1. ATOMIC PHYSICS

2. Early Atomic Theory Democritus 440BC
The smallest indivisible particle of matter is the atom.
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.

3. 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.”

4. 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

5. J.J. Thomson ( )
1897 measured charge to mass ratio of electrons e/m.
Determined that cathode rays were electrons.

6. - Thomson’s plum-pudding model (1897) Rutherford’s model (1909)+ + -
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
Bohr’s model
(1913)
Charge-cloud model
(present)
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1897 J.J. Thomson, a British
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
1911 New Zealander
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1926 Erwin Schrodinger
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
1924 Frenchman Louis
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
1932 James
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
1904 Hantaro Nagaoka, a
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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

12. 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.

13. 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

14. 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

15. shorthand
Atomic number Z = number of protons Mass number A = mass of nucleons

16. 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:
Alpha particles
Beta particles
3. Gamma radiation

17. Isotopes
Isotopes are atoms of the same element with different numbers of neutrons
C C13 C14
Atoms which give out radioactivity are called radioactive isotopes.
The nucleus of a radioactive isotope is unstable.

18. Characteristics of types of radiation

19. 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> \

21. 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

22. 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 g g

24. Uses of Radioactivity Carbon-14 dating Radiotherapy
Sterilisation – medical supplies
Nuclear medicine
Food irradiation
Smoke detectors
Industrial uses
Agriculture applications

25. ELECTROMAGNETIC SPECTRUM