ATOMISM
5th century BC, Ancient Greece – Leucippus, Democritus
Both from the Ionian school of naturalistic philosophy
The earliest proponents of the concept of atomism
“All matter is composed of indivisible particles called atoms.”
Proposed the earliest views on the shapes and connectivity of atoms.
“They reasoned that the solidness of the material corresponded to the
shape of the atoms involved. Thus, iron atoms are solid and strong with
hooks that lock them into a solid; water atoms are smooth and slippery; salt
atoms, because of their taste, are sharp and pointed; and air atoms are
light and whirling, pervading all other materials.”
BILLIARD BALL MODEL – “CHEMICAL ATOMISM”
~1800, Great Britain – John Dalton
Elements are made of tiny particles (small solid spheres)
called atoms.
All atoms of a given element are identical.
Compounds are composed of atoms in specific ratios.
Chemical reactions are rearrangements of atoms
(mass is conserved).
DISCOVERY OF CATHODE RAYS
~1860, Germany
A heated cathode of an electric circuit in a vacuum with a large
potential difference applied between the electrodes emitted a
strange beam that was thought to be an electromagnetic wave.
DISCOVERY OF THE ELECTRON
~1900, Great Britain – J. J. Thompson
Investigated cathode rays.
Can be deflected in their path
by magnetic and electric fields
→ carry a charge.
Concluded that these rays were
actually particles, later called
electrons.
Able to measure their charge-to-mass ratio .e
e
m
Q
THOMSON’S PLUMB PUDDING MODEL
The atoms consist of a number of negatively electrified corpuscles
enclosed in a sphere of uniform positive electrification.
Atoms are neutral → the total neg. charge = the total pos. charge.
~1910, USA – R. A. Millikan
Charged droplets of oil between two metal electrodes
State of mechanical equilibrium between the downward
gravitational force and the upward electric force
The charges were all multiples of a fundamental value,
calculated to be 1.602×10−19 C → charge is quantized.
MILLIKAN’S OIL DROP EXPERIMENT
RUTHERFORD’S GOLD FOIL EXPERIMENT
~1910, New Zealand/Great Britain – E. Rutherford Beam of alpha particles (helium atoms without electrons)
was directed onto a sheet of very thin gold foil.
Expected Results Observed Results
Thomson’s Model Rutherford’s Model
Conclusion: The atom is mostly empty space with a small, dense,
positively charged nucleus surrounded by negative electrons.
DISCOVERY OF THE PROTON
~1920, E. Rutherford
Alpha particles were shot into nitrogen gas → scintillation detectors
showed the signatures of hydrogen nuclei.
This hydrogen could only have come from the nitrogen → nitrogen
must contain hydrogen nuclei.
о The hydrogen nucleus is
present in other nuclei as
an elementary particle
named proton.
(Greek πρτον = first)
DISCOVERY OF THE NEUTRON
~1930, Great Britain – James Chadwick
Bombarded beryllium with alpha particles.
Radiation emitted by beryllium stroke into paraffin wax.
Protons shot out from the paraffin wax.
What was in the “beryllium radiation”?
The radiation not affected by electric or magnetic field → neutral.
Photons would be too “weak” to shoot out protons.
These new neutral particles have approximately the mass of protons.
Named neutrons.
DISCOVERY OF THE NEUTRON
ATOMIC NUCLEUS – SUMMARY
a very dense region at the center of an atom
consisting of nucleons: protons and neutrons
accounts for almost all the mass of an atom
~10-15 m in diameter
The number of protons
in an atomic nucleus
identical to the charge
number of the nucleus
uniquely identifies a chemical
element
in a neutral atom, equal to
the number of electrons
The number of nucleons
in an atomic nucleus
different for each different
isotope of a chemical element
e.g., common has 6 n0
while radioactive has 8 n0
Atomic (Proton) Number Z
Mass (Nucleon) Number A
C126
C146
XAZ
Chemical Element
Is composed of atoms with the same Z
Naturally a mixture of different isotopes
Nuclide
Is composed of atoms with the same Z and A
ELEMENTARY (?) PARTICLES
Electron is not known to be made up of smaller particles
Mass kg, charge C
BUT
Protons and neutrons ARE known to be made up of QUARKS
(though these do not exist separated or isolated)
Mass , charge C
31101.9 em19106.1 eQ
enp mmm 1800 19106.1 pQ
QUESTIONS
1) What is the charge of an electron?
2) What did Millikan find out in his experiment?
3) How was an atomic nucleus discovered?
4) What is the charge of a proton?
5) What is the charge of a neutron?
6) What is the difference between a chemical element and a nuclide?
7) What particles are atoms composed of?
8) What do you know about quarks?
UCHeH 23592
146
242
31 ,,,