400 B.C.
350 B.C
Democritus Aristotle
0
1903
Rutherford
1907
Millikan
1913
Bohr
1743-1794
Lavoisier
1800
Dalton
1900
Thomson

 In 400 B.C. Democritus believed all existence consisted of two things:
 atoms (coined the term atom from the Greek word ‘atomos’ meaning indivisible)
 the void (space)
 Democritus’ theory:
 atoms are particles in space (the void)
 atoms always in motion; make life predictable
 matter could not be divided indefinitely; atoms are indivisible
 all things we see differ because of the shape, size, and arrangement of atoms

 In 350 B.C. Aristotle and his followers dismissed the idea of atoms
 Instead modified an earlier theory that matter was made up of four ‘elements’:
 earth
 fire
 water
 air
 Even though Aristotle was wrong his theory persisted for 2000 years!

 During his lifetime, 1743-1794, Antoine Lavoisier established the Law of Conservation of Matter
 Law= in a chemical reaction, the mass of the reactants is equal to the mass of the products
 Theory stated that mass is concrete and measurable, rather than ‘magical’

 In 1800 John Dalton was examining gases dissolved in water and decided that matter was made of individual particles with spaces between them
 Devised a chemical atomic theory:
 all matter is made of atoms
 atoms of an element are identical
 each element has different atoms
 atoms of different elements combine in constant ratios to form compounds
 atoms are rearranged in reactions

 In 1897, looking at charged particles, J.J Thomson found particles that had a charge that did not change regardless of the metal they came from…
 He called these charged particles electrons
(negatively charged particles)
 Later he proposed a model for the composition of the atom called the ‘raisin bun’ or ‘plum pudding model’ although he was proved to be incorrect

 Materials, when rubbed, can develop a charge difference.
This electricity is called ‘cathode rays’ when passed through an evacuated tube
 These rays have small mass and are negative
 Thomson noted that these negative subatomic particles were a fundamental part of all atoms
 1) Dalton’s ‘Billiard Ball’ model (1800-1900):
 atoms are solid and indivisible

2) Thomson’s ‘Plum Pudding’ Model (1900):
 negative electrons in a positive framework
3) The Rutherford Model (around 1910):
 atoms are mostly empty space
 negative electrons orbit a positive nucleus

 Used radiation to unlock clues to the atom; alpha particle
(α) -positive charge and beta particle(β) -negative charge
 Ernest Rutherford short alpha (α) particles at gold foil
 Most particles passed through; therefore, atoms are mostly empty
 Some positive alpha deflected or bounced back!
 Thus, a ‘nucleus’ is positive and holds most of an atom’s mass

 In 1907 Robert Millikan found that all electrons are identical
 Millikan was able to find the charge of a single electron

 Around 1913 Niels Bohr discovered that electrons orbit the nucleus in ‘shells’
 Electrons can be bumped up to a higher sell if hit by an electron or a photon of light
 It is when electrons fall back down energy level(s) that they release a photon

 There are two types of spectra :
 continuous spectra
 line spectra
 These jumps down from shell to shell account for the line spectra seen in gas discharge tubes
(through spectroscopes)