, whole-number ratios.
When elements react, their atoms sometimes combine in
more than one simple, whole-number ratio.
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Large, negatively
charged spherical
electron ‘cloud’
cloud
Tiny, positively
charged nucleus
That contains ‘all’
of the atom’s mass
99.94-99.98%
Atom is mainly
‘open space’
Prediction based on Rutherford’s nuclear atom:
number of scattered alpha particles is proportional to:
thickness of foil
square of the charge on the nucleus
1/sin4(theta/2)
1/velocity4
Mass Spectroscopy
Emission Spectra
•experimental key to atomic structure: analyze light emitted by high temperature gaseous elements
•experimental setup: spectroscopy
•atoms emit a characteristic set of discrete wavelengths- not a continuous spectrum!
•atomic spectrum can be used as a "fingerprint" for an element
•hypothesis: if atoms emit only discrete wavelengths, maybe atoms can have only discrete energies
•an analogy
A turtle sitting on a ramp can have any height above the ground- and so, any potential energy
A turtle sitting on a staircase can take on only certain discrete energies
•energy is required to move the turtle up the steps (absorption)
•energy is released when the turtle moves down the steps (emission)
•only discrete amounts of energy are absorbed or released (energy is said to be quantized)