The Atom:
An Outline of Historical Atomic Theory
I.
The earliest models
A. Greeks
1. approximately 400 BCE the Greeks classified matter into four substances (a.k.a. the four
humours) – earth, air, fire, and water
2. DEMOCRITUS (ca. 460 BCE – ca. 370 BCE)
a. Greek philosopher who first suggested that matter consisted of small indivisible particles
called atoms.
b. believed that matter could be divided into smaller and smaller particles until a basic
particle of matter was reached.
c. performed no experiments to test his ideas, arrived at conclusion by purely thinking.
B. ALCHEMISTS
1. mystics and fakes claiming they could turn cheap metals into gold.
2. dominated the next two thousand years of chemical history
3. performed experiments & recorded observations, but did not openly share their results
C. ROBERT BOYLE (1627 – 1691) – Father of Chemistry
1. first person to perform quantitative physical experiments
2. studied the quantitative relationship between pressure and volume of a gas.
3. defined an element as something that could not be broken down into simpler substances
4. published a book of his findings, The Skeptical Chemist in 1661
II. Fundamental Chemical Laws
A. by the late 1700’s, combustion had been studied extensively; carbon dioxide, nitrogen, hydrogen,
and oxygen had been discovered
B. JOSEPH PRIESTLY (1733 – 1804)
1. English clergyman and scientist
2. discovered oxygen in 1774 by heating mercuric oxide
3. discovered that the gas produced by the fermentation of grain (later identified as carbon
dioxide) could be dissolved in water to make seltzer
C. ANTOINE LAVOISIER (1743 -1794) – Father of Modern Chemistry
1. performed quantitative experiments that showed that combustion involved oxygen (which he
named ‘oxygine’) and not phlogiston
2. discovered that life was supported by a process that involved oxygen and was similar to
combustion
3. formulated the law of conservation of mass
a. performed quantitative experiments and weighed the reactants and products
b. found that mass of reactants always equaled the mass of products
c. concluded that matter cannot be created or destroyed
4. having been associated with collecting taxes for the government, he was guillotined in 1794
D. JOSEPH PROUST (1754 – 1826)
1. French chemist
2. demonstrated the law of definite proportions
a. a given compound always contains exactly the same proportion of elements by mass
for example: carbon dioxide is always 3 parts carbon to 8 parts oxygen, by mass
III. First experimental atomic theory
A. JOHN DALTON (1776 – 1844)
1. English school teacher
2. fascinated with meteorology, which led to an interest in the gases of the air and their ultimate
components – atoms
3. humble man with severe handicaps
a. very poor
b. inarticulate
c. color blind (Daltonism)
B. Dalton’s atomic theory – The Billiard Ball (Rubber Ball) model
1. each element is made up of tiny particles called atoms
2. the atoms of a given element are identical; the atoms of different elements are different in
some fundamental way
3. chemical compounds are formed when atoms combine or rearrange with each other. a given
compound always has the same number and types of atoms
4. chemical reactions involve reorganization of atoms – changes in the way they are bound to each
other; the atoms are not changed in a chemical reaction
C. Law of Multiple Proportions
1. reasoned that a given compound should always contain the same combination of atoms
2. studied carbon and oxygen and found that they formed two different compounds
3. explained this by formulating the Law of Multiple Proportions – When two elements form a
series of compounds, the ratios of the masses of the second element that combine with 1 gram
of the first element can always be reduced to small whole numbers
IV. Early experiments to characterize the atom
A. Electrical investigations
1. MICHAEL FARADAY (1791 – 1867)
a. investigated the electrical nature of matter
b. used electricity to decompose water into its components
c. disagreed with Dalton’s model of the atom – something had to hold atoms together
d. claimed electricity was the “glue” that held atoms together
e. discovered cathode rays
2. J.J. THOMSON (1856 – 1940)
a. further studied cathode rays and found that they were repelled by the negative pole of an
electric field – thus, negatively charged
b. postulated that the ray was a stream of negatively charged particles – called electrons
c. further studied electron (more to come…)
3. SIR WILLIAM CROOKES
a. developed the Crookes’ tube that produced cathode rays
b. noticed that the rays always move from the cathode to the anode
c. found that the rays traveled in straight lines and possessed energy
d. concluded that a stream of light was made up of particles
4. EUGENE GOLDSTEIN (1850 – 1930)
a. through work with Goldstein tubes (modified CRT), was credited with the discovery of the
proton in 1885
b. called them ‘canal rays’
5. WILLIAM RÖNTGEN (1845 – 1923)
a. discovered penetrating rays given off by cathode ray tubes
b. called them x-rays
c. first person to win the Nobel Prize in Physics
6. HENRI BECQUEREL (1852 – 1908)
a. 1896 – found that a piece of mineral containing uranium could produce an image on a
photographic plate in the absence of light
b. attributed this phenomenon to a spontaneous emission of radiation by the uranium atom –
the spontaneous decay of an unstable atom nucleus with the release of x-rays, gamma rays,
alpha particles, and/or beta particles
i. Named radioactivity by Marie Curie
c. awarded the Nobel Prize in Physics in 1903
B. The electron
1. J.J. THOMSON (1856 – 1940)
a. determined the charge to mass ratio of an electron:
q/m = -1.76 x 108 coulombs/gram
b. concluded that all atoms of all substances contain the same kind of negative charge…
∴ all electrons are equal
c.
Thomson’s Model of the Atom – The Plum Pudding Model
i. The atom consisted of positive material with electrons embedded randomly in it.
ii. Called Plum Pudding Model because the electrons were dispersed like raisins in a
pudding (the positive material), as in plum pudding, a popular English desert
d. awarded the Nobel Prize in Physics in 1906 (subsequently, seven of his assistants were
awarded Nobel Prizes)
2. ROBERT MILLIKAN (1868 – 1953)
a. performed the oil-drop experiment – determined the charge of an electron:
-1.76 x 10-19 coulombs
b. combined with Thomson’s ratio, he calculated the mass of an electron:
9.11 x 10-28 grams
c. won the Nobel Prize in Physics in 1923, the first American to do so
3. ERNEST RUTHERFORD (1871 – 1937)
a. 1908 – carried out an experiment to test Thomson’s model of the atom
i. used alpha particles as ‘bullets’ to probe the atom
(alpha particle = small, dense, fast, positive particle – same as helium nucleus,
although Rutherford did not know this…)
ii. shot the alpha particles at a thin sheet of gold foil
b. based on Thomson’s model, Rutherford predicted the alpha particles would be able to pass
right through the foil
c. actual results:
i. most particles passed straight through
ii. some were deflected
iii. 1 out of every ~20,000 were deflected straight back!!!
iv. formulated a new model of the atom
d. Rutherford’s Model of the Atom – Planetary Model (1911)
i. atom contains a very dense, very small center of concentrated positive charge,
which he called the nucleus
ii. atoms are mostly empty space
iii. electrons must be present in the empty space around the nucleus to maintain
neutrality of matter
e. calculated the entire diameter of the atom to be about 100,000 times the diameter of the
nucleus
f. won the Nobel Prize in Chemistry in 1908
C. JAMES CHADWICK
a. discovered the neutron in 1932
b. awarded the Nobel Prize in Physics in 1935