Transcript atomic physics - mrshillphysics

ATOMIC PHYSICS
Early Atomic Theory
1. Democritus 440BC
The smallest indivisible particle of matter
is the atom.
2. Aristotle 340BC
criticised an alternative model based on
the four elements – fire, water, air and
earth.
This model fitted well with religious beliefs
and held for 1600 years.
Alchemy appeared 400 years after Aristotle.
End of the Dark Age
• Galileo (1564 – 1642)
Returned to the Democritus model with a
world made of void and atoms.
• Lavoisier (1743 – 1794)
Law of Conservation of Mass during a
chemical reaction.
“the sum of the mass of the reactants
equals the sum of the masses of the
products.”
Dalton – a revolutionary theory
In 1803 proposed that:
• Elements are made up of tiny particles
called atoms.
• Elements contain only one type of atom.
• Atoms cannot be created or destroyed – a
chemical reaction being the
re-arrangement of elements.
• Compounds contain more than one type of
atom
J.J. Thomson (1856 -1940)
• 1897 measured
charge to mass ratio
of electrons e/m.
• Determined that
cathode rays were
electrons.
1 2
eV  mv
2
2
e v

m 2V
e
e
+
e
-
+
e
+
+
e
+e
+e
e
+ e + e
-
Thomson’s plum-pudding
model (1897)
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
- +
Rutherford’s model
(1909)
1897 J.J. Thomson, a British
1911 New Zealander
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1904 Hantaro Nagaoka, a
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.
Bohr’s model
(1913)
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
Charge-cloud model
(present)
1926 Erwin Schrodinger
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1924 Frenchman Louis
1932 James
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
Thomson Model
Plum-pudding
model
In the nineteenth century, Thomson
described the atom as a ball of positive
charge containing a number of electrons
uniformly distributed.
meanwhile …….
Spectra
1886 Balmer - when an element is ionized in
a discharge tube the light emitted produces
a line spectrum in a spectroscope. pic
Radioactivity
Bequerel and the Curies (1898) identified
elements that produced radioactive rays
and particles.
Rutherford
Scattering Experiment
• Tried to verify Thomson’s model of atom.
Aim: to prove that atoms in the gold foil were
spheres of positive fluid with negative
charges embedded.
Prediction: less than 1% of alpha particles
would be deflected more than 3º.
The Experiment
• To test this he designed and experiment directing ‘alpha’
particles toward a thin metal foil.
– The screen was coated with a substance that produced flashes
when it was hit by an alpha particle.
Experimental Observations
• most alpha particles passed through the
gold foil undeflected.
• large numbers of α were deflected at
angles >3º
• 1/10,000 α were backscattered through
180º
•
pic
Rutherford’s proposed model
• most of the atom is space
• a very small dense and positive nucleus in
the centre of the atom.
• electrons were around nucleus but at a
distance.
Problems of the model
• line spectra could not be explained
• electrons could not be at rest – they would be
attracted to the nucleus and be neutralized.
• if electrons circled the nucleus, they would be
undergoing centripetal acceleration – energy?
• accelerated charges emit electromagnetic
radiation, so electrons should lose energy
continuously and spiral into the nucleus.
• stability of atoms could not be explained
The Atom
Nucleus
• Positive protons and neutral neutrons
each of atomic mass unit 1. Together they
are the nucleons.
• held together by strong nuclear forces
balanced by strong electric forces
• As the number of protons increases the
nuclear forces decreases and the nucleus
becomes more unstable
shorthand
Atomic number Z = number of protons
Mass number A = mass of nucleons
a
z
x
35
17
Cl
12
6
C
Radioactive Isotopes and
radioactivity
Radioisotopes - isotopes that have unstable
nuclei and will spontaneously disintegrate
and emit radiation.
Becquerel 1896 mineral pitchblende
Curie 1898 radium and polonium
Three types or radiation:
1. Alpha particles
2. Beta particles
3. Gamma radiation
Isotopes
• Isotopes are atoms of the same element
with different numbers of neutrons
• C12
C13
C14
• Atoms which give out radioactivity are
called radioactive isotopes.
• The nucleus of a radioactive isotope is
unstable.
Characteristics of types of
radiation
Alpha particles
• Nuclei of helium atoms He =2p+2n
• Heavy particle – low speeds 2x10 7 ms-1
• Strong ionizers – attracting e from other
molecules
• Low penetration in air
• Deflected magnetic by a and electric field
• Emitted by elements>
• \
Half life of Isotopes
• The rate at which a nucleus decays is
characteristic of that nucleus
• Rate is measure by half life t1/2 and is an
exponential decay function
• The time it takes for half the given mass of
an element to decay into a new element.
• It is constant but random process
• eg. at zero time if there is 10g of a
radioactive element
• eg. at zero time if there is 10g of a
radioactive element with a half life of 6hr
then:
after 6 hrs there will be 5g left
12
2.5g
18
1.25g
Uses of Radioactivity
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Carbon-14 dating
Radiotherapy
Sterilisation – medical supplies
Nuclear medicine
Food irradiation
Smoke detectors
Industrial uses
Agriculture applications
ELECTROMAGNETIC SPECTRUM