Periodic Table
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
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
Prout 1815
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
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
Element
Relative
atomic mass
Lithium
7
Calcium
40
Sodium
23
Strontium
88
Potassium
39
Barium
137
Béguyer de Chancourtois
1862
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
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
Element
Atomic
Element
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
Iron
14
21
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
Element
II
Atomic
weights
Element
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
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
1
H
2
Li
Be
B
C
N
O
F
3
Na
K
Mg
Ca
Al
*
Si
Ti
P
V
S
Cr
Cl
Mn
4
Cu
Rb
Zn
Sr
*
Y
*
Zr
As
Nb
Se
Mo
Br
*
Ag
Cd
In
Sn
Sb
Te
I
5
VII
Fe Co Ni
Ru Rh Pd
Mendeleev predicted the properties of this element, he called eka-silicon
Eka-Silicon
Property
Prediction
Appearance
Relative Atomic
mass
Density
Dark-grey
72
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
Relative Atomic
mass
Density
Dark-grey
73
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
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 of molecule
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 (easie
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.)