Post on 19-Jan-2018
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Historical Development of an Atom
Democritus 470-380 B.C
Democritus was a Greek philosopher (470-380 B.C.) who is the father of modern atomic thought.
He proposed that matter could NOT be divided into smaller pieces forever.
He believed that matter was made of small, hard, particles, that he called “atomos”
John Dalton 1807 He built upon Democritus’s atomic
theory. Matter is made up of atoms Atoms cannot be divided into smaller
pieces. All the atoms of an element are
exactly the same Different elements have different
kinds of atoms Compounds form by combining
atoms
John Dalton 1807
Model DrawingSolid throughout Same throughout
J.J. Thomson 1898
Particles smaller than the atom exists
Discovered the electron, which is negatively charged
If atoms have negative particles, then atoms must also have positive particles.
J.J. Thomson 1898
Model Drawing
Positive and negative particles
Ernest Rutherford 1911
Rutherford performed the gold foil experiment.
In his experiment a radio active beam emitted (+) alpha particles toward gold foil.
Most particles went through, some were deflected, and others bounced back!
This showed that atoms have a very dense (+) center.
Ernest Rutherford 1911
Rutherford showed that atoms have (+) particles in the center and are mostly empty space.
He called the center of the atoms the nucleus.
All the mass of an atom and its positive charge are located in the nucleus.
Ernest Rutherford 1911
Model Drawing
Positive nucleusNegative particles surround nucleus
Niels Bohr 1913
Niels Bohr improved on Rutherford’s model.
He proposed that electrons move around the nucleus in specific layers or shells.
Every atom has a specific number of electron shells.
Niels Bohr 1913
Electrons move in orbits around the nucleus
1st energy level = 2 electrons 2nd energy level = 8 electrons 3rd energy level = 18 electrons 4th energy level = 32 electrons
Niels Bohr 1913
Model Drawing
Electrons orbit the positive nucleus on energy levels
James Chadwick 1932
Chadwick discovered neutrons He worked with Rutherford and
discovered particles with no charge He called those particles neutrons Neutrons are found in the nucleus
of an atom.
James Chadwick 1932
Model Drawing
Electrons orbit the positive nucleus which contains protons and neutrons
Electron Cloud ModelCurrent
Electrons do not follow fixed orbits, but tend to occur more frequently in certain areas around the nucleus
it is impossible to know where an electron is at any given time