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.
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.