20 Chemistry
Chapter 4 Notes
Democritus (460 – 370 BC) - the first person to propose that matter was not infinitely
divisible – that matter was made up of tiny building blocks called atomos. His
ideas however stemmed only from a belief and were not substantiated by any
scientific method. Although correct for the most part (and about 2000 years ahead
of his time), he and his ideas were dismissed (largely by Aristotle who was simply
more popular at the time).
John Dalton ( 1766 – 1844) - his theory was very similar to Democritus’ ideas but had
the added benefit of being supported by experiments.
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All matter is comprised of small particles called atoms
Atoms of a given element are identical ***
Atoms cannot be created divided into smaller particles or destroyed ***
Atoms combine in simple whole number ratios to form compounds
In chemical reactions, atoms are separated combined and rearranged
*** - later it was proven that there were exceptions
The Size of the Atom
The Diameter of a typical atom is in the order of magnitude of 10-10 m. The number of
atoms of copper in a penny and the approximate population of earth respectively are:
29,000,000,000,000,000,000,000
6,000,000,000
Try p. 91 # 1 – 5
The Discovery of Subatomic Particles
The Cathode Ray Tube – it was a discovered that by applying a voltage across the two
ends of a low pressure gas (in a tube), particles would travel from the cathode (negative
terminal) to the anode (positive terminal). That the path of these particles could be
altered by a magnetic field lead scientists to believe that these particles were charged.
When the beam of particles was passed between oppositely charged plates, they deflected
toward the positive plate. This lead scientists to conclude that these cathode rays were
negatively charged.
J.J. Thompson (1856 – 1940) – discovered that the mass of this negatively charged
cathode ray particle was far less than the mass of a hydrogen atom (the smallest atom).
This meant that there were particles smaller than atoms and since they came from atoms,
atoms must be divisible into smaller subatomic particles (in this case the electron) and
that at least one aspect of Dalton’s theory was flawed.
Robert Millikan (1868 – 1953) – determined the charge on an electron (the millikan oil
drop experiment) . Since the charge to mass ratio was already known this lead to the
mass of an electron.
9.1 x 10-28 g
The mass of a proton is about 1.67 x 10-24 g (about 1500 times more than that of an
electron.
Early models of the Atom (p. 94 – 95)
Thompson’s plum pudding model – electrons were evenly distributed throughout a
uniform positive charge.
Rutherford’s model (1912) – the results of his gold foil experiments suggested the
positive charges of an atom concentrated in a very small space in the centre of the atom
(the nucleus). The nucleus is very small and since it contains the majority of the mass of
an atom, it is also very dense. How dense? See middle of page 95.
Elements Revisited
The periodic table of elements arranges the over 110 elements (types of atoms). They are
arranged by the number of protons. All atoms that have one proton are called Hydrogen
and are for the most part identical. Similarly all atoms with two protons are called
Helium etc. The number of protons is identified by the “atomic number” and because
atoms are neutrally charged
Atomic # = # of protons = # of electrons
Try p. 99 # 11 – 13
Isotopes
Isotopes are atoms with the same number of protons (and electrons) but different
numbers of neutrons. So in fact, Dalton’s theory although very impressive was wrong
again. Atoms of the same element are not necessarily the same.
Although an element may have different isotopes, the percentages of each isotope in a
sample of an element remains the same from sample to sample.
Mass number = # of neutrons + # of protons
Example of a symbol for Potassium-39
Try p. 101 # 14
39
19
K
What does this mean?
The atomic mass unit (amu) – because the mass of a proton is so small (1.67 x 10-24 g),
chemists created a different unit for mass based on a very common carbon -12 atom.
Carbon-12 has a mass of 12 amu…..and an amu is 1/12th the mass of a carbon-12 atom.
See p.102 table 4 – 2
Try p. 104 # 15 - 17
Radioactive Decay
While individual atoms remain intact in chemical reactions, they are altered in Nuclear
Reactions. Specifically the nucleus of the atom changes and how it changes depends on
the type of radiation that takes place. There are three types of radiation.
Alpha decay: an alpha particle is a helium nucleus. (example p. 106)
A
Z
X  ZA42Y  24He  00
Beta decay: A beta particle is an electron. This electron however comes from the
neucleus. (example p. 107)
A
Z
X  Z A1Y  01  00
and Gamma radiation.
Try p. 107 # 23 – 27
Review
Vocabulary p. 111
Try p. 112 # 28 – 32, 34 – 40, 42 – 53, 55 – 65, 72, 77 - 82