Atomic Theory

History of the Atom

What the World around us is made from is a question that has been asked since the beginning of recorded history. Over this time, many different ideas have been formulated to explain what could be observed.

Through history there have been three main explanations of matter :-

1. Classical explanation

2. Chemical explanation

3. Modern atomic explanation

Classical Element Theory

One of the first theories used to explain what Matter was was devised by the ancient Greek Philosopher Empedocles, who lived between 492 - 432 BCE.

It was Empedocles who first used scientific ( for his day ) processes to "prove" that all matter in the World around him consisted of four elements - Earth, Water, Air and Fire.

Empedocles thought that all materials could be described as being made of different quantities of the the four elements, for example :-

This classical view of matter continued as the main explanation of nature for nearly 2000 years, until the alchemists of the 17th century began to change this in their quest for eternal life...

Chemical Element Theory

For the past 2000 years, the Greek 4 (or 5) element model was the accepted understanding of Matter, but as the 1600's progressed alchemists such as Robert Boyle, who were studying chemical properties in an attempt to turn base metals into Gold, began to realise that this model was far too simple...

The video below shows an in depth look into Alchemy and Robert Boyle.

It was in his 1661 publication "The Skeptical Chymist" that Robert Boyle abandons the Greek 4 elements and puts forward his own theory of matter - Corpuscularianism. Boyle's theory stated that all Matter consists of atoms and clusters of atoms in motion, and that all reactions were due to collisions of these atoms. Due to this theory, Boyle is regarded as the first modern Chemist.

Boyle's work on these atoms was furthered by John Dalton in the early 1800's. Dalton investigated the proportion of different elements within several gases, concluding that if a pure sample of the gas or fluid was obtained, then every time the same proportion of elements would be found within it. He theorised that this was due to all particles of an element being identical in size, mass and other properties, with each element differing in these quantities from each other.

Key to Dalton's understanding was the idea of the indivisibility of atoms. His view was that all Chemistry could do is join or break connections between atoms, that it was impossible to create or destroy these most basic building blocks of Matter.

"Chemical analysis and synthesis go no farther than to the separation of particles one from another, and to their reunion. No new creation or destruction of matter is within the reach of Chemical agency. We might as well attempt to introduce a new planet into the Solar System, or to annihilate one already in existence, as to create or destroy a particle of Hydrogen. All the change we can produce, consist in separating particles that are in a state of cohesion or combination, and joining those that were previously at a distance." - John Dalton ( A new system of Chemical Philosophy 1808 )

It would take the discovery of the Electron and other subatomic particles nearly 100 years later, to prove this idea wrong...

Modern Atomic Explanation

In 1896, J J Thomson proved that recently discovered Cathode Rays were in fact an unknown negatively charged particle. In his further research, he showed that these unknown particles had masses very much smaller than those of an atom, and therefore, the atom was not the smallest building block of Matter.

Thomson thought that the atom was made up of these negatively charge particles spread throughout a positively charged region. This positively charged region was required to explain why atoms could be made of negatively charged particles and yet be electrically neutral.

This model of the Atom was referred to as the Plum Pudding Model, as the way plums were spread within the Pudding seemed a good analogy of the structure of the atom :-

This Model was disproved in 1909 by one of Thomson's students, Ernest Rutherford. In a experiment performed with Hans Geiger and Ernest Marsden, Rutherford disproved the Plum Pudding Model by showing that almost the entire mass of the atom was located in a dense region at the center - the nucleus. They performed this by bombarding thin gold foil with positively charged Alpha particles. If the Thompson Model was correct, all the Alpha particles would pass through with little deviation, which turned out not to be correct.

The diagram below shows the expected results of the experiment, for each of the two models :-

Within the experiment most of the Alpha particles did pass through with little deviation, however a small fraction actually left the foil at angles consistent with a collision of a small, dense region of positive charge. This dense region is now known as the nucleus of the atom.

Bohr Model of the Atom

With the Rutherford model of the atom, almost all experimental results relating to the structure of the atom could be explained theoretically, except 1 - Line Emission Spectra.

As can be seen in the above image, it was known at the very beginning of the 20th century that light given off by each element, when observed through a grating, gave a unique signature pattern - a Line Spectrum. However, the reason behind this was unknown. Neils Bohr, a Danish scientist working with Rutherford realised that the new structure of the atom could be used to explain this phenomenon. The tweaks he made to the model led to the creation of the Bohr model of the Atom :-

Within his model, Electrons could move between energy levels, by either emitting or absorbing a Photon of equal energy. It was these jumps between energy levels that gave rise to the Emission Spectra he observed.

In the Bohr Model of the atom, Bohr "broke" several rules of Matter. One of the large issues with the Rutherford model of the atom was that in a classical understanding of rotating charges ( such as the electron orbiting the nucleus ), the rotating charge should emit radiation, losing energy. Obviously, this does not happen as the electrons follow stable orbits, and Bohr created a theory as to why. Bohr decided that the standard rules of E-M Radiation did not apply to the atomic scale as long as the electrons remain in a series of fixed energy values. The assumptions he made here form the very beginning of a Quantum understanding of Matter.

He also took this understanding to a more mathematical level. Bohr combined his understanding of Plank's Constant and angular momentum together, as both had identical units. Bohr's theory stated that an electron could enter a stable orbit if that orbit had an angular momentum equal to h/.

Where :-

h = Planck's Constant 6.63 x10-34 m2 kg s-1

n = An integer number ( 1,2,3... )

m = Mass of Electron ( kg )

v = Tangential Speed of the Electron ( ms-1 )

r = Radius of orbit ( m )

Example 1-

Calculate the radius of the first Bohr orbit of a Hydrogen atom if the Electron has a tangential velocity of 2.2 x106 ms-1.

h = 6.63 x10-34 m2 kg s-1

n = 1

m = 9.11 x10-31 kg

v = 2.2 x106 ms-1

r = ?

( 9.11 x10-31 ) x ( 2.2 x106 ) x r = ( 1 x 6.63 x10-34 ) / ( 2π )

r = 5.3 x10-11 m

The video below shows a summary of the history of Atomic Theory.