History of the Atom
Democritus
Dalton
(JJ Thompson)
Rutherford
Bohr
Electron Cloud
Warm Up 2.02
1.
2.
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5.
Draw electron diagrams for the following
elements:
Carbon
Silicon
Titanium
Lithium
According to the 2n2 law, what is the maximum
number of electrons that will fit in the 1,2,3 and
4 electron orbits (respectively) around an
atom?
Warm Up
1. A silicon atom weighs 34 amu’s. How many protons,
neutrons and electrons does this atom contain?
2. A tritium atom ( a form of hydrogen) weighs 3 amu’s.
How many protons, neutrons and electrons does this
atom contain?
3. A uranium atom weighs 238 amu’s. How many
protons, neutrons and electrons does this atom
contain?
4. A thorium atom weighs 232 amu’s. How many protons,
neutrons and electrons does this atom contain?
5. A plutonium atom weighs 244 amu’s. How many
protons, neutrons and electrons does this atom
contain?
Warm Up 090413
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7.
Person’s model that says atoms of the same element are all the
same mass.
Person’s model that says electrons only travel in discrete orbits.
Person’s model that says atoms are made of negative “plums”
stuck inside a “pudding” matrix.
Person’s model that says atoms are mostly empty space, but have
a nucleus that contains neutrons.
Person that performed the “gold foil” test.
Person (or persons) that said that atoms are not dividable.
Person who said that electrons can jump to higher orbits but
eventually fall back and give off energy in the form of light
Democritus (460BC-370BC)
Known as the laughing
philosopher.
Democritus (460BC-370BC)
The theory of Democritus held that
everything is composed of "atoms",
which are physically indivisible; that
between atoms, there lies empty
space; that atoms are indestructible;
have always been, and always will
be, in motion; that there are an infinite
number of atoms, and kinds of atoms,
which differ in shape, and size.
John Dalton (1766-1844)
Dalton’s Model
 (1803) John Dalton
(England) formulated the
modern version of the
atomic theory. In his
model all atoms in a
given chemical element
are exactly alike, while
the atoms of different
elements differ by atomic
weight.

1. Chemical Elements are
made up of atoms.
2. The atoms of an element
are identical in their
masses.
3. Atoms of different
elements have different
masses.
4. Atoms only combine in
small whole number
ratios – 1:1, 1:2, 2:3…
Warm Up
1. List two conclusions that John Dalton
made about atoms!
2. What is the “nickname” given to J.J.
Thompson’s model of the atom? Why do
we use that nickname?
3. What did Rutherford conclude from his
gold foil experiment?
4. What two rules did Neils Bohr say all
atoms followed?
J.J. Thompson and the
Discovery of the Electron
• 1906 Nobel Prize Winner-conduction of
electricity through gases.
• 1856-1940.
• Joseph John.
• First to suggest the existence of subatomic particles.
• Builds the first CRT
The Cathode Ray Tube
J.J. Thompson’s Model
• (1898) J.J. Thompson
(England) discovered the
electron, the component
of the atom with negative
charge.
• His model of the atom
had the negatively
charged electron evenly
distributed throughout a
sphere of positively
charged material.
• This is known as the
“plum pudding” model of
the atom. (electrons are
like raisins in pudding.
Ernest Rutherford and the
Nucleus
Ernest Rutherford (1871-1937)
• Knighted in 1914
• In 1921, while working with Niels Bohr,
Rutherford theorized about the existence
of neutrons, which could somehow
compensate for the repelling effect of the
positive charges of protons by causing an
attractive nuclear force and thus keep the
nuclei from flying apart from the repulsion
between protons
Rutherford’s Model
• Gold Foil and Alpha Particle experiment.
• Rutherford could experimentally identify particles with positive
charge that he called protons.
• Although he could explain the charge of atomic nuclei with the right
number of protons, the mass of the nucleus for large atoms was
always larger than the sum of its protons.
• Therefore he postulated the existence of a neutral particle with a
mass nearly the same as the proton which, when added to the
protons in the nucleus, would give the right mass.
• Rutherford called this hypothetical particle the neutron.
• Later (1930) Rutherford’s colleague James Chadwick was able to
detect the neutron experimentally.
Niels Bohr (1885-1962)
Niels Bohr-Planetary Model
• Born in Denmark.
• Bohr developed his Bohr Model of the
atom with the atomic nucleus at the centre
and electrons in orbit around it, which he
compared to the planets orbiting the Sun.
• Nobel Prize in 1922.
Bohr’s Model
•
•
•
Bohr said, "Here's some rules that
seem impossible, but they
describe the way atoms operate,
so let's pretend they're correct and
use them." Bohr came up with two
rules which agreed with
experiment:
RULE 1: Electrons can orbit only
at certain allowed distances from
the nucleus.
RULE 2: Atoms radiate energy
when an electron jumps from a
higher-energy orbit to a lowerenergy orbit. Also, an atom
absorbs energy when an electron
gets boosted from a low-energy
orbit to a high-energy orbit.
Summary
Our ideas about the nature of atoms have progressed over the last two centuries
(and continue to develop today).
John Dalton introduced a new form of the ancient Greek idea of atoms at the
beginning of the nineteenth century.
In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding'
model of the atom.
In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets
round the Sun.
In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.
We can think of the atom as a positively charged nucleus with negatively charged
electrons orbiting the nucleus in energy levels (or shells).
Reference
http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htm
http://www.nobeliefs.com/atom.htm