Download History of Chemistry

History of the Atom
From Democritus to Bohr
Democritus of Abdera
• Greek 4th century BC.
Democritus is known
as the "Laughing
Philosopher“ because of his
joyous spirit. He was a big
man (relatively speaking)
and enjoyed life
tremendously. He also was
very widely traveled.
• All matter is composed of
atoms, which are bits of
matter too small to be seen.
These atoms CANNOT be
further split into smaller
portions.
• There is a void, which is
empty space between atoms.
• Atoms are completely solid.
• Atoms are homogeneous, with
no internal structure.
• Atoms are different in ...
» 1) ...their sizes
» 2) ...their shapes.
» 3) ...their weight.
John Dalton
• English physicist
• Early 1800’s
• The atom is a
solid mass.
1) chemical elements are made of atoms
2) the atoms of an element are identical in their masses
3) atoms of different elements have different masses
4) atoms only combine in small, whole number ratios
such as 1:1, 1:2, 2:3 and so on. This is called the
THE LAW OF MULTIPLE PROPORTIONS.
5) ENERGY can neither be created nor destroyed.
Atoms store energy (protons, neutrons, electrons)
6) Elements in a compound will always combine in
fixed ratios.
J.J.Thomson
• English Physicist
• 1856-1940
• Experimented
with
cathode rays
• Pictured the atom
as “Plum
Pudding”
• Discovered that cathode rays were made
of negatively charged particles that had a
mass 1000 times smaller than Hydrogen.
• Called particles corpuscles. (now called
electrons)
• Eluded to the idea that if there are
negative particles in matter, there must
also be positive particles.
• Atomic theory; By the way, this is often
referred to as Thomson's "plum pudding
model," where the pudding represents
the sphere of positive electricity and the
bits of plum scattered in the pudding are
the electrons.
Ernst Rutherford
• Enjoyed studying
science for the sole
purpose of learning
new things.
• 1898-1911 studied
alpha-particles
• Developed the Solar
System approach to
the structure of the
atom.
•
Most of the mass of an
atom was located in the
nucleus.
•
When bombarding gold
foil with alpha
particles, his team
discovered these things;
1. The nucleus is so small that the alpha particles will go right
through.
2. Some will go near gold nuclei and be deflected.
3. Very few will ever hit a nucleus “head-on”.
4. The positive charge of the nucleus repels the alpha particles.
Professor James
Chadwick
• English Physicist
• 1932, confirmed
the existence of
the neutron.
Neils Bohr
• Developed new
more modern
theory of the
atom
Bohr’s Atomic theory
1) There is a nucleus (this was Rutherford's
discovery).
2) The electrons move about the nucleus in
"stationary states" which are stable, that is, NOT
radiating energy.
3) When an electron moves from one state to
another, the energy lost or gained is done so ONLY in
very specific amounts of energy, not just any old
amount.
4) Each line in a spectrum is produced when an
electron moves from one stationary state to another.
Bohr Model of the Atom
Electrons rotate about
the nucleus in
"orbitals," like the
planets rotating
around the
sun. Electrons
normally exist only in
well defined regions,
called quantum
energy levels, around
the atom. These
energy levels
surround the atom
like a "cloud."
Atomic Models