Section 2
The Early Atom
By Boundless
![Thumbnail](../../../../../../figures.boundless-cdn.com/16979/square/cg10c3-003.jpg)
Modern scientific usage denotes the atom as composed of constituent particles: the electron, the proton and the neutron.
![Thumbnail](../../../../../../figures.boundless-cdn.com/30730/raw/helium-atom-qm.jpg)
Dalton believed that that matter is composed of discrete units called atoms -- indivisible, ultimate particles of matter.
![Thumbnail](../../../../../../figures.boundless-cdn.com/31663/square/3q9mbr7wqpwuow0au86e.jpg)
Thomson proposed that the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16976/raw/ord-atomic-planetary-model.jpg)
Rutherford confirmed that the atom had a concentrated center of positive charge and relatively large mass.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16984/square/figure-31-03-01a.jpeg)
Bohr suggested that electrons in hydrogen could have certain classical motions only when restricted by a quantum rule.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16990/raw/bohr-atom-par.jpg)
Bohr explained hydrogen's spectrum successfully by adopting a quantization condition and by introducing the Planck constant in his model.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16982/square/figure-31-03-05a.jpeg)
According to Bohr, electrons can only orbit stably, in certain orbits, at a certain discrete set of distances from the nucleus.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16985/square/figure-31-03-03a.jpeg)
Based on his assumptions, Bohr derived several important properties of the hydrogen atom from the classical physics.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16980/raw/hydrogen-transitions.jpg)
The observed hydrogen-spectrum wavelengths can be calculated using the following formula:
![Thumbnail](../../../../../../figures.boundless-cdn.com/16989/square/figure-31-06-00a.jpeg)
By assuming that the electron is described by a wave and a whole number of wavelengths must fit, we derive Bohr's quantization assumption.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16992/raw/compton-scattering.jpg)
Compton explained the X-ray frequency shift during the X-ray/electron scattering by attributing particle-like momentum to "photons".
![Thumbnail](../../../../../../figures.boundless-cdn.com/16999/square/figure-31-04-08aa.jpeg)
X-ray shows its wave nature when radiated upon atomic/molecular structures and can be used to study them.
![Thumbnail](../../../../../../figures.boundless-cdn.com/16991/square/figure-2030-04-02a.jpeg)
The Compton Effect is the phenomenon of the decrease in energy of photon when scattered by a free charged particle.