atomic theory timeline 400 b.c. democritus 350 b.c aristotle 0 1743-1794 lavoisier 1800 dalton 1907...
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Atomic Theory Timeline
400 B.C.
Democritus
350 B.CAristotle
0 1743-1794Lavoisier
1800
Dalton
1907
Millikan
1903
Rutherford
1900Thomson
1913
Bohr
Democritus 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
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!
Lavoisier 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’
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
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
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 (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
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
Millikan In 1907 Robert Millikan found that all electrons
are identical
Millikan was able to find the charge of a single electron
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
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)
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