Atomic Physics: An Introduction and Overview

The purpose of this writeup is to give a general understanding of atomic physics without too much numbers or confusing detail. The atomic model used here is Ernest Rutherford's.

According to Rutherford's atomic model, the atom can be thought of as a miniature solar system. The nucleus is orbited by electrons, as the sun is orbited by the planets. However, the nucleus has an internal structure as well:

The Nucleus

The nucleus itself is composed of nucleons, which are two different kinds of particles: protons and neutrons. Protons have a positive charge (exactly the inverse of an electron - 1.6*10^-19 As), while neutrons are without charge, hence the name. Protons and neutrons are almost two thousand times heavier than electrons, which still doesn't mean much - about 1.7*10^-27 kg.

Now, if you think about it, the nucleus should not be holding together. It's densely packed with positively charged protons, all repelling each other. The reason that does not happen is that the nucleus is held together by a force far more powerful than the electromagnetic force: the so-called strong nuclear force. It acts on all nucleons and holds the nucleus together, since all nucleons are attracted to each other by this force. This is not apparent to us because the strong nuclear force only works over very limited distances.

Atomic Number, Mass Number, Ions and Isotopes

In normal atoms, the number of protons equals the number of electrons. Thus, the positive charge of the protons and the negative charge of the electrons cancel out each other, making atoms electromagnetically neutral. Atoms in which that is not the case are called ions. Depending on whether there are too little or too many electrons in the atom, the ion can have positive or negative charge. Ions with positive charge are called cations, those with negative charge are called anions.

The name of an atom is usually determined by the number of protons (electrons). Thus, atoms with one proton are called hydrogen, these with six protons carbon, and the ones with 92 uranium.

A type of atom, an element, is identified by two numbers - the atomic number and the mass number. The atomic number is the number of protons, while the mass number is the total number of nucleons (protons + electrons).

Both numbers can be found on the periodic table of the elements, next to the short form of the name of the element (for example: Hydrogen, Helium, Uranium).

Actually, it's a bit more complicated than that, because atoms with the same number of protons can have different numbers of neutrons. These different "flavors" of one atom are called isotopes. Hydrogen, for example, has an isotope with no neutrons, one with one neutron, called deuterium, and another with two neutrons, called tritium. That is why the bigger number in the periodic table of the elements, the atomic weight, is often not a whole number. Because the atomic weight of an element is the average of the mass number of the different isotopes of the element. Hydrogen, for example, has an atomic weight of 1.00794. The additional 0.00794 is caused by minute quantities of deuterium in normal hydrogen.

The distinction of different isotopes does not matter to chemistry. The interactions between atoms depend only on the number of electrons. Deuterium is chemically identical to normal hydrogen, though it is twice as heavy, on account of having twice the nucleons

How to Write Them Down

The proper way to write down a specific element is:

atomic number 
              SHORT ELEMENT NAME
  mass number 

2 |   |  __
  |---| /__\
4 |   | \__


Since it's a bit difficult to write, this can also be written as 2-He-4, or only He-4, since the 2 is redundant. (Because 2 means Helium)

Places to go on - more information on nuclear physics:

Rutherford model of the atom
Bohr model of the atom
The Nuclear Physics Project
The Sub-Atomic Physics Project

Nuclear Physics
Atomic Physics
atom, molecule, nucleus, proton, neutron, electron
Louis de Broglie
Werner Heisenberg
Niels Bohr

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