Because similar atoms have similar properties, we can arrange the elements in a table called the periodic table, and we can see relationships and trends in it.
Some properties of matter show clear trends and patterns, and the periodic table of the elements reflects many of the patterns that may be seen in the elements.
The atoms of the elements, ordered by relative mass show regular patterns, which may be seen in any systematic study of the periodic table of the elements.
Similar elements in the periodic table are usually in the same group: the halogens are a typical group, as are the alkali metals, noble gases and alkali earths.
As the periodic table developed, it became possible to find gaps and predict new elements, which chemists could then seek to find, somewhere in nature.
In 1817, Johann Wolfgang Döbereiner drew attention to the existence of triads of elements, pointing to the oxides of calcium, strontium and barium.
In 1828, Jöns Berzelius was able to provide a table of 28 elements which had been identified by then, but it was not enough to allow any patterns to be seen.
By 1829, Döbereiner noted that there were triads, groups of three elements. Chlorine, bromine and iodine made one, lithium, sodium and potassium made another.
In 1865, John Alexander Reina Newlands proposed his 'law of octaves', which became a further helpful step on the way to the first rows of the periodic table.
In 1871, Dmitri Mendeleev systematically examined the periodic table and by identifying gaps, predicted the existence of gallium, scandium, and germanium.
Dmitri Mendeleev had a total of 63 elements to work on in 1869, enough to have a reasonable chance of detecting any periodic tendencies in the elements.
Dmitri Mendeleev studied atomic weight, specific gravity, volume, valence, specific heat and other properties for each of the elements to find trends.
Mendeleev's ideas differed from the earlier schemes to organize the elements because he got the order right, and because he left room for undiscovered elements.
In each case, Mendeleev pointed out that the atomic weight of the middle member in the triad was close to the arithmetic mean of the other two atomic weights.
Norman Lockyer used a spectral analysis of light coming from the Sun to find helium in the Sun before the element was ever discovered here on Earth.
Ramsay and Rayleigh found argon, and reasoned, if there was one new element to fit into the periodic table, there should be more, one for each row of the table.
In 1906, Charles Barkla found each element had a characteristic X-ray and that the penetration of these X-rays was related to the atomic weight of the element.
In 1914, Henry Moseley had shown that nuclear charge was the real basis for numbering the elements, counting for more than average nuclear mass.
Like Mendeleev, Henry Moseley was able to find gaps in his pattern and from these, predicted three undiscovered elements: technetium, promethium, and rhenium.
Moseley's three predicted elements have since been either discovered (technetium and rhenium) or made (promethium has never been found in nature).
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsperiodic.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.