Similar atoms have similar chemical properties which depend mainly on the number of electrons in the outside shell, but also on the size of the nucleus.
Atoms are made of fundamental particles: in simple terms, the nucleus is made up of protons and neutrons, and the electrons are found around an atom in shells.
An ion is an atom or group of atoms which is charged because it has a net deficit or excess of electrons. Ions of an element nearly always have the same charge.
The nucleus of an atom may be thought of as being made of protons and neutrons, although at a certain point in the study of physics, this is seen as too simple.
Atoms have electron shells which can be detected, giving them some reality: much about atoms relates to quantum physics, and is somewhat surreal, as we see it.
The shell structure of the electrons in any given atom is reflected in the successive ionization energy values for that atom, measured as atoms are removed.
Electrons are arranged in a shell structure that influences the chemical properties of the elements, most of the influence coming from the outermost shell.
Chemical elements have atoms that are essentially all the same. Elements occur as isotopes of slightly different mass. Elements generally have several isotopes.
Isotopes are generally considered chemically identical, but some chemical and biochemical processes can separate them or cause one of them to be concentrated.
Johann Balmer took a series of measurements for 'hydrogen lines', as observed in stellar spectra, and found a simple formula linking the values to each other.
Balmer's hydrogen line calculations seemed at first like simple mysticism, but new lines could be predicted, and later they were the key to electron shells.
J. J. Thomson proposed his plum pudding atom model, but soon after it was first suggested, it did not match many observations, so a better model was needed.
Thomson's plum pudding model assumed atoms filled all of the space they existed in, with no spaces, a mix of protons and electrons (neutrons were unknown).
Geiger and Marsden found in 1909 that alpha particles fired at metal foil mostly went through, but 1 in 20,000 bounced back or was deflected by 90 or more.
Rutherford described this result as surprising " . . . it was as if you had fired a 15-inch shell at a piece of tissue-paper and it came back and hit you."
Based on the gold foil and alpha particles experiment, Rutherford proposed an atom with a small massive nucleus of protons surrounded by orbiting electrons.
In 1911, Ernest Rutherford explained the Geiger-Marsden experiment by invoking the nuclear atom, and inferred the nucleus from the alpha scattering result.
By 1911, Rutherford had taken this result, and used it to model an atom where the atom had a diameter about 10,000 times the diameter of the tiny nucleus.
Ernest Rutherford's 1912 model of the atom, which had a positive nucleus with orbiting electrons was both mechanically and electromagnetically unstable.
The Rutherford model of the atom did not fit observations: in particular, circular orbits were simply not possible, but it is still the popular view of an atom.
The simple model of orbiting electrons around an atom fails: a circular orbit involves acceleration, and accelerating charged particles must emit radiation.
In 1913, one year after Rutherford proposed an atom with a positive nucleus and orbiting electrons, Niels Bohr showed how the model could be rendered stable.
In 1915, Arnold Sommerfeld developed a modified Bohr atomic model using elliptical instead of circular orbits to explain relativistic fine structure.
In 1931, Harold Urey discovered deuterium using evaporation concentration techniques and spectroscopy to identify the heavier isotope of hydrogen.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsatomstruc.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.