Atoms, Atoms Everywhere! The History of Atomic Models

Timeline of the Models* ? 460 BC Democritus “Atoms” 1800 John Dalton “Billiard Ball” 1897 JJ Thomson “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.

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

Democritus (c. 460 BC) Democritus asked: If you keep breaking matter in half, how many breaks can you make before you can’t break it apart any further? Democritus believed that there was a smallest bit of matter which is indivisible. He called the smallest possible bits of matter atoms.

John Dalton (1800 AD) English chemist, John Dalton, developed his atomic model based upon experimentation. Through experimentation, he concluded that atoms of different elements are different. Dalton developed the following Laws: The Law of Constant Composition (aka: the Law of Definite Proportions). The Law of Multiple Proportions ( different compounds may be formed by the same elements in different, but fixed) ratios.

John Dalton’s Model “Billiard Ball” or “Marbles” Believed each atom of different elements were different. Dalton's model represented atoms as tiny, indivisible, indestructible particles or spheres.

J.J. Thomson (1897) In 1897, English physicist J.J. Thomson 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.

The Cathode Ray Tube

JJ Thomson’s Model “Plum Pudding” Because the beam of light traveled to the positive end of the tube he concluded that the cathode beam had a negative charge. Because the beam could push a paddle wheel he concluded that the particle had mass. Thomson'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. Alpha particles are Helium nuclei.

Ernest Rutherford (1911) In 1911 Ernest Rutherford hypothesized that alpha particles shot at atoms should go right through them. He devised the now famous “gold foil experiment” to test his hypothesis.

Gold Foil Experiment Rutherford used Radium as the source of the alpha particles which were emitted toward the atoms in gold foil. A fluorescent screen (detector) circled the gold foil enabling him to observe the result of the alpha particles’ impact. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Rutherford’s Model “Planetary Model” Rutherford was forced to conclude that the alpha particles were reflected by what must be a centrally located mass of positive charge… A nucleus. Rutherford’s new model was that negative electrons orbited a positive center (nucleus) like our planets orbit the sun. He suggested that the distance between the positive center (nucleus) and the electrons was huge-like a marble in the center of a football field.

One little problem… The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually radiate (ultraviolet) energy as they spiraled inward toward the nucleus. This meant that atoms were unstable and could not exist very long. This was known as the Ultra-Violet Catastrophe.

Niels Bohr (1912) In 1912 a Danish physicist, Niels Bohr came up with a theory that would not allow electrons to spiral into the nucleus and came up with some rules for what does happen. His model required that electrons had to orbit in exact (quantized) energy levels, as this would support observed spectral emissions of atoms such as Hydrogen and Helium.

Neils Bohr had some help from Louis deBroglie Neils Bohr had some help from Louis deBroglie . Prince deBroglie suggested that the electron acts as a wave, In so doing, only discrete (quantized) energy levels are allowed! 1924

1924

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

Recall: White light gives off all wavelengths of energy- all colors.

Line emission spectrum for helium gas Line emission spectrum for helium gas. Only certain colors are seen which are characteristic of the electron transitions occurring in the Helium atoms.

Spectral line emission is characteristic of a given element. Bohr’s Model of the atom matched the observed spectrum for atoms of small atomic number, but did not match what was observed for larger atoms. It also did not explain why some spectral lines were observed to be brighter than others.

Energy Levels or “shells” 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.

As excited electrons drop down to lower energy levels, electromagnetic energy is given off (emitted). Sometimes this electromagnetic energy corresponds to the visible spectrum of light.

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.