Transcript 1.3

1.3 Early Models of the Atom
pp. 13 - 15
Ancient Models of the Atom
• Democritus: he stated that there must be a smallest particle,
which he called an atom.
• Empedocles: proposed that all matter is composed of four
essential substances: earth, air, water, and fire.
Dalton’s Atomic Theory:
•Dalton (1808): he suggested that atoms are solid spheres like
billiard balls.
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all matter is made up of tiny, indivisible particles called atoms
all atoms of an element are identical
atoms of different elements have different properties
atoms are rearranged to form new substances in chemical
reactions, but they are never created or destroyed (Law of
conservation of mass)
•Problem: Dalton’s theory did not address how things acquire
electrical charge.
Thomson’s Atomic Theory
J. J. Thomsom (1897) gave a new model of the atom which
included electrons spaced evenly in a positively charged sphere:
the “plum pudding model.” The fruit represents the electrons
Thomson’s plum pudding model of
J. J. Thomson used a device
known as a cathode ray tube to the atom could be called the
blueberry muffin model.
perform his experiments
Ernest Rutherford 1909: discovery of the nucleus
•Rutherford tested Thomson’s theory via his famous gold foil
experiment.
•He concluded that an atom contains a positively charged nucleus
(composed of protons) surrounded by mostly empty space. Some
of the empty space is occupied by electrons.
James Chadwick, 1932:
• His experiments confirmed that nuclei contain neutral particles as
well as protons. They were called neutrons.
• A neutron is similar in mass to a proton but has no charge; it is
neither positive nor negative.
• The term isotope is used to distinguish between atoms that
contain the same number of protons (same element) but different
numbers of neutrons.
• e.g. chlorine-35 vs. chlorine-37
Niels Bohr (1962):
•Rutherford’s model could not explain why electrons do not collapse
into the nucleus since they are attracted to the protons.
•Bohr proposed an atomic model that does explain this behaviour:
•The phenomenon that led to this model will be discussed in section
1.4.
Homework:
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