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Lecture 3, Fall 2007.
CHAPTER 2. Atoms,
molecules, and ions.
2.1. The Atomic Theory of Matter.
Democritus (~ 400 BC)
thought that matter was
made of atoms. Aristotle
did not.
Democritus' ideas were
ignored for the next 2000
(460-371 BC):
The Atomic Theory of Matter
John Dalton (1766-1844):
•Elements are made of tiny
particles called atoms
• all atoms of one element are
• atoms are not created nor
destroyed in chemical processes
• atoms of different elements
can combine to form compounds
John Dalton
The Atomic Theory of Matter
Dalton’s Atomic Theory explained..
… The Law of Constant Composition
[in one compound, number and kind of atoms is constant]
… The Law of Conservation of Mass
[total mass before reaction = total mass after reaction]
… The Law of Multiple Proportions
[If two elements form more than one compound,
the ratios of the masses of the second element
which combine with a fixed mass of the first element
will be ratios of small whole numbers]
The Atomic Theory of Matter
The Law of Multiple Proportions:
12 g carbon + 16 g oxygen
12 g carbon + 32 g oxygen
32 g : 16 g
HW: 8
2.2. The Discovery of Atomic
• Cathode Rays and Electrons. Discovery
of the electron.
J.J. Thomson (1856-1940):
• discovered the electron
• atoms of all elements
contain electrons
• electron:
negatively charged
particles of very small mass
Cathode Rays and Electrons.
Discovery of the electron.
Cathode ray tube
J.J Thomson's original
Cathode Ray Tube
TV tubes are cathode-ray tubes. The beam of
electrons is accelerated towards the +ve cathode,
which has a hole in it through which the beam
passes and strikes a target.
J. J. Thompson and the electron:
• J. J. Thompson investigated these
beams, and found they were repelled
by –ve charge, attracted by a +ve
charge, and deflected by a magnetic
field. He concluded these particles
were negatively charged electrons. He
was able only to calculate a charge to
mass ratio from his experiments.
J.J. Thomson's Model:
“Plum Pudding Model”
in +ve charge
like pieces
of fruit in a
fruit cake
"smeared-out" positive charge
Determination of mass of the electron:
Millikan’s oil-drop
experiment (1909)
Determination of mass of the electron:
Robert Millikan performed his famous
oil-drop experiment in which he
measured the charge on the electron,
from which its mass could then be
Mass of electron = 9.10 x 10-28 g
Radioactivity and the structure
of the atom.
• In 1896 Henri
discovered that a
Uranium compound
emitted high-energy
radiation, =
Henri Becquerel
Becquerel left
some urananite
wrapped in a
piece of photographic paper,
and found
(right) that it
had been ‘fogged’
by the radioactivity
Becquerel’s original piece of
photographic paper fogged
by uraninite
Rutherford and the discovery of the
Rutherford investigated the
nature of radioactivity.
Three types of radiation were
emitted, α, β, and γ particles.
Experiments showed that
with electrically charged
plates, α rays were deflected
towards the -ve plate,
γ rays were not affected,
and β rays were attracted
towards the +ve plate.
Earnest Rutherford
Rutherford analyzed particles
emitted by radioactive substances:
It turns out that α particles are positive He nuclei,
β particles are –ve electrons, and γ particles
are neutral photons.
Rutherford and the deflection of
alpha particles by gold foil.
Rutherford examined beams of α
particles emitted by radioactive
substances. Nearly all of the beams
passed straight through a sheet of gold
foil a few thousand atoms thick (next
slide). His undergraduate student
Marsden found that a few were
scattered at large angles. From this
Rutherford proposed the nuclear model
of the atom.
Rutherford’s gold foil experiment:
First page of
Rutherford’s paper
in the Philospohical
Magazine (1911)
outlining his ideas
on the structure of
the atom
Rutherford’s gold foil experiment and the
nuclear model of the atom.
Alpha particle is
a helium nucleus
only one in 20,000 alpha
particles is deflected, so atom
Is mostly empty space
all the protons and
neutrons are
concentrated in the nucleus
electrons orbit
the much heavier
go straight
2.3 The Modern View of Atomic
6 electrons orbit
The basic building
blocks of the atom
are the proton,
neutron, and
electron. The charge
on the electron is
-1.602 x 10-19 C
(Coulombs), and on
the proton is +1.602
x 10-19 C. The
neutron is neutral.
the nucleus
Nucleus has
6 protons and
6 neutrons
An atom –
the carbon atom
• Every atom has an equal number of
protons and electrons = atomic number
(Z). Protons and neutrons reside
together in the nucleus. Protons are
held together by the strong nuclear
force, which is much stronger than
electrostatic forces, but acts over only
very small distances.
• ISOTOPES: These have same Z (same
number of protons) but different
numbers of neutrons in the nucleus.
The amu (atomic mass unit).
Grams are too big to be convenient for
describing the masses of nuclei. We
use the amu, which is 1.60554 x 10-24 g.
(This comes from 1/6.022 x 1023 which
is 1/Avogadro’s number).
Charge Mass (amu)
5.486 x 10-4
Atoms are very small. Have diameters between
100 and 500 pm (pm = picometers = 10-12 m).
Can also be expressed as Angstroms (Å), 1-5 Å
(an Å = 10-10 meters).
0.000000001 meters (1 nanometer)
or about 0.00000004 inches