Download History of Atomic Theory

History of Atomic Theory
5th Century B.C. - Present
Atom
Greek for “uncuttable”
Leucippus – Greek Philosopher
and founder of Atomism
5th Century B.C.
If you divide matter into smaller and smaller
parts eventually you arrive at small particles
which can not be further subdivided. Leucippus
called these indivisible particles atoms. His
atomic theory was further developed by his
disciple, Democritus.
Democritus
Democritus had no instruments to extend the
reach of his senses, so all of his experiments
were just "mind experiments." The ancient
Greeks gave humanity tremendous gifts despite
only having their minds to work with. We
humans tend to be skeptical and want proof
before we believe in most things, so thousands
of years passed before his theories could be
proved.
Democritus – 450 B.C.
• All atoms in a substance are identical
• Atoms of different substances are different
sizes and shapes
• Atoms are in constant motion
• Atoms are indivisible and eternal
John Dalton (Early 1800’s)
• Atoms are indivisible and indestructible
• All atoms of a single substance are identical
• Atoms of different substances differ primarily in
their weights
• In chemical compounds atoms combine in small
specific ratios
Known as the Father
of Atomic Theory
J.J. Thomson (1897)
• Used Cathode Ray tube in his
experiments
• Atoms consist of a sphere of positive
charge
• The electrons are embedded in a “+”
charged sphere (like seeds in a
watermelon)
Thomson’s “Plum Pudding” Atomic Model
Electrons are imbedded into a sphere of positive charge
Ernest Rutherford (1908)
• All atoms have a positively charged central
particle called the nucleus
• Electrons (“-” negatively charged particles)
are scattered around the atoms edge
Gold Foil Experiment
Rutherford’s Model of the Atom
Niels Bohr (1913)
Classical Mechanical Model
• Electrons orbit the nucleus in definite
paths – like planets around the sun
• Shell – the region in an atom where
the electrons travel
• Electrons in the same shell are
approx. the same distance from the
nucleus
Bohr’s Model (continued)
•Energy Levels- electrons in the same shell have
similar energy
•Electrons closest to the nucleus have the least
energy
•8 electrons are needed in the last shell to
stabilize the atom (Happy Atoms)
Bohr’s Model of the Atom
Max Planck
Quantum Mechanical Model
• Energy Levels – his model
predicts the probability of
where an electron may exist
• Energy levels have
sublevels
• Sublevels are the spatial
distribution of electrons in an
atom (describe the cloud
shape)
Sublevels in Electron Energy
Levels
Name of sublevel
s
p
d
f
max. # of e2
6
10
14
orientation
spherical
perpendicular
diagonal
fundamental
Orbitals- the spatial description of the path of an electron
Atomic Orbitals
The regions in space in which electrons are likely to be found
The Neutron
• In 1920 Ernest Rutherford proposed the
possible existence of a neutral particle,
with the approximate mass of a proton,
that could result from the capture of an
electron by a proton
The Neutron (cont.)
• In 1932, Irene Joliot-Curie, one of
Madame Curie’s daughters, and her
husband, Frederic Joliot-Curie, decided to
use a strong polonium alpha source, which
they had developed, to further investigate
penetrating radiation.
The Neutron (cont.)
• James Chadwick repeated the Joliot- Curie
experiments. He bombarded the hydrogen
atoms in paraffin with beryllium along with other
targets. By comparing the energies of recoiling
charged particles from different targets, he
proved that the beryllium emissions contained a
neutral component with a mass approximately
equal to that of the proton. He called it the
neutron in a paper published in the February
17, 1932. For which he received the Nobel
Prize.
James Chadwick’s Apparatus
Quarks (1960-70)
• Believed to be one of the basic building blocks of
matter. Quarks were first discovered in
experiments done in the late 1960's and early
1970's.
• It is believed that nucleons (protons and
neutrons) are made up of quarks.
• Protons – contain two up and one down quark
• Neutrons – contain two down and one up quark
Quarks
• Three families of quarks are known to exist.
Each family contains two quarks. The first family
consists of Up and Down quarks, the quarks that
join together to form protons and neutrons.
• The second family consists of Strange and
Charm quarks and only exist at high energies.
• The third family consists of Top and Bottom
quarks and only exist at very high energies.