Learning Standards
•
Atomic Structure
Broad Concept:
Atomic models are
used to explain
atoms and help us
understand the
interaction of
elements and
compounds
observed on a
macroscopic scale.,
–Recognize discoveries from Dalton (atomic
theory), Thomson (the electron), Rutherford (the
nucleus), and Bohr (planetary model of atom) and
understand how these discoveries lead to the
modern theory.
–Describe Rutherford’s “gold foil” experiment
that led to the discovery of the nuclear atom.
Identify the major components (protons,
neutrons, and electrons) of the nuclear atom and
explain how they interact.
–Write the electron configurations for the first
twenty elements of the periodic table.
Atomic theory proposed by
John Dalton
All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of
atoms
Chemical reactions occur when atoms are
rearranged
Compounds are formed from atoms of the
Atom Definition
Atom
What is inside the atom?
Discovery of Protons
• Eugene Goldstein noted streams of
positively charged particles in cathode
rays in 1886.
–Particles move in opposite direction
of cathode rays.
–Called “Canal Rays” because they
passed through holes (channels or
canals) drilled through the negative
electrode.
Canal rays must be positive.
Goldstein postulated the
existence of a positive
fundamental particle called
the “proton”.
Thomson’s Experiment And
Discovery of Electrons
-
 Passing
Voltage source
+
an electric current makes a beam
appear to move from the negative to the
positive end.
Thomson’s Experiment
Voltage source
+
 By
adding an electric field he found
that the moving pieces were negative.
The
electron was discovered in
1897 by Thomson. He imagined
the atom as a “raisin pudding” with
electrons stuck in a cake of
positive charge.
J.J. Thomson’s Model of Atom
• Plum Pudding Model,
1896
• Thought an atom was
like plum pudding
– Dough was cloud
– Raisins were electrons
– Didn’t know about
neutrons at this time
Rutherford’s experiment and
discovery of nucleus
• English physicist Ernest Rutherford
(1911)
• Shot alpha particles at fluorescent
screen.
• When an alpha particle hits a fluorescent
screen, it glows.
Lead
block
Uranium
Fluorescent
Screen
Gold Foil
What he expected
He Expected
The alpha particles to pass through without
changing direction very much.
What he got
He thought the mass was
evenly distributed in the atom
The Nuclear Atom
Since some
particles were
deflected at
large angles,
Thomson’s
model could
not be correct.
How Rutherford
explained results…..
• Atom is mostly
empty space.
• Small dense,
positive piece at
center is
(NUCLEUS)
RUTHERFORD’S MODEL OF AN
ATOM

In 1912, Rutherford discovered the
nucleus by doing scattering experiments.
He concluded the atom was mostly empty
space, with a large dense body at the
center, and electrons which orbited the
nucleus like planets orbit the Sun.
The model created by Rutherford had
still some serious discordance.
According to the classic science,
electron moving around the nucleus
should emit an electromagnetic wave.
Electron should than move not by the
circle but helical and finally collide with
the nucleus. But atom is stable.
Rutherford also realized that the
nucleus must contain both neutral and
positively charged particles. The
neutron was then discovered in 1932
by Chadwick.
Bohr’s Model of the Atom
• Similar to
Rutherford’s model
• Thought atom was
mostly empty space
• Neils Bohr, 1913
– Nucleus in center
is dense,
positively charge
– Electrons revolve
around the
nucleus.
Following Rutherford’s
planetary model of the atom, it
was realized that the attraction
between the electrons and the
protons should make the atom
unstable
Bohr proposed a model in
which the electrons would
stably occupy fixed orbits, as
long as these orbits had special
quantized locations
Parts of an Atom
Each element has a different number of protons
in its nucleus
Protons have positive charge
Change the number of protons  change
elements
This is called nuclear physics
The element also has the same number of
electrons
Electrons have negative charge
Change the number of electrons  ionize
the element
This is called chemistry
Some elements also have neutrons
Neutrons have no charge
They are in the nuclei of atoms
p
e
n
Subatomic particles
Name Symbol Charge
Actual
mass (g)
Electron
e-
9.11 x 10-28
Proton
p+
1.67 x 10-24
Neutron
n0
1.67 x 10-24
Bohr’s model
• Electrons move around the nucleus at
stable orbits without emitting radiation.
• Electron in one of these stable orbit has
a definite energy.
• Energy is radiated only when electrons
make transitions from high energy orbit
to a low energy orbit.
In the Bohr model, the electron can
change orbits, accompanied by the
absorption or emission of a photon
of a specific color of light.
Bohr’s Model
1913 – Atomic Model with Fixed
Orbits proposed –
The Atom has No
Charge
Number of Protons =
Number of Electrons
An easy way to calculate the total number of electrons
that can be held by a given energy level is to use the
formula 2*n2 ,
Principle energy
level ( n )
Maximum number
of electrons 2*n2
1
2
2
8
3
18
4
32
5
50
6
72
7
98
The most electrons
possible in the first shell
are 2. After the first shell
is filled, the second shell
starts filling up,
according to the number
of positive charges in the
nucleus. The most
allowed in the second
shell is 8 electrons. Then
the third shell starts to
fill.
Study Buddy Review
• Name three
subatomic particles.
• Who is credited with
discovering each
particle?
• Describe the
Rutherford Gold Foil
Experiment.