The Development Of The Periodic Table

Swedish chemist Jöns Jakob Berzelius replaced geometric patterns that were used as chemical symbols with letters that were an abbreviation of the name of the element. He chose to use English names for most elements, though a few retained their Latin names. He also used the weight of hydrogen to develop a coherent system of atomic weights. Hydrogen was given a value of one as it was the lightest element and all remaining elements were believed to have a whole number above one for their value. Berzelius also combined all the knowledge of his time into a single system which enabled chemists across the world to share their thoughts and systems in a unified approach.

German chemist, Johann Dobereiner, was aware of 40 elements. He noted that some groups of three elements had similar properties and he named the groups triads. Three of the triads were:
• Lithium, Sodium and Potassium (All group 1 elements)
• Calcium, Strontium, Barium (All group 2 elements)
• Chlorine, Bromine and Iodine (All group 7 elements)

John Newlands, an English chemist, proposed the ‘Law of Octaves’ which had the elements arranged in order of increasing atomic weight. Newlands believed that each eighth element had similar properties. Many of the eighth elements were part of Dobereiner’s triads (eg. the first, the eighth and fifteenth; the second, the ninth and the sixteenth). The 'Law of Octaves' stated ‘the eighth element starting from a given one is a kind of repetition of the first like the eighth note in an octave of music’.
His ‘Law of Octaves’ only worked up to calcium; eg. Titanium does not have similar properties to Aluminium. The ‘Law of Octaves’ did not include the noble gases as they were not yet discovered. The ‘law’ identified many similarities among elements but required similarities where none existed.

Russian chemist and inventor, Dmitri Mendeleev, is said to be the creator of the modern periodic table. He knew of 63 elements and it’s said he wrote the names and properties of the elements on small cards which he then arranged in order of atomic weight. The cards then got rearranged, maintaining their order, into groups with similar properties. He then proposed the periodic law: ‘Elements have properties that recur or repeat according to their atomic weight.’
Previous scientists had attempted to logically organise the elements but Mendeleev was the first to leave ‘holes’ for elements not yet discovered, the properties of which could be predicted from the first periodic table. He also predicted the properties of six elements that could fill the ‘holes’. Three of those predictions he called ‘eka-silicon’, ‘eka-boron’ and ‘eka-aluminium’ (‘eka’ meaning ‘like’). Germanium, scandium and gallium were discovered with properties very similar to his predictions, respectively. This convinced many scientists of the accuracy, and value, of Mendeleev’s periodic table.

Scottish chemist, William Ramsay, used the ‘new’ technology of refrigeration to liquefy and separate components of air. He successfully removed water, carbon dioxide, oxygen and nitrogen but found he had an unknown gas left behind. This was argon, the first noble gas and further experimentation identified helium, neon, krypton and xenon. Noble gases have a full valence shell of electrons which means they are very stable and unreactive to other elements.

Henry Moseley, a young English physicist, used X-Ray technology to refine the order of some of the elements of Mendeleev’s periodic table and proposed a minor change to the periodic law: ‘Elements have properties that recur or repeat according to their atomic number.’ He proposed this because some elements were not under similar elements because of their atomic weight. This was fixed by atomic numbers. The law put argon and potassium, cobalt and nickel, and tellurium and iodine in their correct orders.