Damon Wang
July 22nd, 2020
HISTORY OF THE ATOM
460 BC
Democritus develops the idea of atoms
He pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
ATOMA
(greek for indivisible)
HISTORY OF THE ATOM
1808
John Dalton
All matter was made up of tiny spheres
that were able to bounce around with
perfect elasticity and called them
ATOMS
Dalton’s Atomic Theory
1. All matter consists of tiny particles, atoms.
2. Atoms of one element can neither be subdivided nor
changed into atoms of any other element.
3. Atoms can neither be created nor destroyed.
4. All atoms of the same element are identical in mass, size,
and other properties.
...
HISTORY OF THE ATOM
1898
Joseph John Thompson
Found that atoms could sometimes eject
a far smaller negative charged particle
which he called an
ELECTRON
HISTORY OF THE ATOM
1904
Atom was made up of electrons scattered unevenly within an
elastic sphere surrounded by a soup of positive charge to
balance the electron's charge like plums surrounded by pudding.
PLUM PUDDING
MODEL
HISTORY OF THE ATOM
1910
Ernest Rutherford
Fired Helium nuclei at a piece of gold
foil which was only a few atoms thick.
Although most of them passed through.
About 1 in 10,000 hits
Rutherford’s Apparatus
Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.
beam of alpha particles
radioactive
substance
circular ZnS - coated
fluorescent screen
gold foil
HISTORY OF THE ATOM
New evidence allowed him to propose a more detailed
model with a central nucleus.
Positive charge was all in a central nucleus. With this
holding the electrons in place by electrical attraction
However, this was not the end of the story.
HISTORY OF THE ATOM
1913
Niels Bohr
Studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's that the electrons
were in orbits.
Rather like planets orbiting the sun, each
orbit is only able to contain a set number of
electrons.
Bohr’s Atom
electrons in orbits
nucleus
Models of the Atom
Greek
Dalton’s
model
model
(400 B.C.)
(1803)
Thomson’s plum-pudding
model (1897)
Bohr’s model
(1913)
Rutherford’s model
(1909)
Charge-cloud model
(present)
Structure of the Atom
There are two regions
The nucleus
• With protons and neutrons
– Positive charge(protons)
– Almost all the mass
Electron cloud
– Most of the volume of an atom
– The region where the electron can be found
Size of an atom
• Atoms are incredibly tiny.
• Measured in picometers (10-12 meters)
– H atom, 32 pm radius
• Nucleus tiny compared to atom
– Radius near 10-15 m.
– Density near 1014 g/cm3
• If the atom was the size of a stadium, the nucleus would
be the size of a marble.
HELIUM ATOM
Shell
proton
+
electron
N
N
+
-
neutron
Subatomic particles
Name
Symbol
Relative
Charge mass
Actual
mass (g)
Electron
e-
-1
1/1840
9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
no
0
1
1.67 x 10-24
Symbols
Contain the symbol of the element, the mass
number and the atomic number
# protons
+ # neutrons
mass number
# protons
Mass
number
Atomic
number
X
Symbols
• Find the
– number of protons = 9 +
– number of neutrons = 10
– number of electrons = 9
– Atomic number = 9
– Mass number = 19
19
9
F
SUMMARY
1. The Atomic Number of an atom = number of
protons in the nucleus.
2. The Atomic Mass of an atom = number of
Protons + Neutrons in the nucleus.
3. The number of Protons = Number of Electrons.
4. Electrons orbit the nucleus in shells.