1. Atomic Theory Timeline
400 B.C.
350 B.C
1800
1900
Democritus
Aristotle
Lavoisier
Dalton
Thomson
1903
1907
1913
Rutherford
Millikan
Bohr

2. Democritus atoms are particles in space (the void)
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

3. Aristotle
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!

4. Lavoisier
During his lifetime, , 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’

5. Dalton
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

6. Thomson
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

7. Adding Electrons to the Model
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 ( ):
atoms are solid and indivisible

8. 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

9. Rutherford
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

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

11. Bohr
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

12. 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)