modern+atom


 * The Modern Nucleus: Protons**

The structure of the atomic nucleus was being studied by Rutherford, as well as a handful of other scientists.

It was first proposed in 1817 by William Prout that all atoms were made of hydrogen atoms. He used the term "protyles" to explain these simple particles.

In 1917, Rutherford carried out the first nuclear //transmutation//, where he bombarded nitrogen with alpha particles, and produced oxygen and hydrogen.



Here, the transmutation is visible on a film:



Rutherford named this hydrogen particle the "proton", partly after Prout's name and partly after the Greek word //proto//, meaning 'first', since hydrogen is the first element on the periodic table.


 * The Modern Nucleus: Neutrons**

Rutherford realized that a nitrogen atom had an atomic mass of 14 amu, based on experiments using a mass spectrometer (see J. J. Thomson's work). However, he was also aware that the nitrogen atom only contained 7 protons.

This missing mass was hypothesized to be from a neutral sub-atomic particle having a similar mass to the proton. This particle was thus named the //neutron//.

In 1932, James Chadwick verified the existence of the neutron.

Nuclear Fission
Large, heavy atoms were thus known to contain many neutrons. If these nuclei were able to be broken apart (fissioned), there could be a large amount of energy released.

It was in Germany in the 1930's where scientists Lise Meitner and Otto Hahn first carried out a controllable fission of a uranium nucleus.



Controllable fission was first carried out at the University of Chicago by Enrico Fermi in 1942.



The protons and neutrons in a nucleus are bound together tightly by what is aptly named the nuclear force. When an atomic nucleus is split in a fission reaction, this nuclear force is released as energy - and lots of energy - as demonstrated by a nuclear explosion.

The nucleus is also extremely dense. On average, an atomic nucleus has a density in the range of 400 trillion grams per cubic centimeter (400,000,000,000,000 g/mL). A teaspoon of atomic nuclei would weigh 2.2 billion tons!

In comparison, lead, which is a relatively dense metal - has a density of 11.34 g/mL. That's one 350 trillionth of the density of its nucleus. This implies that the volume of an atom as a whole is mostly occupied by electrons.



Powerful microscopy has allowed a glimpse into the atomic scale, but the nucleus is far beyond the magnification capabilities of these devices.



Exploring (or Exploding) the Nucleus
The development of fission led to the idea that it could be used as a weapon. This was a big concern for scientists who were watching the world dissolve into chaos at the onset of World War II. A group of physicists, led by Einstein, wrote a letter to the US President Roosevelt, warning him of the dawning nuclear age:



Roosevelt immediately started the Manhattan Project, and eventually the US became the first nation to ever use nuclear weapons in time of war. This set of a nuclear arms race that even today has ramifications in the political and military world.

Atoms for Peace
In the 1960's, it was hypothesized that protons, neutrons, and electrons were composed of even smaller particles. The proton was the first to be explored, when in 1968 the Stanford Linear Accelerator smashed them together and detected smaller particles. media type="custom" key="25060704" align="center"


 * Modern Electron Arrangement**

In the early 1900's, quantum theory was introduced by Max Planck and Albert Einstein to explain the physics of subatomic particles where traditional Newtonian physics failed. The idea was that energy came in "packets", or quantities, and particles could quantum leap from one energy sate to the next.

Niels Bohr developed his planetary atomic model based on the quantum properties of electrons in the hydrogen atom. Electrons were confined to stationary orbits about the nucleus, but were able to quantum leap from one orbit to another.

The planetary model had issues, however. It was only successful in predicting the energies and positions of a single-electron atom such as hydrogen. The model failed to predict the emission spectrum seen with multi-electron atoms.

In the 1920's, the model of the atom was revised yet again. The work of two physicists - Louis de Broglie and Edwin Schrodinger - paved the way for the model of the atom we still use today. The focus of de Broglie and Schrodinger's research was electron arrangement. The most important chemical and physical properties of the elements arise from their electronic arrangements, so this is where the main focus of the course will go from here.

The difficulty in determining the electronic structure of an atom stems from the Heisenberg Uncertainty Principle. This principle states that the more you know about an electron's position, the less you know about its momentum, and vice versa.

To determine electronic structure, it is important to know where an electron is and how fast it is traveling. But because electrons do not emit any known energy on their own, we have to supply energy to perform a measurement. Any change in energy results in a change in position or momentum for an electron. Their elusiveness has led to attempts to mathematically predict electron position and speed.

This is where de Broglie and Schrodinger come in.