Transcript Development of Atomic Theory

Development of Atomic Theory
400 B.C. -
1808 -
Democritus was first to use the word : atom
atomos meaning “indivisible”
Aristotle (famous philosopher) disputed atoms;
all matter is made up of: earth, air, water, and fire
Dalton (1st Atomic Theory proposed)
i- all matter is made up of small particles (atoms)
ii- all atoms in an element are identical
iii- atoms in different elements are different
iv- atoms can’t be created/destroyed
v- atoms combine in simple whole number ratios
to make compounds
Dalton’s Model was known as the Billiard Ball Model
since atoms were nondescript spheres.
Points 4 & 5 were proposed from experimental work by:
Lavoisier – Law of Conservation of Mass
Proust – Law of Constant Composition
However, points 2 & 4 were determined to be not entirely
correct because of the existence of isotopes and subatomic
particles, respectively.
1897 – Thomson (Raisin Bun Model)
-negative particles were embedded in sphere of
positive charge
-discovered the electron through the use of cathode
ray tubes or Crookes tube
-only able to measure charge/mass ratio of electron
Millikan subsequently measured charge and mass of
electron from famous Oil Drop Experiment
1909 – Rutherford (Nuclear Model)
Gold foil experiment where a stream of positively
charged alpha particles shot at a micro thin sheet of
gold foil and pathways detected on coated screen.
Observations:
i) most particles (99.99%) went straight through foil
ii) some particles (0.01%) were slightly deflected
iii)a few particles deflected straight back (0.0001%)
Conclusions:
i) most of the atom is empty space
ii) something positively charged to deflect “+” alpha particles
iii)dense, positive core in atom to cause massive deflection
nucleus describes this region of atom; also contains
neutrons discovered by Chadwick in
1932.
Problem: Where are the
electrons?
1914 – Bohr Model (Planetary Model)
- electrons travel around the nucleus in specific
pathways called orbits
- concept of energy levels came from Planck &
Einstein who proposed that energy is quantized
(specific values) in packets called quanta (photons for
light)
- experimental evidence from line spectra of elements
supported this. Energy is released (emission) or
absorbed (absorption) by electrons at certain
wavelengths.
- electrons travel in these orbits without losing energy
(stationary state) but could gain energy and jump into a
higher orbit (excited or transition state) or could lose energy
and fall to a lower orbit, or the lowest orbit (ground state)
- there is a specific number of electrons that can fit into each
energy level or orbit: 2,8,18,32
Problem:
Only explained Hydrogen!
Quantum Atomic Theory / Wave Mechanical Model
(1924)
Following the groundwork of Bohr, DeBroglie (along with
Planck & Einstein) noticed the dual behaviour of electrons as
both a form of energy and as a particle of matter.
E=hn
E = mc2
Heisenberg added to this concept that the position and
velocity of an electron could never be simultaneously
determined (Uncertainty Principle). All of these
concepts/findings led to the development of our current
model of the atom proposed by Schrodinger.
Schrodinger’s model of the atom is just a more in depth
approach to Bohr’s model, but involves mathematically
derived differential equations.
These calculations simply suggest the maximum probability
of finding an electron in a given region of space with a
particular quantity of energy. This region is known as an
orbital.
Orbitals can have different sizes, shapes, orientations and
properties. There are 4 parameters that define the
characteristics of these orbitals and the electrons within.
These parameters can be defined as quantum numbers and
provide the basis of our understanding of chemical bonding.