Dobereiners Triads: – A German chemist Doberneiner observed the relationship between atomic masses of several groups of three elements in which the central or middle element had atomic mass average of the other two elements.
Newlands Octaves; – In 1984, Newland put forward his observations in the form of law of octaves. According to this law “there is a repetition in chemical properties of every eight element if they are arranged by increasing their atomic masses”.
Li Be B C N O F
7 9 11 12 14 16 19
Na Mg Al Si P S Cl
23 24 27 28 31 32 35.5
The properties of Sodium (Na) are similar to those of Lithium (Li). Similarly the properties of Chlorine ( Cl ) are similar to Fluorine(F).
Drawbacks of Newlands law of octaves:- The law of octaves failed because of two reasons.
- No space was left for undiscovered elements.
- At that time noble gases were also not known.
Mendeleev’s periodic table;- In 1869, a Russian chemist Mendeleev for the first time introduced the name periodic table. He arranged the known 63 elements in order of their increasing atomic masses. The vertical columns were called groups and the horizontal rows were called periods.
Elements having same properties are placed in the same group. This periodic table got the name Mendeleev’s periodic table due to name of presenter scientist and was based on Mendeleev’s law, this law states.
“Properties of elements are periodic functions of their atomic masses”.
Merits of Mendeleev’s periodic table:-
1. Systematic arrangements of 63 elements in groups and periods.
2. Corrected doubtful masses of some elements like Be, Pd, Pt etc.
3. Predicted properties of elements which were not discovered -like scandium, gallium, germanium, which was corrected and left gaps for them.
Demerits of Mendeleev’s periodic table:-
Controversial position of hydrogen.
2. Position of isotopes was not correct.
3. Two similar elements like Cu & Hg were grouped differently.
4. Co with higher atomic no. was placed before lower atomic no. Ni which was contradicting arrangement of element in increasing atomic no.
5. Did not provide space for noble gases which were discovered later.
- No space left for isotopes.