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Periodic Table

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Traite Elementaire de Chimie.

Produced the first table of elements

Introduced a logical system for naming compounds

and helped introduce the metric system

May 8th 1794

Dalton, 1803, was the first chemist to use the term ‘atom’

He used this idea to explain how elements react together to

form molecules.

Dalton suggested that it should be possible to compare the masses

of atoms.

All atomic weights are multiples of Hydrogen and that

Hydrogen is the fundamental element from which all other elements are formed.

However, it was found that atomic weights

are not whole numbers. E.g. Cl 35.46

More on atomic numbers

Noticed that certain groups of 3 elements,

e.g. Ca, Sr, Ba the atomic weight of 2nd was

approximately the mean the 1st and 3rd.

Dobëreiner’s triads (5 in total)

Telluric Screw

He also realised that when elements are arranged in order of their atomic weight there was a repeating pattern of elements at regular intervals.

That this happens when there are multiplies of 8x the atomic weight of hydrogen.

So he arranged them in a spiral around a vertical cylinder divided into 16 vertical sections.

Newlands in 1865, using Cannizzaro’s system (elements in order of

succession) of atomic weights noticed a pattern, noticed that

the 8th one was a ‘kind of repetition of the 1st.

He called this the ‘Law of Octaves’. OK for the first 15 or so elements

Meyer in 1869, independently, put forward a similar list of elements.

Meyer plotted an ‘atomic volume’ curve, showing that a quantitative

property alternatively rises and falls over definite periods of the

Elements.

Unlike Meyer, Mendeleyev believed in his convictions.

In 1869 he published ‘Principles of Chemistry’

Mendeleev, without knowing about Meyer’s work, predicted as yet undiscovered elements.

Meyer recognised Mendeleev’s work and both where awarded The Davy medal for Chemistry in 1882.

Mendeleev predicted the properties of this element, he called eka-silicon

Mendeleev also predicted the properties of Gallium and Scandium

What was the faulty reasoning that led to inaccurate relative atomic masses

(atomic weights)?

There were two main faults.

First chemists were not distinguishing between the weights of atoms and of molecules.

Seven common elements exist as diatomic molecules. Of special importance

was hydrogen, the original standard for atomic weights.

If a molecule of H2 is given a relative mass of 1 instead of 2,

then when other elements are compared with it, their relative atomic masses are

halved.

Second, at the time chemists used a term called equivalent, or combining weight.

This was the number of grams of an element that combined with 8 g of oxygen (easier

to do than with hydrogen)

(They used this because 8 g of oxygen combine with 1 g hydrogen so 8 g of oxygen

was equivalent to 1 g hydrogen.)