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Periodic Table. http://www.markbardsleyillustration.co.uk/. Lavoisier 1789. Traite Elementaire de Chimie. http://web.bilkent.edu.tr/. Produced the first table of elements. Introduced a logical system for naming compounds and helped introduce the metric system. May 8th 1794. Atomic Weights.

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periodic table
Periodic Table

http://www.markbardsleyillustration.co.uk/

lavoisier 1789
Lavoisier 1789

Traite Elementaire de Chimie.

http://web.bilkent.edu.tr/

Produced the first table of elements

Introduced a logical system for naming compounds

and helped introduce the metric system

May 8th 1794

atomic weights
Atomic Weights

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.

www.bioanalytical.com

prout 1815
Prout 1815

All atomic weights are multiples of Hydrogen and that

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

elmoscow.ru

However, it was found that atomic masses

are not whole numbers. E.g. Cl 35.46

More on atomic masses

dob reiner 1817
Dobëreiner 1817

www.eoearth.org

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)

b guyer de chancourtois 1862
Béguyer de Chancourtois 1862

Telluric Screw

fr.wikipedia.org

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
Newlands

www.chemsoc.org

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
Meyer

www.apsidium.com

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.

mendeleev 1871
Mendeleev 1871

Unlike Meyer, Mendeleev 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.

elmoscow.ru

mendeleev 18711
Mendeleev 1871

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

germanium
Germanium

Mendeleev also predicted the properties of Gallium and Scandium

relative atomic masses
Relative Atomic Masses

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 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

(easierto 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.)