1. JOHANN DOBEREINER’S TRIAD
Arranged the elements in groups of three based on similarity in their properties
Noted that if elements are arranged in order of increasing atomic mass, the average of the mass of the lightest and heaviest is very close to the mass of the middle member
Element
Atomic Mass
Average Li
6.941
23.02 Na
22.99 K
39.10

3. JOHN NEWLAND’S LAW OF OCTAVES
- if elements are arranged in terms of increasing atomic mass, every eight element had similar properties.
- Compared this pattern with the musical scale 1 2 3 4 5 6 7 8 Li Be B C N O F Na Mg He Si P S Cl K do re mi fa
sol la ti

4.                                                                              
To Newlands, Li to Na is an octave of eight elements.

5. FEATURES OF THE PERIODIC TABLE PROPOSED BY DMITRI MENDELEEV
1. Arranged the elements in order of increasing atomic mass (Meyer did the same)
2. Gave attention to periodicity of chemical properties (Meyer did the same)
3. Left vacant spaces where an element should fit even when such an element was not yet known (Meyer did the same
4. Did not restrict the length of the periods to 8 elements but allowed the known properties to determine the length of the periods.
5. Allowed the chemical properties to prevail over the atomic mass.
6. Predicted the presence of missing elements based on the periodicity of properties.

7. PROPERTIES OF EKA-SILICON AND GERMANIUM
PROPERTY
Eka-silicon as predicted by Mendeleev
Germanium (Ge) as actually determined later
Atomic Mass 72
72.6
Physical Appearance
Grey metal
Density
5.5 g/ml
5.36 g/ml
Melting Point
High
958 oC
Formation of Oxide
EsO2
GeO2
Density of Oxide
4.7
4.70
Reaction with HCl
slight
None at 25 oC

8. THEORETICAL BASIS OF THE PERIODIC TABLE
1. Elements with the similar properties are placed in a vertical columns called groups, which are numbered from 1 to 18.
2. Elements in the same group have the same number of valence electrons and have similar electronic configurations.
3. A new period or row is started with the element which has one electron in its outermost main energy level.

9. Inner transition metals

11. PROPOSED
VERSIONS
OF THE
PERIODIC TABLE

17. Same atomic number but different atomic mass number
ISOTOPES
Particles having the same number of protons but different numbers of neutrons.
Same atomic number but different atomic mass number

20. Weighted average atomic mass
Weighted average of the masses of the isotopes of an element
Element
Symbol
Natural abundance
Mass (amu)
Weighted average atomic mass
Oxygen
16 8 O
99.759
15.995
( X )
+ ( X )
+ ( X ) =
17 8 O
0.037
16.995
18 8 O
0.204
17.999