Atoms, Atoms Everywhere! The History of Atomic Models

Timeline of the Models* ? 460 BC Democritus “Atoms” 1800 John Dalton “Billiard Ball” 1897 JJ Thompson “Plum Pudding” 1911 Rutherford “Planetary” 1912 Niels Bohr “Bohr Model” Today Many scientists “Modern” *There are many more models. These are the ones we’ll cover in class.

Democritus (c. 460 BC) Democritus asked: If you keep breaking matter in half, how many breaks will you have to make before you can’t break it apart any further? Democritus called the smallest possible bits of matter atoms. (indivisible in Greek) He had no experiments to support his theory. Democritus was supposedly known as ‘the laughing philosopher’ because of his wry amusement at human foibles.

The Ancients – B.C. Believed Aristotle's theory that everything was made up of the fundamental “elements” Earth Wind (air) Fire Water

Aristotle’s Folly Unfortunately Aristotle (the more popular Greek philosopher) dismissed the atomic idea of Democritus as worthless. (What?!) For more than 2000 years nobody did anything to continue Democritus’ work. No surprise, we call these the “Dark Ages” of atomic theory.

John Dalton (c. 1800 AD) English chemist, John Dalton, performed experiments with various chemicals and showed matter seemed to consist of “indivisible” particles (atoms). Though he didn’t know about atoms’ structure, he did know about the Law of Conservation of Matter and based his theory on this. Dalton was an avid weather watcher and discoverer of color blindness among other things.

Dalton’s Ideas “Billiard Ball” or “Marbles” Dalton's model says atoms are tiny, indivisible, indestructible particles Believed each atom had a certain mass, size, and chemical behavior determined by what kind of elements they make up. See next slide for the details….

John Dalton Theory --1800 (a.k.a “the marble guy”) Atoms are the smallest particles of nature-indivisible and indestructible All atoms of the same element are identical Atoms of different kinds can combine to form compounds Chemical reactions are atoms recombining to form new substances

J.J. Thomson (c. 1897) In 1897, English physicist J.J. Thompson discovered the electron and proposed a model for the structure of the atom. Using a CATHODE RAY TUBE, Thomson discovered electrons have a negative charge and thought that the rest of matter must have a positive charge to offset the negative electron.

His Experiment

JJ Thompson’s Model “Plum Pudding” Because the beam of light traveled from to the positive end of the tube he concluded that the light had a negative charge Because the beam could push a paddle wheel he concluded that the particle had mass. Thompson's model says atoms are positively charged spheres with negatively charged electrons randomly located throughout.

Side Trip…Alpha Particles! Around this time scientists also discovered alpha rays (particles), which had a positive charge. Some physicists thought these alpha particles were made up of the positive parts of JJ Thompson’s atom.

Ernest Rutherford (1911) Rutherford as a student worked under J.J. Thompson supervision at the famous Cavendish Laboratories. In 1911 Ernest Rutherford bombarded atoms with alpha rays to investigate the inside of the atom. The results were, to say the least, unexpected!

Gold Foil Experiment Rutherford used Radium as the source of the alpha particles and shot them at a thin gold foil like aluminum foil but made of gold A fluorescent screen sat behind the gold foil on which he could observe the alpha particles’ impact.

When the particles bounced back or were deflected Rutherford reasoned that it hit something massive and positive. This mass became know as the nucleus. When the alpha particles went straight through it hit nothing. This happened most often so the atom is mostly empty space.

Rutherford’s “Planetary Model Rutherford’s model said the negative electrons orbited a positive center (NUCLEUS)like our planets orbit the sun. The nucleus contained most of the mass of the atom And the distance between the positive center (nucleus) and the electrons was huge-like a marble in the center of a football field. The atom was mostly empty space!!!!

One little problem… The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually lose energy and spiral inward toward the nucleus. (BOOM! No more atom.)

Niels Bohr (1912) In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. This was a pretty radical approach, because for the first time rules had to fit the observation regardless of how they conflicted with the theories of the time! (Aristotle would have been furious).

Previous experiments-White light gives off all wavelengths of energy- all colors.

Previous experiments-helium gas only gives off certain colors thus the line emission spectra for helium is produced.

The explanation- An electron absorbs energy it jumps farther away form the nucleus As the electron falls back closer to the nucleus it gives off the energy as colored light.

How Light Relates to Electron Location Bohr observed that only certain colors were given off Therefore the electron could only orbit at certain distances from the nucleus

Bohr’s Rules RULE 1: Electrons can orbit only at certain allowed distances from the nucleus (energy-levels). RULE 2: An atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit. Rule 3: Atoms radiate energy when an electron jumps down from a higher-energy orbit to a lower-energy orbit.

See light excite an atom! Bohr’s Model of the atom. Light can excite electrons around atoms and this gives rise to “quantum levels”. See light excite an atom!

Are We Done Yet? Almost… Cliff Notes version is that Niels Bohr came really close, and when you add the works of Arnold Sommerfeld, Wolfgang Pauli, Louis de Broglie, Erwin Schrödinger, Max Born, and Werner Heisenberg, we arrive at today’s model…

Today’s Model!-Electron Cloud Today's model says electrons are not confined to fixed orbits. They occupy volumes of space outside the nucleus.