Periodic Table

Lavoisier 1789

Traite Elementaire de Chimie.

Produced the first table of elements

Introduced a logical system for naming compoundsand 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 toform molecules.

Dalton suggested that it should be possible to compare the massesof atoms.

Hydrogen 1

Carbone 4.2

Oxygen 5.5

Water 6.5

Sulphur 14.4

Sulphuric Acid

25.4

Atomic Weights

Prout 1815

All atomic weights are multiples of Hydrogen and thatHydrogen is the fundamental element from which all other elements are formed.

However, it was found that atomic weightsare not whole numbers. E.g. Cl 35.46

More on atomic numbers

Dobëreiner 1817

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

Calcium 40

Strontium 88

Barium 137

Element Relative atomic mass

Lithium 7

Sodium 23

Potassium 39

Béguyer de Chancourtois 1862

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.

Telluric Screw

Newlands

Element Atomic weights

Element Atomic Weights

Element 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

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

ElementI

Atomic weights

ElementII

Atomic weights

ElementIII

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

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

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.

Mendeleyev 1871

period I II III IV V VI VII VII

1 H

2 Li Be B C N O F

3 NaK

MgCa

Al*

SiTi

PV

SCr

ClMn

Fe Co Ni

4 CuRb

ZnSr

*Y

*Zr

AsNb

SeMo

Br* Ru Rh Pd

5 Ag Cd In Sn Sb Te I

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

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.

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

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 (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.)

Relative Atomic Masses