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.