Download Atomic Theory - North Bergen School District

Atomic Theory
Atomic Theory Timeline
  The atomic model has changed over time
  For over 2 centuries scientists have created different
models of the atom
  As scientists have learned more and more about
atoms, the atomic model has changed.
Atomic Theory Timeline
  The timeline of major ideas goes like this:
Dalton
Thomson
Rutherford
Bohr
But before all these guys there was Democritus
  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
  This was before scientists could prove things in the
lab
  He claimed that matter was made of small, hard
particles that he called “atomos”
John Dalton 1808
  John Dalton created the first atomic theory
  He was an English school teacher
  Dalton viewed atoms as tiny, solid balls
Dalton’s Theory
Dalton’s atomic theory had 4 parts:
1. Atoms are tiny, invisible particles.
2. Atoms of one element are all the same.
3. Atoms of different elements are different.
4. Compounds form by combining atoms
The Cathode Ray Tube, Crookes 1870
  He used a glass tube with
almost no air in it.
  He put two pieces of metal
(electrodes) inside and connected
the electrodes to a battery
  He put an object in the middle
of the tube
  When the battery was connected the shadow of the object
was projected from the cathode (negative electrode) onto
the anode (positive electrode)
  This showed that something was moving in a straight line
from the cathode to the anode
What were these particles?
  How did the scientists know if these were particles or
light?
J.J. Thomson put a magnet beside the tube and
saw that the stream of particles bent in the
direction of the magnet. Magnets do not bend
light.
Therefore, the beam in the cathode ray tube must be
made up of charged particles of matter that came from
the cathode.
J.J. Thomson 1897
  J.J. Thomson discovered electrons
  He also proposed the existence of a (+) particle
  His atomic model was known as the “raisin bun
model”
  He was the first scientist to show that the atom was
made of even smaller things
Thomson’s Model
  Atoms are made mostly out of (+) charged material,
like dough in a bun.
  The (-) charged electrons are found inside the (+)
dough.
Ernest Rutherford 1911
  Rutherford discovered protons and the nucleus
  He showed that atoms have (+) particles in the
center, and are mostly empty space
  He called these (+) particles protons
  He called the center of atoms the nucleus
Rutherford’s Experiment
Most particles (alpha particles, positive charge) went
through, but oddly, some were deflected
Rutherford’s Experiment
Most alpha particles
went through the gold
foil; the atom is mostly
empty space
Rutherford’s Experiment
  The atom had a very
dense (+) center,
Rutherford called it the
Nucleus.
When the alpha particles
hit the nucleus, they
deflected because
the nucleus was also
positively charged.
What about neutrons?
  Rutherford’s experiments failed to answer one big
question.
If the only other particle in the atom was
the proton then why was the mass of most
atoms at least twice the mass of the protons?
The answer was the neutron. The neutron has the
same mass as a proton but has no charge. It is also in
the 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.
The Electron Cloud Model
  Electrons travel around the nucleus in the electron
cloud
  The electrons are going
to be as close to the
nucleus as possible
because they are attracted
to the positive charge of
the protons.