1. Periodic Table

2. Lavoisier 1789 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

3. 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.
Hydrogen 1
Carbone
4.2
Oxygen
5.5
Water
6.5
Sulphur
14.4
Sulphuric Acid
25.4

4. Prout 1815 All atomic weights are multiples of Hydrogen and that
Hydrogen is the fundamental element from which all other elements are formed.
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However, it was found that atomic masses
are not whole numbers. E.g. Cl 35.46
More on atomic masses

5. Dobëreiner 1817 Noticed that certain groups of 3 elements,
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)
Element
Relative atomic mass
Lithium 7
Sodium 23
Potassium 39
Element
Relative atomic mass
Calcium 40
Strontium 88
Barium
137

6. Béguyer de Chancourtois 1862
Telluric Screw
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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.

7. Newlands
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
Element
Atomic weights
Atomic Weights
Hydrogen 1
Fluorine 8
Chlorine 15
Lithium 2
Sodium 9
Potassium 16
Beryllium 3
Magnesium 10
Calcium 17
Boron 4
Aluminium 11
Chromium 18
Carbon 5
Silicon 12
Titanium 19
Nitrogen 6
Phosphorus 13
Manganese 20
Oxygen 7
Sulphur 14
Iron 21

8. Meyer
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.
Element I
Atomic weights II
III
Atomic Weights B 11 Al
27.3 C
11.97 Si 28 N
14.01 P
30.9 O
15.96 S
31.98 F
19.1 Cl
35.38 Li
7.01 Na
22.99 K
39.04
?Be
9.3 Mg
23.9 Ca
39.9

9. 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.
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10. Mendeleev 1871 period I II III IV V VI VII 1 H 2 Li Be B C N O F 3 NaK Mg Ca Al * Si Ti P S Cr Cl Mn
Fe Co Ni 4 Cu Rb Zn Sr Y Zr As Nb Se Mo Br
Ru Rh Pd 5 Ag Cd In Sn Sb Te
Mendeleev predicted the properties of this element, he called eka-silicon

11. Eka-Silicon Property Prediction Appearance Dark-grey
Relative Atomic mass 72
Density
5.5 g cm-3
Reaction with water
None
Reaction with alkali
More than with acid
Reaction with Acid
Very little
Oxide
Basic
Chloride
Liquid, <100oC b.p.

12. Germanium Property Germanium 1885 Appearance Dark-grey
Relative Atomic mass 73
Density
5.35 g cm-3
Reaction with water
None
Reaction with alkali
More than with acid
Reaction with Acid
Very little
Oxide
Basic
Chloride
Liquid, <100oC b.p.
Mendeleev also predicted the properties of Gallium and Scandium

13. 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.)