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Introducing Atomic physics


History of the Atomic Model

Quantum Atomic Mode

Electronic configuration

Atomic Spectra

   

Introduction

Atomic physics: The field of physics studying atoms as an atomic nucleus and a system of electrons.

Atomic physics  meanly deals with the way electrons are arranged around the atomic nucleus and how they change. It further includes an atoms energy states, how it interacts with outside particles and electromagnetic fields. It is one of the most successful application of quantum mechanics.


History of the Atomic Model

While is was perfected during the 20th century, atomic theory actually dates back to the ancient Greek philosophers Leucippus of Miletus and Democritus of Thrace in about 440 BC. Their atomist school of the ancient Greek philosophers proposed the idea that matter is made of vary small indivisible and indestructible atoms. They saw atoms as being in continual motion through a surrounding void, colliding with each other like billiard balls. The void between the atoms resulted in questions that they could not answer causing the idea of the atom to be rejected by Aristotle in favor of a continuous matter idea. The atom was not seen again for 400 years when the Roman poet Lucretius wrote about it.

In 1687 Isaac Newton showed that Boyle’s law; the principle that at a constant temperature the product of a gas’ volume and pressure is constant; fits well with the idea that a gas is made of particles. Over a century later in 1808 John Dalton suggested the atoms of each if the element are identical. This was shortly followed in 1811 by Amedeo Avogadro who suggested that each elements’ particles may be two or more atoms stuck together in what he called molecules. As the 19th century progressed came the theory that there were a finite number of elements, each made of a different type of atom, with nearly limitless ways that these atoms can combine to form chemical compounds. This lead to the kinetic theory of gases that showed that pressure and viscosity result from the the motion of atoms and molecules.

At this time the accepted atomic model was one known as the “plum-pudding” model proposed by Joseph John Thomson. This model has negatively charged electrons embedded in relatively large positively charge mass. To test this idea Ernest Rutherford. suggested what is known as the Gold Foil Experiment. It was actually conducted in 1911 by Hans Geiger and Ernest Marsden.



The Gold Foil Experiment involved shooting alpha particles at a gold foil to observe how they are scattered by the gold on a screen. The experiment probed the structure of the Atom.


If negative electron were embedded in a thin positive mass all would get through.


However most get through with some being scattered by the small positive nucleus.

Rutherford proposed that the atom is mostly empty space.
Consisting of a small positive nucleus surrounded by negative electrons scattered around it.


From Concepts of Modern Physics 4thed
Arthur Beisher, 1987, fair use.

The Rutherford model led to the planetary model of the atom. In this model the electron do not stand still because if an electron that stood still would fall into the nucleus. So this led to modeling the atom as a solar system with electrons orbiting the nucleus. The planetary model of the atom ultimately led to the Quantum Atomic Model and its problems helped lead to the development of Quantum Mechanics.
It is still used to illustrate the atom.


Quantum Atomic Model


From Concepts of Modern Physics 4thed
Arthur Beisher, 1987, fair use.

However the Planetary model had a major flaw in that electrons are charged particles. Since an accelerating charge emits light loosing energy this model is vary unstable  because the electron would quickly spiral into the nucleus. This would happen so quickly that no atoms would be observed because they would collapse in well under a second. This fact led to the Bohr Atomic Model

The Bohr Atomic Mode was  based on the fact that Broglie had shown earlier that particles have wave like propertied. In his model Bohr then showed that these waves could be bent into orbits.

From Concepts of Modern Physics 4thed
Arthur Beisher, 1987, fair use.

From Concepts of Modern
Physics 4thed, Arthur Beisher,
 1987, fair use.

From Concepts of Modern
Physics 4thed, Arthur Beisher,
 1987, fair use.

From Concepts of Modern
Physics 4thed, Arthur Beisher,
 1987, fair use.

In the Bohr Atomic Model Electrons can only occupy those orbits with exactly a whole number of wavelengths. In such orbits the waves are add together with constructive interference.
 

From Concepts of Modern Physics 4thed
Arthur Beisher, 1987, fair use.

In the Bohr Atomic Model a fractional number of wavelengths. Would destructively interfere and not persist as a result an electron can not occupy such an bits.
The result is that that electrons jump from orbit to orbit, emitting photons when they go to a lower orbit and absorbing photons when they go to a higher orbit.

From Concepts of Modern Physics 4thed
Arthur Beisher, 1987, fair use.

It turns out that transitions between the possible electron orbits match the spectral lines that are actually observed in hydrogen.

Studying elements other hydrogen included the effects of multiple electrons resulting in the electron orbitals shown here. The shapes of these orbitals were originally found mathematically and were long viewed as purely mathematical abstractions. The orbitals were never expected to be seen and physics students have long been told this. However field-emission electron microscopy has changed this.

Imaging atomic orbitals

Mathematically generated s orbital image

Field-emission electron microscopy s orbital image http://www.chymist.com/Imaging atomic orbitals.pdf
 

Mathematically generated p orbital image
 
 

Field-emission electron microscopy p orbital image http://www.chymist.com/Imaging atomic orbitals.pdf

The imaging of actual electron orbitals was a surprise and it vividly shows the accuracy of Quantum Atomic Model and of quantum mechanics as a whole.
 

Electron configuration: The distribution of electrons with in an atom or molecule in terms of atomic or molecular orbitals.

Example of electron configurations

Hydrogen

1s1

Helium:

1s2

Carbon

1s2 2s2 2p2

Nitrogen:

1s2 2s2 2p3

Oxygen

1s2 2s2 2p4


Atomic Spectra

Atomic spectra are the emissions and absorption lines of each element an compound. They result from electrons changing atomic or molecular orbitals and are fully explained by the Quantum Atomic Model.


Conclusion

Atomic physics studies atoms dealing meanly with the electrons around the atomic nucleus. This includes how those electrons are arranged and those arrangements change. Our understanding of the atom took over 2,000 years to develop, however that understanding has provided many benefits

 


 

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