Transcript Atomic Models: Developing the Structure of the Atom

Atomic Models:
Developing the Structure
of the Atom
The Greek Model
 Around 440 BC, Leucippus originated the
atom concept. He and his pupil, Democritus,
refined and extended it in future years.
 Five major points to their atomic idea.
– All matter is composed of atoms, bits of matter too
small to be seen. These atoms CANNOT be
broken apart
– There is a void (empty space) between atoms
– Atoms are completely solid
– Atoms of different materials are different in size,
shape, and weight
The Dalton Model (1803)
 John Dalton of England introduced the
atomic idea to “Modern” chemistry
 The basic ideas in Dalton's atomic theory:
– Chemical elements are made of atoms
– The atoms of an element are identical
– Atoms of different elements have different
masses
– Atoms only combine in small, whole number
ratios such as 1:1, 1:2, 2:3 and so on.
Dalton’s Atom, 1807
The atom is a solid indivisible sphere
that can undergo chemical reactions
but is unchanged by the reaction.
The Thomson Model,1903
 In 1897, J.J. Thomson discovered
the first subatomic particle, the
electron, using a cathode ray tube.
 First model showing an atom that
is divisible (has smaller parts).
Plum Pudding Model
The Gold Foil Experiment
The nucleus was
discovered.
The Rutherford Model
1. The atom contains a tiny dense center:
“nucleus”
– the nucleus is about 1/10 trillionth the volume of
the atom
2. The nucleus contains almost the entire mass of the atom.
3. The nucleus is positively charged.
4. The electrons move around in the empty
space (the void?) surrounding the nucleus
Rutherford’s Atom
1908
e-
3p+
e-
e-
The Bohr Model (1913)
 Niels Bohr applies
mathematics to Rutherford's
model. This eventually led
to the idea of energy levels
for the electrons.
 Led scientists to the
understanding that valence
electrons, those in the
highest energy level,
determine chemical
reactivity.
Chadwick (1935)
 Hypothesized that the mass of the nucleus
could not be explained by the masses of
protons alone.
 Be foil experiment discovered neutrons,
particles that had mass but no charge.
Subatomic Particles
Particle
Proton
Neutron
Charge
1
0
Mass (kg)
1.67E-27
1.67E-27
Electron
-1
9.11E-31
Location
nucleus
nucleus
moving around the
outside of the nucleus
3 protons
4 neutrons
3 electrons
Lithium
Chemical versus Nuclear Reactions
Chemical
Stable atoms (Not radioactive)
Electrons exchanged between
atoms
Nuclear
Radioactive nuclei = unstable,
which decays
Nucleus ejects/absorbs neutrons,
protons, electrons.
Atoms are conserved (unchanged
but rearranged)
Atoms change into new
elements/isotopes!
Atomic # or
mass # changes
Reactions are affected by
temperature and pressure
Reactions NOT affected by
temperature or pressure.